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Thesyntax of C++ isthe set of rules defining how aC++ program is written and compiled.

C++ syntax is largely inherited from the syntax of its ancestor languageC, and has influenced the syntax of several later languages including but not limited toJava,C#, andRust.

Much of C++'s syntax aligns withC syntax, as C++ provides backwards compatibility with C.

The C++"Hello, World!" program program is as follows:^[1]

importstd;intmain(){std::println("Hello, world!");}

Anidentifier is the name of an element in thecode. There are certain standardnaming conventions to follow when selecting names for elements. Identifiers in C++ arecase-sensitive.

An identifier can contain:

• Any Unicode character that is a letter (including numeric letters likeRoman numerals) or digit.
• Currency sign (such as ¥).
• Connecting punctuation character (such as_).

An identifier cannot:

• Start with a digit.
• Be equal to a reserved keyword, null literal orBoolean literal.

The identifiernullptr is not a reserved word, but is a global constant that refers to anull pointer literal. Similarly, the wordstrue andfalse refer to theBoolean values true and false respectively.

The following words may not be used as identifier names or redefined.^[2]

The following words may be used as identifier names, but bear special meanings in certain contexts.

The following tokens are recognised by thepreprocessor in the context of preprocessor directives.

The separators{ and} signify a code block and a new scope. Class members and the body of amethod are examples of what can live inside these braces in various contexts.

Inside of method bodies, braces may be used to create new scopes, as follows:

voiddoSomething(){inta;{intb;a=1;}a=2;b=3;// Illegal because the variable b is declared in an inner scope.}

C++ has two kinds ofcomments:traditional comments andend-of-line comments.

Traditional comments, also known as block comments, start with/* and end with*/, they may span across multiple lines.

/* This is a multi-line comment.It may occupy more than one line. */

End-of-line comments start with// and extend to the end of the current line.

// This is an end-of-line comment

Documentation comments in the source files are processed by the externalDoxygen tool to generate documentation. This type of comment is identical to traditional comments, except it starts with/** and follows conventions defined by the Doxygen tool. Technically, these comments are a special kind of traditional comment and they are not specifically defined in the language specification.

/** * This is a documentation comment. * * @author John Doe */

Much like in C, theparameters given on acommand line are passed to a C++ program with two predefined variables - the count of the command-line arguments inargc and the individualarguments ascharacter strings in the pointer arrayargv. So the command:

myFilt p1 p2 p3

results in something like:

While individual strings are arrays of contiguous characters, there is no guarantee that the strings are stored as a contiguous group.

The name of the program,argv[0], may be useful when printing diagnostic messages or for making one binary serve multiple purposes. The individual values of the parameters may be accessed withargv[1],argv[2], andargv[3], as shown in the following program:

importstd;intmain(intargc,char*argv[]){std::println("{}",argc);for(size_ti=0;i<argc;++i)std::println("argv[{}] = {}",i,argv[i]);}

C++ introducesobject-oriented programming (OOP) features to C. It offersclasses, which provide the four features commonly present in OOP (and some non-OOP) languages:abstraction,encapsulation,inheritance, andpolymorphism. One distinguishing feature ofC++ classes compared to classes in other programming languages is support for deterministicdestructors, which in turn provide support for theResource Acquisition is Initialization (RAII) concept.

As in C, C++ supports four types ofmemory management: static storage duration objects, thread storage duration objects, automatic storage duration objects, and dynamic storage duration objects.^[3]

Static storage duration objects are created beforemain() is entered (see exceptions below) and destroyed in reverse order of creation aftermain() exits. The exact order of creation is not specified by the standard (though there are some rules defined below) to allow implementations some freedom in how to organize their implementation. More formally, objects of this type have a lifespan that "shall last for the duration of the program".^[4]

Static storage duration objects are initialized in two phases. First, "static initialization" is performed, and onlyafter all static initialization is performed, "dynamic initialization" is performed. In static initialization, all objects are first initialized with zeros; after that, all objects that have a constant initialization phase are initialized with the constant expression (i.e. variables initialized with a literal orconstexpr). Though it is not specified in the standard, the static initialization phase can be completed at compile time and saved in the data partition of the executable. Dynamic initialization involves all object initialization done via a constructor or function call (unless the function is marked withconstexpr, in C++11). The dynamic initialization order is defined as the order of declaration within the compilation unit (i.e. the same file). No guarantees are provided about the order of initialization between compilation units.

