This articlemay havetoo many section headers. Please help consolidate the article.(March 2017) (Learn how and when to remove this message)

This articlemay be too technical for most readers to understand. Pleasehelp improve it tomake it understandable to non-experts, without removing the technical details.(March 2017) (Learn how and when to remove this message)

C++11 is a version of a jointtechnical standard, ISO/IEC 14882, by theInternational Organization for Standardization (ISO) andInternational Electrotechnical Commission (IEC), for theC++ programming language. C++11 replaced the prior version of the C++ standard, namedC++03,^[1] and was later replaced byC++14. The name follows the tradition of naming language versions by the publication year of the specification, though it was formerly namedC++0x because it was expected to be published before 2010.^[2]

Although one of the design goals was to prefer changes to the libraries over changes to thecore language,^[3] C++11 does make several additions to the core language. Areas of the core language that were significantly improved include multithreading support,generic programming support, uniform initialization, and performance. Significant changes were also made to theC++ Standard Library, incorporating most of theC++ Technical Report 1 (TR1)libraries, except the library of mathematical special functions.^[4]

C++11 was published asISO/IEC 14882:2011^[5] in September 2011 and is available for a fee. The working draft most similar to the published C++11 standard is N3337, dated 16 January 2012;^[6] it has only editorial corrections from the C++11 standard.^[7]

C++11 is fully supported byClang 3.3 and later.^[8] C++11 is fully supported byGNU Compiler Collection (GCC) 4.8.1 and later.^[9]

The design committee attempted to stick to a number of goals in designing C++11:

• Maintain stability and compatibility with older code
• Prefer introducing new features via the standard library, rather than extending the core language
• Improve C++ to facilitate systems and library design, rather than introduce new features useful only to specific applications
• Increase type safety by providing safer alternatives to earlier unsafe techniques
• Increase performance and the ability to work directly with hardware
• Provide proper solutions for real-world problems
• Make C++ easy to teach and to learn without removing any utility needed by expert programmers

Attention to beginners is considered important, because most computer programmers will always be such, and because many beginners never widen their knowledge, limiting themselves to work in aspects of the language in which they specialize.^[2][3]

One function of the C++ committee is the development of the language core. Areas of the core language that were significantly improved includemultithreading support,generic programming support, uniform initialization, and performance.

These language features primarily exist to provide some kind ofruntime performance benefit, either of memory or of computing speed.^{[citation needed]}

In C++03 (and before), temporaries (termed "rvalues", as they often lie on the right side of an assignment) were intended to never be modifiable — just as in C — and were considered to be indistinguishable fromconst T& types; nevertheless, in some cases, temporaries could have been modified, a behavior that was even considered to be a useful loophole.^[10] C++11 adds a new non-constreference type called anrvalue reference, identified byT&&. This refers to temporaries that are permitted to be modified after they are initialized, for the purpose of allowing "move semantics".

A chronic performance problem with C++03 is the costly and unneededdeep copies that can happen implicitly when objects are passed by value. To illustrate the issue, consider that anstd::vector<T> is, internally, a wrapper around a C-style array with a defined size. If anstd::vector<T> temporary is created or returned from a function, it can be stored only by creating a newstd::vector<T> and copying all the rvalue's data into it. Then the temporary and all its memory is destroyed. (For simplicity, this discussion neglects thereturn value optimization.)

In C++11, amove constructor ofstd::vector<T> that takes an rvalue reference to anstd::vector<T> can copy the pointer to the internal C-style array out of the rvalue into the newstd::vector<T>, then set the pointer inside the rvalue to null. Since the temporary will never again be used, no code will try to access the null pointer, and because the pointer is null, its memory is not deleted when it goes out of scope. Hence, the operation not only forgoes the expense of a deep copy, but is safe and invisible.

Rvalue references can provide performance benefits to existing code without needing to make any changes outside the standard library. The type of the returned value of a function returning anstd::vector<T> temporary does not need to be changed explicitly tostd::vector<T> && to invoke the move constructor, as temporaries are considered rvalues automatically. (However, ifstd::vector<T> is a C++03 version without a move constructor, then the copy constructor will be invoked with aconst std::vector<T>&, incurring a significant memory allocation.)

For safety reasons, some restrictions are imposed. A named variable will never be considered to be an rvalue even if it is declared as such. To get an rvalue, the function templatestd::move() should be used. Rvalue references can also be modified only under certain circumstances, being intended to be used primarily with move constructors.

Due to the nature of the wording of rvalue references, and to some modification to the wording for lvalue references (regular references), rvalue references allow developers to provide perfect function forwarding. When combined withvariadic templates, this ability allows for function templates that can perfectly forward arguments to another function that takes those particular arguments. This is most useful for forwarding constructor parameters, to create factory functions that will automatically call the correct constructor for those particular arguments. This is seen in theemplace_back set of the C++ standard library methods.

C++ has always had the concept of constant expressions. These are expressions such as3+4 that will always yield the same results, at compile time and at runtime. Constant expressions are optimization opportunities for compilers, and compilers frequentlyexecute them at compile time and hardcode the results in the program. Also, in several places, the C++ specification requires using constant expressions. Defining an array requires a constant expression, and enumerator values must be constant expressions.

However, a constant expression has never been allowed to contain a function call or object constructor. So a piece of code as simple as this is invalid:

intget_five(){return5;}intsome_value[get_five()+7];// Create an array of 12 integers. Ill-formed C++

This was not valid in C++03, becauseget_five() + 7 is not a constant expression. A C++03 compiler has no way of knowing ifget_five() actually is constant at runtime. In theory, this function could affect a global variable, call other non-runtime constant functions, etc.

C++11 introduced the keywordconstexpr, which allows the user to guarantee that a function or object constructor is a compile-time constant.^[11] The above example can be rewritten as follows:

constexprintget_five(){return5;}intsome_value[get_five()+7];// Create an array of 12 integers. Valid C++11

This allows the compiler to understand, and verify, thatget_five() is a compile-time constant.

Usingconstexpr on a function imposes some limits on what that function can do. First, the function must have a non-void return type. Second, the function body cannot declare variables or define new types. Third, the body may contain only declarations, null statements and a single return statement. There must exist argument values such that, after argument substitution, the expression in the return statement produces a constant expression.

Before C++11, the values of variables could be used in constant expressions only if the variables are declared const, have an initializer which is a constant expression, and are of integral or enumeration type. C++11 removes the restriction that the variables must be of integral or enumeration type if they are defined with theconstexpr keyword:

constexprdoubleearth_gravitational_acceleration=9.8;constexprdoublemoon_gravitational_acceleration=earth_gravitational_acceleration/6.0;

Such data variables are implicitly const, and must have an initializer which must be a constant expression.

To construct constant expression data values from user-defined types, constructors can also be declared withconstexpr. Aconstexpr constructor's function body can contain only declarations and null statements, and cannot declare variables or define types, as with aconstexpr function. There must exist argument values such that, after argument substitution, it initializes the class's members with constant expressions. The destructors for such types must be trivial.

The copy constructor for a type with anyconstexpr constructors should usually also be defined as aconstexpr constructor, to allow objects of the type to be returned by value from a constexpr function. Any member function of a class, such as copy constructors, operator overloads, etc., can be declared asconstexpr, so long as they meet the requirements for constexpr functions. This allows the compiler to copy objects at compile time, perform operations on them, etc.

If a constexpr function or constructor is called with arguments which aren't constant expressions, the call behaves as if the function were not constexpr, and the resulting value is not a constant expression. Likewise, if the expression in the return statement of a constexpr function does not evaluate to a constant expression for a given invocation, the result is not a constant expression.

constexpr differs fromconsteval, introduced inC++20, in that the latter must always produce a compile time constant, whileconstexpr does not have this restriction.

