C++ is standardized by theInternational Organization for Standardization (ISO), with the latest standard version ratified and published by ISO in October 2024 asISO/IEC 14882:2024 (informally known asC++23).[17] The C++ programming language was initially standardized in 1998 asISO/IEC 14882:1998, which was then amended by theC++03,C++11,C++14,C++17, andC++20 standards. The currentC++23 standard supersedes these with new features and an enlargedstandard library. Before the initial standardization in 1998, C++ was developed by Stroustrup atBell Labs since 1979 as an extension of the C language; he wanted an efficient and flexible language similar to C that also providedhigh-level features for program organization.[18] Since 2012, C++ has been on a three-year release schedule[19] withC++26 as the next planned standard.[20]
Bjarne Stroustrup, the creator of C++, in his AT&T New Jersey office,c. 2000
In 1979,Bjarne Stroustrup, a Danishcomputer scientist, began work on "C withClasses", the predecessor to C++.[29] The motivation for creating a new language originated from Stroustrup's experience in programming for his PhD thesis. Stroustrup found thatSimula had features that were very helpful for large software development, but the language was too slow for practical use, whileBCPL was fast but too low-level to be suitable for large software development. When Stroustrup started working inAT&T Bell Labs, he had the problem of analyzing theUNIXkernel with respect todistributed computing. Remembering his PhD experience, Stroustrup set out to enhance theC language with Simula-like features.[30] C was chosen because it was general-purpose, fast, portable, and widely used. In addition to C and Simula's influences, other languages influenced this new language, includingALGOL 68,Ada,CLU, andML.[citation needed]
A quiz on C++11 features being given in Paris in 2015
In 1982, Stroustrup started to develop a successor to C with Classes, which he named "C++" (++ being theincrement operator in C) after going through several other names. New features were added, includingvirtual functions, function name andoperator overloading,references, constants, type-safe free-store memory allocation (new/delete), improved type checking, and BCPL-style single-line comments with two forward slashes (//). Furthermore, Stroustrup developed a new, standalone compiler for C++,Cfront.
In 1984, Stroustrup implemented the first stream input/output library. The idea of providing an output operator rather than a named output function was suggested byDoug McIlroy[2] (who had previously suggestedUnix pipes).
In 1985, the first edition ofThe C++ Programming Language was released, which became the definitive reference for the language, as there was not yet an official standard.[32] The first commercial implementation of C++ was released in October of the same year.[29]
In 1989, C++ 2.0 was released, followed by the updated second edition ofThe C++ Programming Language in 1991.[33] New features in 2.0 included multiple inheritance, abstract classes, static member functions,const member functions, and protected members. In 1990,The Annotated C++ Reference Manual was published. This work became the basis for the future standard. Later feature additions includedtemplates,exceptions,namespaces, newcasts, and aBoolean type.
In 1998, C++98 was released, standardizing the language, and a minor update (C++03) was released in 2003.
After C++98, C++ evolved relatively slowly until, in 2011, theC++11 standard was released, adding numerous new features, enlarging the standard library further, and providing more facilities to C++ programmers. After a minorC++14 update released in December 2014, various new additions were introduced inC++17.[34] After becoming finalized in February 2020,[35] a draft of the C++20 standard was approved on 4 September 2020, and officially published on 15 December 2020.[36][37]
On January 3, 2018, Stroustrup was announced as the 2018 winner of theCharles Stark Draper Prize for Engineering, "for conceptualizing and developing the C++ programming language".[38]
In December 2022, C++ ranked third on theTIOBE index, surpassingJava for the first time in the history of the index. As of November 2024[update], the language ranks second afterPython, with Java being in third.[39]
According to Stroustrup, "the name signifies the evolutionary nature of the changes from C."[40] This name is credited to Rick Mascitti (mid-1983)[31] and was first used in December 1983. When Mascitti was questioned informally in 1992 about the naming, he indicated that it was given in atongue-in-cheek spirit. The name comes from C's++operator (whichincrements thevalue of avariable) and a commonnaming convention of using "+" to indicate an enhanced computer program.