Variables of this type are very similar to static storage duration objects. The main difference is the creation time is just before thread creation, and destruction is done after the thread has been joined.^[5]

The most common variable types in C++ arelocal variables inside afunction or block, and temporary variables.^[6] The common feature about automatic variables is that they have a lifetime that is limited to the scope of the variable. They are created and potentially initialized at the point of declaration (see below for details) and destroyed in thereverse order of creation when the scope is left. This is implemented by allocation on thestack.

Local variables are created as the point of execution passes the declaration point. If the variable has a constructor or initializer this is used to define the initial state of the object. Local variables are destroyed when the local block or function that they are declared in is closed. C++ destructors for local variables are called at the end of the object lifetime, allowing a discipline for automatic resource management termedRAII, which is widely used in C++.

Member variables are created when the parent object is created. Array members are initialized from 0 to the last member of the array in order. Member variables are destroyed when the parent object is destroyed in the reverse order of creation. i.e. If the parent is an "automatic object" then it will be destroyed when it goes out of scope which triggers the destruction of all its members.

Temporary variables are created as the result of expression evaluation and are destroyed when the statement containing the expression has been fully evaluated (usually at the; at the end of a statement).

These objects have a dynamic lifespan and can be created directly with a call tonew and destroyed explicitly with a call todelete.^[7] C++ also supportsmalloc andfree, from C, but these are not compatible withnew anddelete. Use ofnew returns an address to the allocated memory. The C++ Core Guidelines advise against usingnew directly for creating dynamic objects in favor of smart pointers throughmake_unique<T> for single ownership andmake_shared<T> for reference-counted multiple ownership,^[8] which were introduced in C++11.

C++ is often considered to be a superset ofC but this is not strictly true.^[9] Most C code can easily be made to compile correctly in C++ but there are a few differences that cause some valid C code to be invalid or behave differently in C++. For example, C allows implicit conversion fromvoid* to other pointer types but C++ does not (for type safety reasons). Also, C++ defines many new keywords, such asnew andclass, which may be used as identifiers (for example, variable names) in a C program.

Some incompatibilities have been removed by the 1999 revision of the C standard (C99), which now supports C++ features such as line comments (//) and declarations mixed with code. On the other hand, C99 introduced a number of new features that C++ did not support that were incompatible or redundant in C++, such asvariable-length arrays, native complex-number types (however, thestd::complex class in the C++ standard library provides similar functionality, although not code-compatible), designated initializers,compound literals, and therestrict keyword.^[10] Some of the C99-introduced features were included in the subsequent version of the C++ standard,C++11 (out of those which were not redundant).^[11][12][13] However, the C++11 standard introduces new incompatibilities, such as disallowing assignment of a string literal to a character pointer, which remains valid C.

To intermix C and C++ code, any function declaration or definition that is to be called from/used both in C and C++ must be declared with C linkage by placing it within anextern"C"{/*...*/} block. Such a function may not rely on features depending onname mangling (i.e., function overloading).

Programs developed in C or C++ often utilize inline assembly to take advantage of its low-level functionalities, greater speed, and enhanced control compared to high-level programming languages^[14][15] when optimizing for performance is essential. C++ provides support for embeddingassembly language using asm declarations^[16], but the compatibility ofinline assembly varies significantly betweencompilers and architectures. Unlike high-level language features such asPython orJava, assembly code is highly dependent on the underlying processor and compiler implementation.

Different C++ compilers implement inline assembly in distinct ways.

• GCC (GNU Compiler Collection) andClang^[17]: Use the GCC extended inline assembly syntax. Using__asm__ keyword instead ofasm when writing code that can be compiled with-ansi and-std options, which allows specifying input/output operands and clobbered registers. This approach is widely adopted, including by Intel^[18] and IBM^[19] compilers.

• MSVC (Microsoft Visual C++): The inline assembler is built into the compiler. Previously supported inline assembly via the__asm keyword, but this support has been removed in 64-bit mode, requiring separate .asm modules instead^[20].