In C++03, a class or struct must follow a number of rules for it to be considered aplain old data (POD) type. Types that fit this definition produce object layouts that are compatible with C, and they could also be initialized statically. The C++03 standard has restrictions on what types are compatible with C or can be statically initialized despite there being no technical reason a compiler couldn't accept the program; if someone were to create a C++03 POD type and add a non-virtual member function, this type would no longer be a POD type, could not be statically initialized, and would be incompatible with C despite no change to the memory layout.

C++11 relaxed several of the POD rules, by dividing the POD concept into two separate concepts:trivial andstandard-layout.

A type that istrivial can be statically initialized. It also means that it is valid to copy data around viamemcpy, rather than having to use a copy constructor. The lifetime of atrivial type begins when its storage is defined, not when a constructor completes.

A trivial class or struct is defined as one that:

1. Has a trivial default constructor. This may use thedefault constructor syntax (SomeConstructor() = default;).
2. Has trivial copy and move constructors, which may use the default syntax.
3. Has trivial copy and move assignment operators, which may use the default syntax.
4. Has a trivial destructor, which must not be virtual.

Constructors are trivial only if there are no virtual member functions of the class and no virtual base classes. Copy/move operations also require all non-static data members to be trivial.

A type that isstandard-layout means that it orders and packs its members in a way that is compatible with C. A class or struct is standard-layout, by definition, provided:

1. It has novirtual functions
2. It has no virtual base classes
3. All its non-static data members have the same access control (public, private, protected)
4. All its non-static data members, including any in its base classes, are in the same one class in the hierarchy
5. The above rules also apply to all the base classes and to all non-static data members in the class hierarchy
6. It has no base classes of the same type as the first defined non-static data member

A class/struct/union is considered POD if it is trivial, standard-layout, and all of its non-static data members and base classes are PODs.

By separating these concepts, it becomes possible to give up one without losing the other. A class with complex move and copy constructors may not be trivial, but it could be standard-layout and thus interoperate with C. Similarly, a class with public and private non-static data members would not be standard-layout, but it could be trivial and thusmemcpy-able.

In C++03, the compiler must instantiate a template whenever a fully specified template is encountered in a translation unit. If the template is instantiated with the same types in many translation units, this can dramatically increase compile times. There is no way to prevent this in C++03, so C++11 introduced extern template declarations, analogous to extern data declarations.

C++03 has this syntax to oblige the compiler to instantiate a template:

templateclassstd::vector<MyClass>;

C++11 now provides this syntax:

externtemplateclassstd::vector<MyClass>;

which tells the compilernot to instantiate the template in this translation unit.

These features exist for the primary purpose of making the language easier to use. These can improve type safety, minimize code repetition, make erroneous code less likely, etc.

C++03 inherited the initializer-list feature from C. A struct or array is given a list of arguments in braces, in the order of the members' definitions in the struct. These initializer-lists are recursive, so an array of structs or struct containing other structs can use them.

structObject{floatfirst;intsecond;};Objectscalar={0.43f,10};//One Object, with first=0.43f and second=10ObjectanArray[]={{13.4f,3},{43.28f,29},{5.934f,17}};//An array of three Objects

This is very useful for static lists, or initializing a struct to some value. C++ also provides constructors to initialize an object, but they are often not as convenient as the initializer list. However, C++03 allows initializer-lists only on structs and classes that conform to the Plain Old Data (POD) definition; C++11 extends initializer-lists, so they can be used for all classes including standard containers likestd::vector.

C++11 binds the concept to a template, calledstd::initializer_list. This allows constructors and other functions to take initializer-lists as parameters. For example:

classSequenceClass{public:SequenceClass(std::initializer_list<int>list);};

This allowsSequenceClass to be constructed from a sequence of integers, such as:

SequenceClasssome_var={1,4,5,6};

This constructor is a special kind of constructor, called an initializer-list-constructor. Classes with such a constructor are treated specially during uniform initialization (seebelow)

The template classstd::initializer_list<> is afirst-class C++11 standard library type. They can be constructed statically by the C++11 compiler via use of the{} syntax without a type name in contexts where such braces will deduce to anstd::initializer_list, or by explicitly specifying the type likestd::initializer_list<SomeType>{args} (and so on for other varieties of construction syntax).

The list can be copied once constructed, which is cheap and will act as a copy-by-reference (the class is typically implemented as a pair of begin/end pointers). Anstd::initializer_list is constant: its members cannot be changed once it is created, and nor can the data in those members be changed (which rules out moving from them, requiring copies into class members, etc.).

Although its construction is specially treated by the compiler, anstd::initializer_list is a real type, and so it can be used in other places besides class constructors. Regular functions can take typedstd::initializer_lists as arguments. For example:

voidfunction_name(std::initializer_list<float>list);// Copying is cheap; see abovefunction_name({1.0f,-3.45f,-0.4f});

Examples of this in the standard library include thestd::min() andstd::max() templates takingstd::initializer_lists of numeric type.

Standard containers can also be initialized in these ways:

std::vector<std::string>v={"xyzzy","plugh","abracadabra"};std::vector<std::string>v({"xyzzy","plugh","abracadabra"});std::vector<std::string>v{"xyzzy","plugh","abracadabra"};// see "Uniform initialization" below

C++03 has a number of problems with initializing types. Several ways to do this exist, and some produce different results when interchanged. The traditional constructor syntax, for example, can look like a function declaration, and steps must be taken to ensure that the compiler'smost vexing parse rule will not mistake it for such. Only aggregates and POD types can be initialized with aggregate initializers (usingSomeType var = {/*stuff*/};).

C++11 provides a syntax that allows for fully uniform type initialization that works on any object. It expands on the initializer list syntax:

structBasicStruct{intx;doubley;};structAltStruct{AltStruct(intx,doubley):x_{x},y_{y}{}private:intx_;doubley_;};BasicStructvar1{5,3.2};AltStructvar2{2,4.3};

The initialization ofvar1 behaves exactly as though it were aggregate-initialization. That is, each data member of an object, in turn, will be copy-initialized with the corresponding value from the initializer-list. Implicit type conversion will be used where needed. If no conversion exists, or only a narrowing conversion exists, the program is ill-formed. The initialization ofvar2 invokes the constructor.

One can also do this:

structIdString{std::stringname;intidentifier;};IdStringget_string(){return{"foo",42};//Note the lack of explicit type.}

Uniform initialization does not replace constructor syntax, which is still needed at times. If a class has an initializer list constructor (TypeName(initializer_list<SomeType>);), then it takes priority over other forms of construction, provided that the initializer list conforms to the sequence constructor's type. The C++11 version ofstd::vector has an initializer list constructor for its template type. Thus this code:

std::vector<int>the_vec{4};

will call the initializer list constructor, not the constructor ofstd::vector that takes a single size parameter and creates the vector with that size. To access the latter constructor, the user will need to use the standard constructor syntax directly.

In C++03 (and C), to use a variable, its type must be specified explicitly. However, with the advent of template types and template metaprogramming techniques, the type of something, particularly the well-defined return value of a function, may not be easily expressed. Thus, storing intermediates in variables is difficult, possibly needing knowledge of the internals of a given metaprogramming library.

C++11 allows this to be mitigated in two ways. First, the definition of a variable with an explicit initialization can use theauto keyword.^[12][13] This creates a variable of the specific type of the initializer:

autosome_strange_callable_type=std::bind(&some_function,_2,_1,some_object);autoother_variable=5;

The type ofsome_strange_callable_type is simply whatever the particular template function override ofstd::bind returns for those particular arguments. This type is easily determined procedurally by the compiler as part of its semantic analysis duties, but is not easy for the user to determine upon inspection.The type ofother_variable is also well-defined, but it is easier for the user to determine. It is anint, which is the same type as the integer literal.

This use of the keywordauto in C++ re-purposes the semantics of this keyword, which was originally used in the typeless predecessor languageB in a related role of denoting an untypedautomatic variable definition.