During C++'s development period, the language had been referred to as "new C" and "C with Classes"[31][41] before acquiring its final name.
Throughout C++'s life, its development and evolution has been guided by a set of principles:[30]
It must be driven by actual problems and its features should be immediately useful in real world programs.
Every feature should be implementable (with a reasonably obvious way to do so).
Programmers should be free to pick their own programming style, and that style should be fully supported by C++.
Allowing a useful feature is more important than preventing every possible misuse of C++.
It should provide facilities for organising programs into separate, well-defined parts, and provide facilities for combining separately developed parts.
No implicit violations of thetype system (but allow explicit violations; that is, those explicitly requested by the programmer).
User-created types need to have the same support and performance as built-in types.
Unused features should not negatively impact created executables (e.g. in lower performance).
C++ is standardized by anISO working group known asJTC1/SC22/WG21. So far, it has published seven revisions of the C++ standard and is currently working on the next revision,C++26.
Scene during the C++ standards committee meeting in Stockholm in 1996
In 1998, the ISO working group standardized C++ for the first time asISO/IEC 14882:1998, which is informally known asC++98. In 2003, it published a new version of the C++ standard calledISO/IEC 14882:2003, which fixed problems identified in C++98.
The next major revision of the standard was informally referred to as "C++0x", but it was not released until 2011.[48]C++11 (14882:2011) included many additions to both the core language and the standard library.[44]
In 2014,C++14 (also known as C++1y) was released as a small extension to C++11, featuring mainly bug fixes and small improvements.[49] The Draft International Standard ballot procedures completed in mid-August 2014.[50]
After C++14, a major revisionC++17, informally known as C++1z, was completed by the ISO C++ committee in mid July 2017 and was approved and published in December 2017.[51]
It has been suggested that this section besplit into articles titledC++ andC++ syntax. (Discuss)(March 2025)
The C++ language has two main components: a direct mapping of hardware features provided primarily by the C subset, and zero-overhead abstractions based on those mappings. Stroustrup describes C++ as "a light-weight abstraction programming language [designed] for building and using efficient and elegant abstractions";[15] and "offering both hardware access and abstraction is the basis of C++. Doing it efficiently is what distinguishes it from other languages."[71]
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".[75]
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.[76]
The most common variable types in C++ arelocal variables inside afunction or block, and temporary variables.[77] 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.[78] 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,[79] which were introduced in C++11.
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.[80] 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.
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.[81][82]
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.
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.
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.[80]
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[83] 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.
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.[84] It permits this to be done in a uniform manner and separately from the main code, while detecting all errors.[85] 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.[86] At the same time, an exception is presented as an object carrying the data about the detected problem.[87]
Some C++ style guides, such as Google's,[88] LLVM's,[89] and Qt's,[90] 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.[91]
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.[92]
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.
C++ has enumeration types that are directly inherited from C's and work mostly like these, except that an enumeration is a real type in C++, giving added compile-time checking. Also (as with structs), the C++enum keyword is combined with atypedef, so that instead of naming the typeenum name, simply name itname. This can be simulated in C using a typedef:typedefenum{Value1,Value2}name;
C++11 also provides a second kind of enumeration, called ascoped enumeration. These are type-safe: the enumerators are not implicitly converted to an integer type. Among other things, this allows I/O streaming to be defined for the enumeration type. Another feature of scoped enumerations is that the enumerators do not leak, so usage requires prefixing with the name of the enumeration (e.g.,Color::Red for the first enumerator in the example below), unless ausing enum declaration (introduced inC++20) has been used to bring the enumerators into the current scope. A scoped enumeration is specified by the phraseenum class (orenum struct). For example:
enumclassColor{Red,Green,Blue};
Theunderlying type of an enumeration is an implementation-defined integral type that is large enough to hold all enumerated values; it does not have to be the smallest possible type. The underlying type can be specified directly, which allows "forward declarations" of enumerations:
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.