• TI ARM Clang and Embedded Compilers^[21]: Some embedded system compilers, like Texas Instruments' TI Arm Clang, allow inline assembly but impose stricter rules to avoid conflicts with register conventions and calling conventions.

C++ provides two primary methods of integrating ASM code.

1. Standalone assembly files – Assembly code is written separately and linked with C++ code.^[22]

2.Inline assembly – Assembly code is embedded within C++ code using compiler-specific extensions.

Example Code for ASM Compatibility

• When calling an assembly function from C++, useextern"C" to prevent C++ name mangling.

//main.cppimportstd;extern"C"intadd_asm(int,int);// Declare the assembly functionintmain(){intresult=add_asm(5,7);std::println("Result from ASM: {}",result);return0;}

#asm code using RISC-V architecture.section.text.globaladd_asmadd_asm:adda0,a0,a1#Addfirstargument(a0)andsecondargument(a1),storeina0ret#Return(a0holdsreturnvalue)

• Global variables in assembly must be declared asextern in C++ and marked.global in assembly.

// main.cppimportstd;extern"C"intglobal_var;// Declare global variable from assemblyintmain(){std::println("Global variable from ASM: {}",global_var);return0;}

#asm using RISC-V architecture.section.data.globalglobal_var.align4global_var:.word42#Defineintegervalue

• Inline assembly allows embedding ASM directly in C++ using theasm keyword.

//main.cpp (using GCC/CLANG compiler)importstd;intmain(){intx=10,y=20,sum;asmvolatile("add %0, %1, %2":"=r"(sum)// Output operand (stored in a register):"r"(x),"r"(y)// Input operands (stored in registers));std::println("Sum using inline ASM: {}",sum);return0;}

Encapsulation is the hiding of information to ensure that data structures and operators are used as intended and to make the usage model more obvious to the developer. C++ provides the ability to define classes and functions as its primary encapsulation mechanisms. Within a class, members can be declared as either public, protected, or private to explicitly enforce encapsulation. A public member of the class is accessible to any function. A private member is accessible only to functions that are members of that class and to functions and classes explicitly granted access permission by the class ("friends"). A protected member is accessible to members of classes that inherit from the class in addition to the class itself and any friends.

The object-oriented principle ensures the encapsulation of all and only the functions that access the internal representation of a type. C++ supports this principle via member functions and friend functions, but it does not enforce it. Programmers can declare parts or all of the representation of a type to be public, and they are allowed to make public entities not part of the representation of a type. Therefore, C++ supports not just object-oriented programming, but other decomposition paradigms such asmodular programming.

It is generally considered good practice to make alldata private or protected, and to make public only those functions that are part of a minimal interface for users of the class. This can hide the details of data implementation, allowing the designer to later fundamentally change the implementation without changing the interface in any way.^[23][24]

Inheritance allows one data type to acquire properties of other data types. Inheritance from abase class may be declared as public, protected, or private. This access specifier determines whether unrelated and derived classes can access the inherited public and protected members of the base class. Only public inheritance corresponds to what is usually meant by "inheritance". The other two forms are much less frequently used. If the access specifier is omitted, a "class" inherits privately, while a "struct" inherits publicly. Base classes may be declared as virtual; this is calledvirtual inheritance. Virtual inheritance ensures that only one instance of a base class exists in the inheritance graph, avoiding some of the ambiguity problems of multiple inheritance.

Multiple inheritance is a C++ feature allowing a class to be derived from more than one base class; this allows for more elaborate inheritance relationships. For example, a "Flying Cat" class can inherit from both "Cat" and "Flying Mammal". Some other languages, such asC# orJava, accomplish something similar (although more limited) by allowing inheritance of multipleinterfaces while restricting the number of base classes to one (interfaces, unlike classes, provide only declarations of member functions, no implementation or member data). An interface as in C# and Java can be defined inC++ as a class containing only pure virtual functions, often known as anabstract base class or "ABC". The member functions of such an abstract base class are normally explicitly defined in the derived class, not inherited implicitly. C++ virtual inheritance exhibits an ambiguity resolution feature calleddominance.