Further, the keyworddecltype can be used to determine the type of expression at compile-time. For example:

intsome_int;decltype(some_int)other_integer_variable=5;

This is more useful in conjunction withauto, since the type of auto variable is known only to the compiler. However,decltype can also be very useful for expressions in code that makes heavy use ofoperator overloading and specialized types.

auto is also useful for reducing the verbosity of the code. For instance, instead of writing

for(std::vector<int>::const_iteratoritr=myvec.cbegin();itr!=myvec.cend();++itr)

the programmer can use the shorter

for(autoitr=myvec.cbegin();itr!=myvec.cend();++itr)

which can be further compacted since "myvec" implements begin/end iterators:

for(constauto&x:myvec)

This difference grows as the programmer begins to nest containers, though in such casestypedefs are a good way to decrease the amount of code.

The type denoted bydecltype can be different from the type deduced byauto.

#include<vector>intmain(){conststd::vector<int>v(1);autoa=v[0];// a has type intdecltype(v[0])b=1;// b has type const int&, the return type of//   std::vector<int>::operator[](size_type) constautoc=0;// c has type intautod=c;// d has type intdecltype(c)e;// e has type int, the type of the entity named by cdecltype((c))f=c;// f has type int&, because (c) is an lvaluedecltype(0)g;// g has type int, because 0 is an rvalue}

C++11 extends the syntax of thefor statement to allow for easy iteration over a range of elements:

intmy_array[5]={1,2,3,4,5};// double the value of each element in my_array:for(int&x:my_array)x*=2;// similar but also using type inference for array elementsfor(auto&x:my_array)x*=2;

This form offor, called the “range-based for”, will iterate over each element in the list. It will work for C-style arrays, initializer lists, and any type that hasbegin() andend() functions defined for it that return iterators. All the standard library containers that have begin/end pairs will work with the range-based for statement.

C++11 provides the ability to createanonymous functions, called lambda functions.^[14]These are defined as follows:

[](intx,inty)->int{returnx+y;}

The return type (-> int in this example) can be omitted as long as allreturn expressions return the same type.A lambda can optionally be aclosure.

Standard C function declaration syntax was perfectly adequate for the feature set of the C language. As C++ evolved from C, it kept the basic syntax and extended it where needed. However, as C++ grew more complex, it exposed several limits, especially regarding template function declarations. For example, in C++03 this is invalid:

template<classLhs,classRhs>Retadding_func(constLhs&lhs,constRhs&rhs){returnlhs+rhs;}//Ret must be the type of lhs+rhs

The typeRet is whatever the addition of typesLhs andRhs will produce. Even with the aforementioned C++11 functionality ofdecltype, this is not possible:

template<classLhs,classRhs>decltype(lhs+rhs)adding_func(constLhs&lhs,constRhs&rhs){returnlhs+rhs;}//Not valid C++11

This is not valid C++ becauselhs andrhs have not yet been defined; they will not be valid identifiers until after the parser has parsed the rest of the function prototype.

To work around this, C++11 introduced a new function declaration syntax, with atrailing-return-type:^[15]

template<classLhs,classRhs>autoadding_func(constLhs&lhs,constRhs&rhs)->decltype(lhs+rhs){returnlhs+rhs;}

This syntax can be used for more mundane function declarations and definitions:

structSomeStruct{autofunc_name(intx,inty)->int;};autoSomeStruct::func_name(intx,inty)->int{returnx+y;}

The use of the “auto” keyword in this case is just part of the syntax and does not perform automatic type deduction in C++11. However, starting with C++14, the trailing return type can be removed entirely and the compiler will deduce the return type automatically.^[16]

In C++03, constructors of a class are not allowed to call other constructors in an initializer list of that class. Each constructor must construct all of its class members itself or call a common member function, as follows:

classSomeType{public:SomeType(intnew_number){Construct(new_number);}SomeType(){Construct(42);}private:voidConstruct(intnew_number){number=new_number;}intnumber;};

Constructors for base classes cannot be directly exposed to derived classes; each derived class must implement constructors even if a base class constructor would be appropriate. Non-constant data members of classes cannot be initialized at the site of the declaration of those members. They can be initialized only in a constructor.

C++11 provides solutions to all of these problems.

C++11 allows constructors to call other peer constructors (termeddelegation). This allows constructors to utilize another constructor's behavior with a minimum of added code. Delegation has been used in other languages e.g.,Java andObjective-C.

This syntax is as follows:

classSomeType{intnumber;public:SomeType(intnew_number):number(new_number){}SomeType():SomeType(42){}};

In this case, the same effect could have been achieved by makingnew_number a default parameter. The new syntax, however, allows the default value (42) to be expressed in the implementation rather than the interface — a benefit to maintainers of library code since default values for function parameters are “baked in” to call sites, whereas constructor delegation allows the value to be changed without recompilation of the code using the library.

This comes with a caveat: C++03 considers an object to be constructed when its constructor finishes executing, but C++11 considers an object constructed onceany constructor finishes execution. Since multiple constructors will be allowed to execute, this will mean that each delegating constructor will be executing on a fully constructed object of its own type. Derived class constructors will execute after all delegation in their base classes is complete.

For base-class constructors, C++11 allows a class to specify that base class constructors will be inherited. Thus, the C++11 compiler will generate code to perform the inheritance and the forwarding of the derived class to the base class. This is an all-or-nothing feature: either all of that base class's constructors are forwarded or none of them are. Also, an inherited constructor will beshadowed if it matches the signature of a constructor of the derived class, and restrictions exist for multiple inheritance: class constructors cannot beinherited from two classes that use constructors with the same signature.

The syntax is as follows:

classBaseClass{public:BaseClass(intvalue);};classDerivedClass:publicBaseClass{public:usingBaseClass::BaseClass;};

For member initialization, C++11 allows this syntax:

classSomeClass{public:SomeClass(){}explicitSomeClass(intnew_value):value(new_value){}private:intvalue=5;};

Any constructor of the class will initializevalue with 5, if the constructor does not override the initialization with its own. So the above empty constructor will initializevalue as the class definition states, but the constructor that takes an int will initialize it to the given parameter.

It can also use constructor or uniform initialization, instead of the assignment initialization shown above.

In C++03, it is possible to accidentally create a new virtual function, when one intended to override a base class function. For example:

structBase{virtualvoidsome_func(float);};structDerived:Base{virtualvoidsome_func(int);};

Suppose theDerived::some_func is intended to replace the base class version. But instead, because it has a differentsignature, it creates a second virtual function. This is a common problem, particularly when a user goes to modify the base class.

C++11 provides syntax to solve this problem.

structBase{virtualvoidsome_func(float);};structDerived:Base{virtualvoidsome_func(int)override;// ill-formed - doesn't override a base class method};

Theoverride special identifier means that the compiler will check the base class(es) to see if there is a virtual function with this exact signature. And if there is not, the compiler will indicate an error.

C++11 also adds the ability to prevent inheriting from classes or simply preventing overriding methods in derived classes. This is done with the special identifierfinal. For example:

structBase1final{};structDerived1:Base1{};// ill-formed because the class Base1 has been marked final

structBase2{virtualvoidf()final;};structDerived2:Base2{voidf();// ill-formed because the virtual function Base2::f has been marked final};

In this example, thevirtual void f() final; statement declares a new virtual function, but it also prevents derived classes from overriding it. It also has the effect of preventing derived classes from using that particular function name and parameter combination.

Neitheroverride norfinal are language keywords. They are technically identifiers for declarator attributes:

• they gain special meaning as attributes only when used in those specific trailing contexts (after all type specifiers, access specifiers, member declarations (for struct, class and enum types) and declarator specifiers, but before initialization or code implementation of each declarator in a comma-separated list of declarators);
• they do not alter the declared type signature and do not declare or override any new identifier in any scope;
• the recognized and accepted declarator attributes may be extended in future versions of C++ (some compiler-specific extensions already recognize added declarator attributes, to provide code generation options or optimization hints to the compiler, or to generate added data into the compiled code, intended for debuggers, linkers, and deployment of the compiled code, or to provide added system-specific security attributes, or to enhancereflective programming (reflection) abilities at runtime, or to provide added binding information for interoperability with other programming languages and runtime systems; these extensions may take parameters between parentheses after the declarator attribute identifier; for ANSI conformance, these compiler-specific extensions should use the double underscore prefix convention).
• In any other location, they can be valid identifiers for new declarations (and later use if they are accessible).