C++ provides more than 35 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and others. Almost all operators can beoverloaded for user-defined types, with a few notable exceptions such as member access (. and.*) and the conditional operator. The rich set of overloadable operators is central to making user-defined types in C++ seem like built-in types.

Overloadable operators are also an essential part of many advanced C++ programming techniques, such assmart pointers. Overloading an operator does not change the precedence of calculations involving the operator, nor does it change the number of operands that the operator uses (any operand may however be ignored by the operator, though it will be evaluated prior to execution). Overloaded "&&" and "||" operators lose theirshort-circuit evaluation property.

Polymorphism enables one common interface for many implementations, and for objects to act differently under different circumstances.

C++ supports several kinds ofstatic (resolved atcompile-time) anddynamic (resolved atrun-time)polymorphisms, supported by the language features described above.Compile-time polymorphism does not allow for certain run-time decisions, whileruntime polymorphism typically incurs a performance penalty.

Variable pointers and references to a base class type in C++ can also refer to objects of any derived classes of that type. This allows arrays and other kinds of containers to hold pointers to objects of differing types (references cannot be directly held in containers). This enables dynamic (run-time) polymorphism, where the referred objects can behave differently, depending on their (actual, derived) types.

C++ also provides thedynamic_cast operator, which allows code to safely attempt conversion of an object, via a base reference/pointer, to a more derived type:downcasting. Theattempt is necessary as often one does not know which derived type is referenced. (Upcasting, conversion to a more general type, can always be checked/performed at compile-time viastatic_cast, as ancestral classes are specified in the derived class's interface, visible to all callers.)dynamic_cast relies onrun-time type information (RTTI), metadata in the program that enables differentiating types and their relationships. If adynamic_cast to a pointer fails, the result is thenullptr constant, whereas if the destination is a reference (which cannot be null), the cast throws an exception. Objectsknown to be of a certain derived type can be cast to that withstatic_cast, bypassing RTTI and the safe runtime type-checking ofdynamic_cast, so this should be used only if the programmer is very confident the cast is, and will always be, valid.

Ordinarily, when a function in a derived classoverrides a function in a base class, the function to call is determined by the type of the object. A given function is overridden when there exists no difference in the number or type of parameters between two or more definitions of that function. Hence, at compile time, it may not be possible to determine the type of the object and therefore the correct function to call, given only a base class pointer; the decision is therefore put off until runtime. This is calleddynamic dispatch.Virtual member functions ormethods^[25] allow the most specific implementation of the function to be called, according to the actual run-time type of the object. In C++ implementations, this is commonly done usingvirtual function tables. If the object type is known, this may be bypassed by prepending afully qualified class name before the function call, but in general calls to virtual functions are resolved at run time.

In addition to standard member functions, operator overloads and destructors can be virtual. An inexact rule based on practical experience states that if any function in the class is virtual, the destructor should be as well. As the type of an object at its creation is known at compile time, constructors, and by extension copy constructors, cannot be virtual. Nonetheless, a situation may arise where a copy of an object needs to be created when a pointer to a derived object is passed as a pointer to a base object. In such a case, a common solution is to create aclone() (or similar) virtual function that creates and returns a copy of the derived class when called.

A member function can also be made "pure virtual" by appending it with=0 after the closing parenthesis and before the semicolon. A class containing a pure virtual function is called anabstract class. Objects cannot be created from an abstract class; they can only be derived from. Any derived class inherits the virtual function as pure and must provide a non-pure definition of it (and all other pure virtual functions) before objects of the derived class can be created. A program that attempts to create an object of a class with a pure virtual member function or inherited pure virtual member function is ill-formed.

Function overloading allows programs to declare multiple functions having the same name but with different arguments (i.e.ad hoc polymorphism). The functions are distinguished by the number or types of theirformal parameters. Thus, the same function name can refer to different functions depending on the context in which it is used. The type returned by the function is not used to distinguish overloaded functions and differing return types would result in a compile-time error message.

When declaring a function, a programmer can specify for one or more parameters adefault value. Doing so allows the parameters with defaults to optionally be omitted when the function is called, in which case the default arguments will be used. When a function is called with fewer arguments than there are declared parameters, explicit arguments are matched to parameters in left-to-right order, with any unmatched parameters at the end of the parameter list being assigned their default arguments. In many cases, specifying default arguments in a single function declaration is preferable to providing overloaded function definitions with different numbers of parameters.

C++ templates enablegeneric programming.C++ supports function, class, alias, and variable templates. Templates may be parameterized by types, compile-time constants, and other templates. Templates are implemented byinstantiation at compile-time. To instantiate a template, compilers substitute specific arguments for a template's parameters to generate a concrete function or class instance. Some substitutions are not possible; these are eliminated by an overload resolution policy described by the phrase "Substitution failure is not an error" (SFINAE). Templates are a powerful tool that can be used forgeneric programming,template metaprogramming, and code optimization, but this power implies a cost. Template use may increaseobject code size, because each template instantiation produces a copy of the template code: one for each set of template arguments, however, this is the same or smaller amount of code that would be generated if the code were written by hand.^[26] This is in contrast to run-time generics seen in other languages (e.g.,Java) where at compile-time the type is erased and a single template body is preserved.

Templates are different frommacros: while both of these compile-time language features enable conditional compilation, templates are not restricted to lexical substitution. Templates are aware of the semantics and type system of their companion language, as well as all compile-time type definitions, and can perform high-level operations including programmatic flow control based on evaluation of strictly type-checked parameters. Macros are capable of conditional control over compilation based on predetermined criteria, but cannot instantiate new types, recurse, or perform type evaluation and in effect are limited to pre-compilation text-substitution and text-inclusion/exclusion. In other words, macros can control compilation flow based on pre-defined symbols but cannot, unlike templates, independently instantiate new symbols. Templates are a tool for staticpolymorphism (see below) andgeneric programming.

In addition, templates are a compile-time mechanism in C++ that isTuring-complete, meaning that any computation expressible by a computer program can be computed, in some form, by atemplate metaprogram before runtime.

In summary, a template is a compile-time parameterized function or class written without knowledge of the specific arguments used to instantiate it. After instantiation, the resulting code is equivalent to code written specifically for the passed arguments. In this manner, templates provide a way to decouple generic, broadly applicable aspects of functions and classes (encoded in templates) from specific aspects (encoded in template parameters) without sacrificing performance due to abstraction.

Templates in C++ provide a sophisticated mechanism for writing generic, polymorphic code (i.e.parametric polymorphism). In particular, through thecuriously recurring template pattern, it is possible to implement a form of static polymorphism that closely mimics the syntax for overriding virtual functions. Because C++ templates are type-aware andTuring-complete, they can also be used to let the compiler resolve recursive conditionals and generate substantial programs throughtemplate metaprogramming. Contrary to some opinion, template code will not generate a bulk code after compilation with the proper compiler settings.^[26]

C++ provides support foranonymous functions, also known aslambda expressions, with the following form:

[capture](parameters)->return_type{function_body}

Since C++20, the keywordtemplate is optional for template parameters of lambda expressions:

[capture]<template_parameters>(parameters)->return_type{function_body}

If the lambda takes no parameters, and no return type or other specifiers are used, the () can be omitted; that is,

[capture]{function_body}

The return type of a lambda expression can be automatically inferred, if possible; e.g.:

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

The[capture] list supports the definition ofclosures. Such lambda expressions are defined in the standard assyntactic sugar for an unnamedfunction object.

Exception handling is used to communicate the existence of a runtime problem or error from where it was detected to where the issue can be handled.^[27] It permits this to be done in a uniform manner and separately from the main code, while detecting all errors.^[28] Should an error occur, an exception is thrown (raised), which is then caught by the nearest suitable exception handler. The exception causes the current scope to be exited, and also each outer scope (propagation) until a suitable handler is found, calling in turn the destructors of any objects in these exited scopes.^[29] At the same time, an exception is presented as an object carrying the data about the detected problem.^[30]

Some C++ style guides, such as Google's,^[31] LLVM's,^[32] and Qt's,^[33] forbid the usage of exceptions.