For the purposes of this section and this section alone, every occurrence of "0" is meant as "a constant expression which evaluates to0, which is of type int". In reality, the constant expression can be of any integral type.

Since the dawn of C in 1972, the constant0 has had the double role of constant integer andnull pointer constant. The ambiguity inherent in the double meaning of0 was dealt with in C by using the preprocessor macroNULL, which commonly expands to either((void*)0) or0. C++ forbids implicit conversion fromvoid * to other pointer types, thus removing the benefit of casting0 tovoid *. As a consequence, only0 is allowed as a null pointer constant. This interacts poorly withfunction overloading:

voidfoo(char*);voidfoo(int);

IfNULL is defined as0 (which is usually the case in C++), the statementfoo(NULL); will callfoo(int), which is almost certainly not what the programmer intended, and not what a superficial reading of the code suggests.

C++11 corrects this by introducing a new keyword to serve as a distinguished null pointer constant:nullptr. It is of typenullptr_t, which is implicitly convertible and comparable to any pointer type or pointer-to-member type. It is not implicitly convertible or comparable to integral types, except forbool. While the original proposal specified that an rvalue of typenullptr_t should not be convertible tobool, the core language working group decided that such a conversion would be desirable, for consistency with regular pointer types. The proposed wording changes were unanimously voted into the Working Paper in June 2008.^[1] A similar proposal was also brought to the C standard working group and was accepted for inclusion inC23.^[17]

For backwards compatibility reasons,0 remains a valid null pointer constant.

char*pc=nullptr;// OKint*pi=nullptr;// OKboolb=nullptr;// OK. b is false.inti=nullptr;// errorfoo(nullptr);// calls foo(nullptr_t), not foo(int);/*  Note that foo(nullptr_t) will actually call foo(char *) in the example above using an implicit conversion,  only if no other functions are overloading with compatible pointer types in scope.  If multiple overloadings exist, the resolution will fail as it is ambiguous,  unless there is an explicit declaration of foo(nullptr_t).  In standard types headers for C++11, the nullptr_t  type should be declared as:      typedef decltype(nullptr) nullptr_t;  but not as:      typedef int nullptr_t; // prior versions of C++ which need NULL to be defined as 0      typedef void *nullptr_t; // ANSI C which defines NULL as ((void*)0)*/

In C++03, enumerations are not type-safe. They are effectively integers, even when the enumeration types are distinct. This allows the comparison between two enum values of different enumeration types. The only safety that C++03 provides is that an integer or a value of one enum type does not convert implicitly to another enum type. Further, the underlying integral type is implementation-defined; code that depends on the size of the enumeration is thus non-portable. Lastly, enumeration values are scoped to the enclosing scope. Thus, it is not possible for two separate enumerations in the same scope to have matching member names.

C++11 allows a special classification of enumeration that has none of these issues. This is expressed using theenum class (enum struct is also accepted as a synonym) declaration:

enumclassEnumeration{Val1,Val2,Val3=100,Val4// = 101};

This enumeration is type-safe. Enum class values are not implicitly converted to integers. Thus, they cannot be compared to integers either (the expressionEnumeration::Val4 == 101 gives a compile error).

The underlying type of enum classes is always known. The default type isint; this can be overridden to a different integral type as can be seen in this example:

enumclassEnum2:unsignedint{Val1,Val2};

With old-style enumerations the values are placed in the outer scope. With new-style enumerations they are placed within the scope of the enum class name. So in the above example,Val1 is undefined, butEnum2::Val1 is defined.

There is also a transitional syntax to allow old-style enumerations to provide explicit scoping, and the definition of the underlying type:

enumEnum3:unsignedlong{Val1=1,Val2};

In this case the enumerator names are defined in the enumeration's scope (Enum3::Val1), but for backwards compatibility they are also placed in the enclosing scope.

Forward-declaring enums is also possible in C++11. Formerly, enum types could not be forward-declared because the size of the enumeration depends on the definition of its members. As long as the size of the enumeration is specified either implicitly or explicitly, it can be forward-declared:

enumEnum1;// Invalid in C++03 and C++11; the underlying type cannot be determined.enumEnum2:unsignedint;// Valid in C++11, the underlying type is specified explicitly.enumclassEnum3;// Valid in C++11, the underlying type is int.enumclassEnum4:unsignedint;// Valid in C++11.enumEnum2:unsignedshort;// Invalid in C++11, because Enum2 was formerly declared with a different underlying type.

C++03's parser defines “>>” as the right shift operator or stream extraction operator in all cases. However, with nested template declarations, there is a tendency for the programmer to neglect to place a space between the two right angle brackets, thus causing a compiler syntax error.

C++11 improves the specification of the parser so that multiple right angle brackets will be interpreted as closing the template argument list where it is reasonable. This can be overridden by using parentheses around parameter expressions using the “>”, “>=” or “>>” binary operators:

template<boolTest>classSomeType;std::vector<SomeType<1>2>>x1;// Interpreted as a std::vector of SomeType<true>,// followed by "2 >> x1", which is not valid syntax for a declarator. 1 is true.std::vector<SomeType<(1>2)>>x1;// Interpreted as std::vector of SomeType<false>,// followed by the declarator "x1", which is valid C++11 syntax. (1>2) is false.

C++98 added theexplicit keyword as a modifier on constructors to prevent single-argument constructors from being used as implicit type conversion operators. However, this does nothing for actual conversion operators. For example, a smart pointer class may have anoperator bool() to allow it to act more like a primitive pointer: if it includes this conversion, it can be tested withif (smart_ptr_variable) (which would be true if the pointer was non-null and false otherwise). However, this allows other, unintended conversions as well. Because C++bool is defined as an arithmetic type, it can be implicitly converted to integral or even floating-point types, which allows for mathematical operations that are not intended by the user.

In C++11, theexplicit keyword can now be applied to conversion operators. As with constructors, it prevents using those conversion functions in implicit conversions. However, language contexts that specifically need a Boolean value (the conditions of if-statements and loops, and operands to the logical operators) count as explicit conversions and can thus use a bool conversion operator.

For example, this feature solves cleanly thesafe bool issue.

In C++03, it is possible to define a typedef only as a synonym for another type, including a synonym for a template specialization with all actual template arguments specified. It is not possible to create a typedef template. For example:

template<typenameFirst,typenameSecond,intThird>classSomeType;template<typenameSecond>typedefSomeType<OtherType,Second,5>TypedefName;// Invalid in C++03

This will not compile.

C++11 adds this ability with this syntax:

template<typenameFirst,typenameSecond,intThird>classSomeType;template<typenameSecond>usingTypedefName=SomeType<OtherType,Second,5>;

Theusing syntax can also be used as type aliasing in C++11:

typedefvoid(*FunctionType)(double);// Old styleusingFunctionType=void(*)(double);// New introduced syntax

In C++03, there are restrictions on what types of objects can be members of aunion. For example, unions cannot contain any objects that define a non-trivial constructor or destructor. C++11 lifts some of these restrictions.^[2]

If aunion member has a non trivialspecial member function, the compiler will not generate the equivalent member function for theunion and it must be manually defined.

This is a simple example of a union permitted in C++11:

#include<new> // Needed for placement 'new'.structPoint{Point(){}Point(intx,inty):x_(x),y_(y){}intx_,y_;};unionU{intz;doublew;Pointp;// Invalid in C++03; valid in C++11.U(){}// Due to the Point member, a constructor definition is now needed.U(constPoint&pt):p(pt){}// Construct Point object using initializer list.U&operator=(constPoint&pt){new(&p)Point(pt);return*this;}// Assign Point object using placement 'new'.};

The changes will not break any existing code since they only relax current rules.

These features allow the language to do things that were formerly impossible, exceedingly verbose, or needed non-portable libraries.

In C++11, templates can take variable numbers of template parameters. This also allows the definition of type-safevariadic functions.

C++03 offers two kinds ofstring literals. The first kind, contained within double quotes, produces a null-terminated array of typeconst char. The second kind, defined asL"", produces a null-terminated array of typeconst wchar_t, wherewchar_t is a wide-character of undefined size and semantics. Neither literal type offers support for string literals withUTF-8,UTF-16, or any other kind ofUnicodeencodings.

C++11 supports three Unicode encodings: UTF-8, UTF-16, andUTF-32. The definition of the typechar has been modified to explicitly express that it is at least the size needed to store an eight-bit coding of UTF-8, and large enough to contain any member of the compiler's basic execution character set. It was formerly defined as only the latter in the C++ standard itself, then relying on the C standard to guarantee at least 8 bits. Furthermore, C++11 adds two new character types:char16_t andchar32_t. These are designed to store UTF-16 and UTF-32 respectively.

Creating string literals for each of the supported encodings can be done thus:

u8"I'm a UTF-8 string."u"This is a UTF-16 string."U"This is a UTF-32 string."

The type of the first string is the usualconst char[]. The type of the second string isconst char16_t[] (note lower case 'u' prefix). The type of the third string isconst char32_t[] (upper case 'U' prefix).

When building Unicode string literals, it is often useful to insert Unicode code points directly into the string. To do this, C++11 allows this syntax:

u8"This is a Unicode Character:\u2018."u"This is a bigger Unicode Character:\u2018."U"This is a Unicode Character:\U00002018."

The number after the\u is a hexadecimal number; it does not need the usual0x prefix. The identifier\u represents a 16-bit Unicode code point; to enter a 32-bit code point, use\U and a 32-bit hexadecimal number. Only valid Unicode code points can be entered. For example, code points on the range U+D800–U+DFFF are forbidden, as they are reserved for surrogate pairs in UTF-16 encodings.

It is also sometimes useful to avoid escaping strings manually, particularly for using literals ofXML files, scripting languages, or regular expressions. C++11 provides a raw string literal:

R"(The String Data \ Stuff " )"R"delimiter(The String Data \ Stuff " )delimiter"

In the first case, everything between the"( and the)" is part of the string. The" and\ characters do not need to be escaped. In the second case, the"delimiter( starts the string, and it ends only when)delimiter" is reached. The stringdelimiter can be any string up to 16 characters in length, including the empty string. This string cannot contain spaces, control characters,(,), or the\ character. Using this delimiter string, the user can have the sequence)" within raw string literals. For example,R"delimiter("(a-z)")delimiter" is equivalent to"\"(a-z)\"".

Raw string literals can be combined with the wide literal or any of the Unicode literal prefixes:

u8R"XXX(I'm a "raw UTF-8" string.)XXX"uR"*(This is a "raw UTF-16" string.)*"UR"(This is a "raw UTF-32" string.)"

C++03 provides a number of literals. The characters12.5 are a literal that is resolved by the compiler as a typedouble with the value of 12.5. However, the addition of the suffixf, as in12.5f, creates a value of typefloat that contains the value 12.5. The suffix modifiers for literals are fixed by the C++ specification, and C++03 code cannot create new literal modifiers.

By contrast, C++11 enables the user to define new kinds of literal modifiers that will construct objects based on the string of characters that the literal modifies.

Transformation of literals is redefined into two distinct phases: raw and cooked. A raw literal is a sequence of characters of some specific type, while the cooked literal is of a separate type. The C++ literal1234, as a raw literal, is this sequence of characters'1','2','3','4'. As a cooked literal, it is the integer 1234. The C++ literal0xA in raw form is'0','x','A', while in cooked form it is the integer 10.

Literals can be extended in both raw and cooked forms, with the exception of string literals, which can be processed only in cooked form. This exception is due to the fact that strings have prefixes that affect the specific meaning and type of the characters in question.

All user-defined literals are suffixes; defining prefix literals is not possible. All suffixes starting with any character except underscore (_) are reserved by the standard. Thus, all user-defined literals must have suffixes starting with an underscore (_).^[18]

User-defined literals processing the raw form of the literal are defined via a literal operator, which is written asoperator "". An example follows:

OutputTypeoperator""_mysuffix(constchar*literal_string){// assumes that OutputType has a constructor that takes a const char *OutputTyperet(literal_string);returnret;}OutputTypesome_variable=1234_mysuffix;// assumes that OutputType has a get_value() method that returns a doubleassert(some_variable.get_value()==1234.0)

The assignment statementOutputType some_variable = 1234_mysuffix; executes the code defined by the user-defined literal function. This function is passed"1234" as a C-style string, so it has a null terminator.

An alternative mechanism for processing integer and floating point raw literals is via avariadic template:

template<char...>OutputTypeoperator""_tuffix();OutputTypesome_variable=1234_tuffix;OutputTypeanother_variable=2.17_tuffix;

This instantiates the literal processing function asoperator "" _tuffix<'1', '2', '3', '4'>(). In this form, there is no null character terminating the string. The main purpose for doing this is to use C++11'sconstexpr keyword to ensure that the compiler will transform the literal entirely at compile time, assumingOutputType is a constexpr-constructible and copyable type, and the literal processing function is aconstexpr function.

For numeric literals, the type of the cooked literal is eitherunsigned long long for integral literals orlong double for floating point literals. (Note: There is no need for signed integral types because a sign-prefixed literal is parsed as an expression containing the sign as a unary prefix operator and the unsigned number.) There is no alternative template form:

OutputTypeoperator""_suffix(unsignedlonglong);OutputTypeoperator""_suffix(longdouble);OutputTypesome_variable=1234_suffix;// Uses the 'unsigned long long' overload.OutputTypeanother_variable=3.1416_suffix;// Uses the 'long double' overload.

In accord with the formerly mentioned new string prefixes, for string literals, these are used:

OutputTypeoperator""_ssuffix(constchar*string_values,size_tnum_chars);OutputTypeoperator""_ssuffix(constwchar_t*string_values,size_tnum_chars);OutputTypeoperator""_ssuffix(constchar16_t*string_values,size_tnum_chars);OutputTypeoperator""_ssuffix(constchar32_t*string_values,size_tnum_chars);OutputTypesome_variable="1234"_ssuffix;// Uses the 'const char *' overload.OutputTypesome_variable=u8"1234"_ssuffix;// Uses the 'const char *' overload.OutputTypesome_variable=L"1234"_ssuffix;// Uses the 'const wchar_t *'  overload.OutputTypesome_variable=u"1234"_ssuffix;// Uses the 'const char16_t *' overload.OutputTypesome_variable=U"1234"_ssuffix;// Uses the 'const char32_t *' overload.

There is no alternative template form. Character literals are defined similarly.

C++11 standardizes support formultithreaded programming.

There are two parts involved: a memory model which allows multiple threads to co-exist in a program and library support for interaction between threads. (See this article's section onthreading facilities.)

The memory model defines when multiple threads may access the same memory location, and specifies when updates by one thread become visible to other threads.

In a multi-threaded environment, it is common for every thread to have some uniquevariables. This already happens for the local variables of a function, but it does not happen for global and static variables.

A newthread-local storage duration (in addition to the existingstatic,dynamic andautomatic) is indicated by the storage specifierthread_local.

Any object which could have static storage duration (i.e., lifetime spanning the entire execution of the program) may be given thread-local duration instead. The intent is that like any other static-duration variable, a thread-local object can be initialized using a constructor and destroyed using a destructor.

In C++03, the compiler provides, for classes that do not provide them for themselves, a default constructor, a copy constructor, a copy assignment operator (operator=), and a destructor. The programmer can override these defaults by defining custom versions. C++ also defines several global operators (such asoperator new) that work on all classes, which the programmer can override.