The exception-causing code is placed inside atry block. The exceptions are handled in separatecatch blocks (the handlers); eachtry block can have multiple exception handlers, as it is visible in the example below.^[34]

importstd;intmain(){try{std::vector<int>vec{3,4,3,1};inti{vec.at(4)};// Throws an exception, std::out_of_range (indexing for vec is from 0-3 not 1-4)}catch(conststd::out_of_range&e){// An exception handler, catches std::out_of_range, which is thrown by vec.at(4)std::println(stderr,"Accessing a non-existent element: {}",e.what());}catch(conststd::exception&e){// To catch any other standard library exceptions (they derive from std::exception)std::println(stderr,"Exception thrown: {}",e.what());}catch(...){// Catch any unrecognised exceptions (i.e. those which don't derive from std::exception)std::println(stderr,"Some fatal error");}}

It is also possible to raise exceptions purposefully, using thethrow keyword; these exceptions are handled in the usual way. In some cases, exceptions cannot be used due to technical reasons. One such example is a critical component of an embedded system, where every operation must be guaranteed to complete within a specified amount of time. This cannot be determined with exceptions as no tools exist to determine the maximum time required for an exception to be handled.^[35]

Unlikesignal handling, in which the handling function is called from the point of failure, exception handling exits the current scope before the catch block is entered, which may be located in the current function or any of the previous function calls currently on the stack.

enumclassColor{Red,Green,Blue};

enumclassColor:long{Red,Green,Blue};// must fit in size and memory layout the type 'long'enumclassShapes:char;// forward declaration. If later there are values defined that don't fit in 'char' it is an error.

Concepts are an extension to thetemplates feature provided by theC++ programming language. Concepts are namedBoolean predicates on template parameters, evaluated atcompile time. A concept may be associated with a template (class template,function template,member function of a class template,variable template, oralias template), in which case it serves as aconstraint: it limits the set of arguments that are accepted as template parameters.

The main uses of concepts are:

• introducing type-checking to template programming
• simplified compiler diagnostics for failed template instantiations
• selecting function template overloads and class template specializations based on type properties
• constraining automatic type deduction

There are five different places in a function template signature where a constraint can be used (labeled below as C1 to C5):^[36]

template<C1T>requiresC2<T>C3autoFun(C4autoparam)requiresC5<T>;

• C1: A type-constraint. This kind replacesclass ortypename for declaring atype template parameter. When using a concept instead of the former two the type is constraint.
• C2: A requires-clause. Whenever a type-constraint does not work, for example, because the concept takes multiple parameters, a requires-clause can be used to apply more elaborated constraints.
• C3 / C4: A constrained placeholder type. The same syntax is available forplaceholder variable aka.auto variable. C++20 addedabbreviated function templates which useauto as a placeholder type in the parameter declaration.^[37] A constrained placeholder type allows to put constraints on the automatically deduced return type of a function or a variable.
• C5: A trailing requires-clause. This form is similar toC2 with one notable exception. A trailing requires-clause can be applied to a function in a class template. This allows the function to remain a regular, template-free function, which can be enabled or disabled depending on the functions trailing requires-clause.

The constraint formsC1 andC2 can be used in all kinds of templates.

Traditionally (prior toC++20), code inclusion in C++ followed the ways of C, in which code was imported into another file using the preprocessor directive#include, which would copy the contents of the file into the other file.

Traditionally, C++ code would be divided between a header file (typically with extension.h,.hpp or.hh) and a source file (typically with extension.cpp or.cc). The header file usually contained declarations of symbols while the source file contained the actual implementation, such as function implementations. This separation was often enforced because#includeing code into another file would result in it being reprocessed for each file it was included by, resulting in increased compilation times if the compiler had to reprocess the same source repeatedly.

Headers often also forced the usage of#include guards or#pragma once to prevent a header from potentially being included into a file multiple times.

The C++ standard library remains accessible through headers, however since C++23 it has been made accessible using modules as well.^[38][39] Even with the introduction of modules, headers continue to play a role in modern C++, as existing codebases have not completely migrated to modules.

A simple example of using C++ modules is as follows:

Hello.cppm

exportmodulemyproject.Hello;importstd;exportnamespacehello{voidprintHello(){std::println("Hello world!");}}

Main.cpp

importmyproject.Hello;intmain(){hello::printHello();}

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