However, there is very little control over creating these defaults. Making a class inherently non-copyable, for example, may be done by declaring a private copy constructor and copy assignment operator and not defining them. Attempting to use these functions is a violation of theOne Definition Rule (ODR). While a diagnostic message is not required,^[19] violations may result in a linker error.

In the case of the default constructor, the compiler will not generate a default constructor if a class is defined withany constructors. This is useful in many cases, but it is also useful to be able to have both specialized constructors and the compiler-generated default.

C++11 allows the explicit defaulting and deleting of these special member functions.^[20] For example, this class explicitly declares that a default constructor can be used:

classSomeType{SomeType()=default;//The default constructor is explicitly stated.SomeType(OtherTypevalue);};

A function can be explicitly disabled. This is useful for preventing implicit type conversions.The= delete specifier can be used to prohibit calling a function with particular parameter types.^[20] For example:

voidnoInt(doublei);voidnoInt(int)=delete;

An attempt to callnoInt() with anint parameter will be rejected by the compiler, instead of performing a silent conversion todouble. CallingnoInt() with afloat still works.

It is possible to prohibit calling the function with any type other thandouble by using a template:

doubleonlyDouble(doubled){returnd;}template<typenameT>doubleonlyDouble(T)=delete;

callingonlyDouble(1.0) will work, whileonlyDouble(1.0f) will generate a compiler error.

Class member functions and constructors can also be deleted. For example, it is possible to prevent copying class objects by deleting the copy constructor andoperator =:

classNonCopyable{NonCopyable();NonCopyable(constNonCopyable&)=delete;NonCopyable&operator=(constNonCopyable&)=delete;};

In C++03, the largest integer type islong int. It is guaranteed to have at least as many usable bits asint. This resulted inlong int having size of 64 bits on some popular implementations and 32 bits on others. C++11 adds a new integer typelong long int to address this issue. It is guaranteed to be at least as large as along int, and have no fewer than 64 bits. The type was originally introduced byC99 to the standard C, and most C++ compilers supported it as an extension already.^[21][22]

C++03 provides two methods to testassertions: the macroassert and the preprocessor directive#error. However, neither is appropriate for use in templates: the macro tests the assertion at execution-time, while the preprocessor directive tests the assertion during preprocessing, which happens before instantiation of templates. Neither is appropriate for testing properties that are dependent on template parameters.

The new utility introduces a new way to test assertions at compile-time, using the new keywordstatic_assert.The declaration assumes this form:

static_assert (constant-expression,error-message);

Here are some examples of howstatic_assert can be used:

static_assert((GREEKPI>3.14)&&(GREEKPI<3.15),"GREEKPI is inaccurate!");

template<classT>structCheck{static_assert(sizeof(int)<=sizeof(T),"T is not big enough!");};

template<classIntegral>Integralfoo(Integralx,Integraly){static_assert(std::is_integral<Integral>::value,"foo() parameter must be an integral type.");}

When the constant expression isfalse the compiler produces an error message. The first example is similar to the preprocessor directive#error, although the preprocessor does only support integral types.^[23] In contrast, in the second example the assertion is checked at every instantiation of the template classCheck.

Static assertions are useful outside of templates also. For instance, a given implementation of an algorithm might depend on the size of along long being larger than anint, something the standard does not guarantee. Such an assumption is valid on most systems and compilers, but not all.

In C++03, thesizeof operator can be used on types and objects. But it cannot be used to do this:

structSomeType{OtherTypemember;};sizeof(SomeType::member);// Does not work with C++03. Okay with C++11

This should return the size ofOtherType. C++03 disallows this, so it is a compile error. C++11 allows it. It is also allowed for thealignof operator introduced in C++11.

C++11 allows variable alignment to be queried and controlled withalignof andalignas.

Thealignof operator takes the type and returns the power of 2 byte boundary on which the type instances must be allocated (as astd::size_t). When given a reference typealignof returns the referenced type's alignment; for arrays it returns the element type's alignment.

Thealignas specifier controls the memory alignment for a variable. The specifier takes a constant or a type; when supplied a typealignas(T) is shorthand foralignas(alignof(T)). For example, to specify that a char array should be properly aligned to hold a float:

alignas(float)unsignedcharc[sizeof(float)]

Prior C++ standards provided for programmer-driven garbage collection viaset_new_handler, but gave no definition of object reachability for the purpose of automatic garbage collection. C++11 defines conditions under which pointer values are "safely derived" from other values. An implementation may specify that it operates understrict pointer safety, in which case pointers that are not derived according to these rules can become invalid.

C++11 provides a standardized syntax for compiler/tool extensions to the language. Such extensions were traditionally specified using#pragma directive or vendor-specific keywords (like__attribute__ for GNU and__declspec for Microsoft). With the new syntax, added information can be specified in a form of an attribute enclosed in double square brackets. An attribute can be applied to various elements of source code:

int[[attr1]]i[[attr2,attr3]];[[attr4(arg1,arg2)]]if(cond){[[vendor::attr5]]returni;}

In the example above, attributeattr1 applies to the type of variablei,attr2 andattr3 apply to the variable itself,attr4 applies to theif statement andvendor::attr5 applies to the return statement. In general (but with some exceptions), an attribute specified for a named entity is placed after the name, and before the entity otherwise, as shown above, several attributes may be listed inside one pair of double square brackets, added arguments may be provided for an attribute, and attributes may be scoped by vendor-specific attribute namespaces.

It is recommended that attributes have no language semantic meaning and do not change the sense of a program when ignored. Attributes can be useful for providing information that, for example, helps the compiler to issue better diagnostics or optimize the generated code.

C++11 provides two standard attributes itself:noreturn to specify that a function does not return, andcarries_dependency to help optimizing multi-threaded code by indicating that function arguments or return value carry a dependency.^{[clarification needed]}

A number of new features were introduced in the C++11 standard library. Many of these could have been implemented under the old standard, but some rely (to a greater or lesser extent) on new C++11 core features.

A large part of the newlibraries was defined in the documentC++ Standards Committee's Library Technical Report (called TR1), which was published in 2005. Various full and partial implementations of TR1 are currently available using the namespacestd::tr1. For C++11 they were moved to namespacestd. However, as TR1 features were brought into the C++11 standard library, they were upgraded where appropriate with C++11 language features that were not available in the initial TR1 version. Also, they may have been enhanced with features that were possible under C++03, but were not part of the original TR1 specification.

C++11 offers a number of new language features that the currently existing standard library components can benefit from. For example, most standard library containers can benefit from Rvalue reference based move constructor support, both for quickly moving heavy containers around and for moving the contents of those containers to new memory locations. The standard library components were upgraded with new C++11 language features where appropriate. These include, but are not necessarily limited to:

• Rvalue references and the associated move support
• Support for the UTF-16 encoding unit, and UTF-32 encoding unit Unicode character types
• Variadic templates (coupled with Rvalue references to allow for perfect forwarding)
• Compile-time constant expressions
• decltype
• explicit conversion operators
• Functions declared defaulted or deleted

Further, much time has passed since the prior C++ standard. Much code using the standard library has been written. This has revealed parts of the standard libraries that could use some improving. Among the many areas of improvement considered were standard libraryallocators. A new scope-based model of allocators was included in C++11 to supplement the prior model.

While the C++03 language provides a memory model that supports threading, the primary support for actually using threading comes with the C++11 standard library.

A thread class (std::thread) is provided, which takes afunction object (and an optional series of arguments to pass to it) to run in the new thread. It is possible to cause a thread to halt until another executing thread completes, providing thread joining support via thestd::thread::join() member function. Access is provided, where feasible, to the underlying native thread object(s) forplatform-specific operations by thestd::thread::native_handle() member function.

For synchronization between threads, appropriatemutexes (std::mutex,std::recursive_mutex, etc.) andcondition variables (std::condition_variable andstd::condition_variable_any) are added to the library. These are accessible viaResource Acquisition Is Initialization (RAII) locks (std::lock_guard andstd::unique_lock) and locking algorithms for easy use.

For high-performance, low-level work, communicating between threads is sometimes needed without the overhead of mutexes. This is done usingatomic operations on memory locations. These can optionally specify the minimum memory visibility constraints needed for an operation. Explicitmemory barriers may also be used for this purpose.

The C++11 thread library also includesfutures and promises for passing asynchronous results between threads, andstd::packaged_task for wrapping up a function call that can generate such an asynchronous result. The futures proposal was criticized because it lacks a way to combine futures and check for the completion of one promise inside a set of promises.^[24]

Further high-level threading facilities such asthread pools have been remanded to a future C++technical report. They are not part of C++11, but their eventual implementation is expected to be built entirely on top of the thread library features.

The newstd::async facility provides a convenient method of running tasks and tying them to astd::future. The user can choose whether the task is to be run asynchronously on a separate thread or synchronously on a thread that waits for the value. By default, the implementation can choose, which provides an easy way to take advantage of hardware concurrency without oversubscription, and provides some of the advantages of a thread pool for simple usages.

Tuples are collections composed of heterogeneous objects of pre-arranged dimensions. A tuple can be considered a generalization of a struct's member variables.

The C++11 version of the TR1 tuple type benefited from C++11 features likevariadic templates. To implement reasonably, the TR1 version required an implementation-defined maximum number of contained types, and substantial macro trickery. By contrast, the implementation of the C++11 version requires no explicit implementation-defined maximum number of types. Though compilers will have an internal maximum recursion depth for template instantiation (which is normal), the C++11 version of tuples will not expose this value to the user.

Usingvariadic templates, the declaration of the tuple class looks as follows:

template<class...Types>classtuple;

An example of definition and use of the tuple type:

typedefstd::tuple<int,double,long&,constchar*>test_tuple;longlengthy=12;test_tupleproof(18,6.5,lengthy,"Ciao!");lengthy=std::get<0>(proof);// Assign to 'lengthy' the value 18.std::get<3>(proof)=" Beautiful!";// Modify the tuple’s fourth element.

It's possible to create the tupleproof without defining its contents, but only if the tuple elements' types possess default constructors. Moreover, it's possible to assign a tuple to another tuple: if the two tuples’ types are the same, each element type must possess a copy constructor; otherwise, each element type of the right-side tuple must be convertible to that of the corresponding element type of the left-side tuple or that the corresponding element type of the left-side tuple has a suitable constructor.

typedefstd::tuple<int,double,string>tuple_1t1;typedefstd::tuple<char,short,constchar*>tuple_2t2('X',2,"Hola!");t1=t2;// Ok, first two elements can be converted,// the third one can be constructed from a 'const char *'.

Just likestd::make_pair forstd::pair, there existsstd::make_tuple to automatically createstd::tuples using type deduction andauto helps to declare such a tuple.std::tie creates tuples of lvalue references to help unpack tuples.std::ignore also helps here. See the example:

autorecord=std::make_tuple("Hari Ram","New Delhi",3.5,'A');std::stringname;floatgpa;chargrade;std::tie(name,std::ignore,gpa,grade)=record;// std::ignore helps drop the place namestd::cout<<name<<' '<<gpa<<' '<<grade<<std::endl;

Relational operators are available (among tuples with the same number of elements), and two expressions are available to check a tuple's characteristics (only during compilation):

• std::tuple_size<T>::value returns the number of elements in the tupleT,
• std::tuple_element<I, T>::type returns the type of the object numberI of the tupleT.

Includinghash tables (unordered associative containers) in the C++ standard library is one of the most recurring requests. It was not adopted in C++03 due to time constraints only. Although hash tables are less efficient than abalanced tree in the worst case (in the presence of many collisions), they perform better in many real applications.

Collisions are managed only vialinear chaining because the committee didn't consider it to be opportune to standardize solutions ofopen addressing that introduce quite a lot of intrinsic problems (above all when erasure of elements is admitted). To avoid name clashes with non-standard libraries that developed their own hash table implementations, the prefix “unordered” was used instead of “hash”.

The new library has four types of hash tables, differentiated by whether or not they accept elements with the same key (unique keys or equivalent keys), and whether they map each key to an associated value. They correspond to the four existingbinary search tree based associative containers, with anunordered_ prefix.

The new classes fulfill all the requirements of acontainer class, and have all the methods needed to access elements:insert,erase,begin,end.

This new feature didn't need any C++ language core extensions (though implementations will take advantage of various C++11 language features), only a small extension of the header<functional> and the introduction of headers<unordered_set> and<unordered_map>. No other changes to any existing standard classes were needed, and it doesn't depend on any other extensions of the standard library.

In addition to the hash tables two more containers was added to the standard library. The std::array is a fixed size container that is more efficient than std::vector but safer and easier to use than a c-style array. The std::forward_list is a single linked list that provides more space efficient storage than the double linked std::list when bidirectional iteration is not needed.

The new library, defined in the new header<regex>, is made of a couple of new classes:

• regular expressions are represented by instance of the template classstd::regex;
• occurrences are represented by instance of the template classstd::match_results,
• std::regex_iterator is used to iterate over all matches of a regex

The functionstd::regex_search is used for searching, while for ‘search and replace’ the functionstd::regex_replace is used which returns a new string.^[25]

Here is an example of the use ofstd::regex_iterator:

#include<regex>constchar*pattern=R"([^ ,.\t\n]+)";// find words separated by space, comma, period tab newlinestd::regexrgx(pattern);// throws exception on invalid patternconstchar*target="Unseen University - Ankh-Morpork";// Use a regex_iterator to identify all words of 'target' separated by characters of 'pattern'.autoiter=std::cregex_iterator(target,target+strlen(target),rgx);// make an end of sequence iteratorautoend=std::cregex_iterator();for(;iter!=end;++iter){std::stringmatch_str=iter->str();std::cout<<match_str<<'\n';}

The library<regex> requires neither alteration of any existing header (though it will use them where appropriate) nor an extension of the core language. InPOSIX C, regular expressions are also available via theC POSIX library#regex.h.

C++11 providesstd::unique_ptr, and improvements tostd::shared_ptr andstd::weak_ptr from TR1.std::auto_ptr is deprecated.

The C standard library provides the ability to generatepseudorandom numbers via the functionrand. However, the algorithm is delegated entirely to the library vendor. C++ inherited this functionality with no changes, but C++11 provides a new method for generating pseudorandom numbers.

C++11's random number functionality is split into two parts: a generator engine that contains the random number generator's state and produces the pseudorandom numbers; and a distribution, which determines the range andmathematical distribution of the outcome. These two are combined to form a random number generator object.

Unlike the C standardrand, the C++11 mechanism will come with three base generator engine algorithms:

• linear_congruential_engine,
• subtract_with_carry_engine, and
• mersenne_twister_engine.

C++11 also provides a number of standard distributions:

• uniform_int_distribution,
• uniform_real_distribution,
• bernoulli_distribution,
• binomial_distribution,
• geometric_distribution,
• negative_binomial_distribution,
• poisson_distribution,
• exponential_distribution,
• gamma_distribution,
• weibull_distribution,
• extreme_value_distribution,
• normal_distribution,
• lognormal_distribution,
• chi_squared_distribution,
• cauchy_distribution,
• fisher_f_distribution,
• student_t_distribution,
• discrete_distribution,
• piecewise_constant_distribution and
• piecewise_linear_distribution.

The generator and distributions are combined as in this example:

#include<random>#include<functional>std::uniform_int_distribution<int>distribution(0,99);std::mt19937engine;// Mersenne twister MT19937autogenerator=std::bind(distribution,engine);intrandom=generator();// Generate a uniform integral variate between 0 and 99.intrandom2=distribution(engine);// Generate another sample directly using the distribution and the engine objects.

Awrapper reference is obtained from an instance of the class templatereference_wrapper. Wrapper references are similar to normal references (‘&’) of the C++ language. To obtain a wrapper reference from any object the function templateref is used (for a constant referencecref is used).

Wrapper references are useful above all for function templates, where references to parameters rather than copies are needed:

// This function will take a reference to the parameter 'r' and increment it.voidfunc(int&r){r++;}// Template function.template<classF,classP>voidg(Ff,Pt){f(t);}intmain(){inti=0;g(func,i);// 'g<void (int &r), int>' is instantiated// then 'i' will not be modified.std::cout<<i<<std::endl;// Output -> 0g(func,std::ref(i));// 'g<void(int &r),reference_wrapper<int>>' is instantiated// then 'i' will be modified.std::cout<<i<<std::endl;// Output -> 1}

This new utility was added to the existing<functional> header and didn't need further extensions of the C++ language.

Polymorphicwrappers forfunction objects are similar tofunction pointers in semantics and syntax, but are less tightly bound and can indiscriminately refer to anything which can be called (function pointers, member function pointers, or functors) whose arguments are compatible with those of the wrapper.

An example can clarify its characteristics:

std::function<int(int,int)>func;// Wrapper creation using// template class 'function'.std::plus<int>add;// 'plus' is declared as 'template<class T> T plus( T, T ) ;'// then 'add' is type 'int add( int x, int y )'.func=add;// OK - Parameters and return types are the same.inta=func(1,2);// NOTE: if the wrapper 'func' does not refer to any function,// the exception 'std::bad_function_call' is thrown.std::function<bool(short,short)>func2;if(!func2){// True because 'func2' has not yet been assigned a function.booladjacent(longx,longy);func2=&adjacent;// OK - Parameters and return types are convertible.structTest{booloperator()(shortx,shorty);};Testcar;func=std::ref(car);// 'std::ref' is a template function that returns the wrapper// of member function 'operator()' of struct 'car'.}func=func2;// OK - Parameters and return types are convertible.

The template classfunction was defined inside the header<functional>, without needing any change to the C++ language.

Metaprogramming consists of creating a program that creates or modifies another program (or itself). This can happen during compilation or during execution. TheC++ Standards Committee has decided to introduce a library for metaprogramming during compiling via templates.

Here is an example of a meta-program using the C++03 standard: arecursion of template instances for calculating integer exponents:

template<intB,intN>structPow{// recursive call and recombination.enum{value=B*Pow<B,N-1>::value};};template<intB>structPow<B,0>{// ''N == 0'' condition of termination.enum{value=1};};intquartic_of_three=Pow<3,4>::value;

Many algorithms can operate on different types of data; C++'stemplates supportgeneric programming and make code more compact and useful. Nevertheless, it is common for algorithms to need information on the data types being used. This information can be extracted during instantiation of a template class usingtype traits.

Type traits can identify the category of an object and all the characteristics of a class (or of a struct). They are defined in the new header<type_traits>.

In the next example there is the template function ‘elaborate’ which, depending on the given data types, will instantiate one of the two proposed algorithms (Algorithm::do_it).

// First way of operating.template<boolB>structAlgorithm{template<classT1,classT2>staticintdo_it(T1&,T2&){/*...*/}};// Second way of operating.template<>structAlgorithm<true>{template<classT1,classT2>staticintdo_it(T1,T2){/*...*/}};// Instantiating 'elaborate' will automatically instantiate the correct way to operate.template<classT1,classT2>intelaborate(T1A,T2B){// Use the second way only if 'T1' is an integer and if 'T2' is// in floating point, otherwise use the first way.returnAlgorithm<std::is_integral<T1>::value&&std::is_floating_point<T2>::value>::do_it(A,B);}

Viatype traits, defined in header<type_traits>, it's also possible to create type transformation operations (static_cast andconst_cast are insufficient inside a template).

This type of programming produces elegant and concise code; however, the weak point of these techniques is the debugging: it's uncomfortable during compilation and very difficult during program execution.

Determining the return type of a template function object at compile-time is not intuitive, particularly if the return value depends on the parameters of the function. As an example:

structClear{intoperator()(int)const;// The parameter type isdoubleoperator()(double)const;// equal to the return type.};template<classObj>classCalculus{public:template<classArg>Argoperator()(Arg&a)const{returnmember(a);}private:Objmember;};

Instantiating the class templateCalculus<Clear>, the function object ofcalculus will have always the same return type as the function object ofClear. However, given classConfused below:

structConfused{doubleoperator()(int)const;// The parameter type is notintoperator()(double)const;// equal to the return type.};

Attempting to instantiateCalculus<Confused> will cause the return type ofCalculus to not be the same as that of classConfused. The compiler may generate warnings about the conversion fromint todouble and vice versa.

TR1 introduces, and C++11 adopts, the template classstd::result_of that allows one to determine and use the return type of a function object for every declaration. The objectCalculusVer2 uses thestd::result_of object to derive the return type of the function object:

template<classObj>classCalculusVer2{public:template<classArg>typenamestd::result_of<Obj(Arg)>::typeoperator()(Arg&a)const{returnmember(a);}private:Objmember;};

In this way in instances of function object ofCalculusVer2<Confused> there are no conversions, warnings, or errors.

The only change from the TR1 version ofstd::result_of is that the TR1 version allowed an implementation to fail to be able to determine the result type of a function call. Due to changes to C++ for supportingdecltype, the C++11 version ofstd::result_of no longer needs these special cases; implementations are required to compute a type in all cases.

For compatibility withC, from C99, these were added:^[26]

• Preprocessor:^[27]
  • variadic macros,
  • concatenation of adjacent narrow/wide string literals,
  • _Pragma() – equivalent of#pragma.
• long long – integer type that is at least 64 bits long.
• __func__ – macro evaluating to the name of the function it is in.
• Headers:
  • cstdbool (stdbool.h),
  • cstdint (stdint.h),
  • cinttypes (inttypes.h).

Heading for a separate TR:

• Modules
• Decimal types
• Math special functions

Postponed:

• Concepts
• More complete or required garbage collection support
• Reflection
• Macro scopes

The termsequence point was removed, being replaced by specifying that either one operation is sequenced before another, or that two operations are unsequenced.^[28]

The former use of the keywordexport was removed.^[29] The keyword itself remains, being reserved for potential future use.

Dynamicexception specifications are deprecated.^[29] Compile-time specification of non-exception-throwing functions is available with thenoexcept keyword, which is useful for optimization.

std::auto_ptr is deprecated, having been superseded bystd::unique_ptr.

Function object base classes (std::unary_function,std::binary_function), adapters to pointers to functions and adapters to pointers to members, and binder classes are all deprecated.

Annex D.2 states: "The use of theregister keyword as a storage-class-specifier (§7.1.1) is deprecated."

• C11

This section'suse ofexternal links may not follow Wikipedia's policies or guidelines. Pleaseimprove this article by removingexcessive orinappropriate external links, and converting useful links where appropriate intofootnote references.(October 2018) (Learn how and when to remove this message)

• The C++ Standards Committee
• C++0X: The New Face of Standard C++
• Herb Sutter's blog coverage of C++11
• Anthony Williams' blog coverage of C++11
• A talk on C++0x given by Bjarne Stroustrup at the University of WaterlooArchived 2009-01-23 at theWayback Machine
• The State of the Language: An Interview with Bjarne Stroustrup (15 August 2008)Archived 31 January 2009 at theWayback Machine
• Wiki page to help keep track of C++ 0x core language features and their availability in compilers
• Online C++11 standard library reference
• Online C++11 compiler
• Bjarne Stroustrup'sC++11 FAQ
• More information on C++11 features:range-based for loop, why auto_ptr is deprecated, etc.^[usurped]

• C++
• Programming language standards
• C++ programming language family
• IEC standards
• ISO standards

• Articles with short description
• Short description matches Wikidata
• Articles needing cleanup from March 2017
• All pages needing cleanup
• Wikipedia articles that are too technical from March 2017
• All articles that are too technical
• All articles with unsourced statements
• Articles with unsourced statements from August 2013
• Wikipedia articles needing clarification from September 2014
• Wikipedia external links cleanup from October 2018
• Webarchive template wayback links
• Articles with example C++ code