The C++standard consists of two parts: the core language and the standard library. C++ programmers expect the latter on every major implementation of C++; it includes aggregate types (vectors, lists, maps, sets, queues, stacks, arrays, tuples),algorithms (find,for_each,binary_search, random_shuffle, etc.), input/output facilities (iostream, for reading from and writing to the console and files), filesystem library, localisation support,smart pointers for automatic memory management,regular expression support,multi-threading library, atomics support (allowing a variable to be read or written to by at most one thread at a time without any external synchronisation), time utilities (measurement, getting current time, etc.), a system for converting error reporting that does not use C++exceptions into C++ exceptions, arandom number generator, and a slightly modified version of theC standard library (to make it comply with the C++ type system).
A large part of the C++ library is based on theStandard Template Library (STL). Useful tools provided by the STL includecontainers as the collections of objects (such asvectors andlists),iterators that provide array-like access to containers, andalgorithms that perform operations such as searching and sorting.
Furthermore, (multi)maps (associative arrays) and (multi)sets are provided, all of which export compatible interfaces. Therefore, using templates it is possible to write generic algorithms that work with any container or on any sequence defined by iterators.
As in C, thefeatures of thelibrary may be accessed by using the#includedirective to include astandard header. TheC++ Standard Library provides 105 standard headers, of which 27 are deprecated. With the introduction ofmodules inC++20, these headers may be accessed withimport, and inC++23, the entire standard library can now be directly imported as module itself, withimportstd;. Currently, the C++ standard library provides two modules,std andstd.compat (a compatibility module forstd which exports C standard library facilities into the global namespace).
The standard incorporates the STL that was originally designed byAlexander Stepanov, who experimented with generic algorithms and containers for many years. When he started with C++, he finally found a language where it was possible to create generic algorithms (e.g., STL sort) that perform even better than, for example, the C standard library qsort, thanks to C++ features like using inlining and compile-time binding instead of function pointers. The standard does not refer to it as "STL", as it is merely a part of the standard library, but the term is still widely used to distinguish it from the rest of the standard library (input/output streams, internationalization, diagnostics, the C library subset, etc.).[93]
Most C++ compilers, and all major ones, provide a standards-conforming implementation of the C++ standard library.
The C++ Core Guidelines[94] are an initiative led by Bjarne Stroustrup, the inventor of C++, and Herb Sutter, the convener and chair of the C++ ISO Working Group, to help programmers write 'Modern C++' by using best practices for the language standards C++11 and newer, and to help developers of compilers and static checking tools to create rules for catching bad programming practices.
The main aim is to efficiently and consistently write type and resource safe C++.
The Core Guidelines were announced[95] in the opening keynote at CPPCon 2015.
The Guidelines are accompanied by the Guideline Support Library (GSL),[96] a header only library of types and functions to implement the Core Guidelines and static checker tools for enforcing Guideline rules.[97]
To give compiler vendors greater freedom, the C++ standards committee decided not to dictate the implementation ofname mangling,exception handling, and other implementation-specific features. The downside of this decision is thatobject code produced by differentcompilers is expected to be incompatible. There are, however, attempts to standardize compilers for particular machines oroperating systems. For example, the Itanium C++ ABI is processor-independent (despite its name) and is implemented by GCC and Clang.[98]
C++ is often considered to be a superset ofC but this is not strictly true.[99] 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.[100] 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).[101][102][103] 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[104][105] when optimizing for performance is essential. C++ provides support for embeddingassembly language using asm declarations[106], 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.
Variations Across CompilersDifferent C++ compilers implement inline assembly in distinct ways.
GCC (GNU Compiler Collection) andClang[107]: 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[108] and IBM[109] 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[110].
TI ARM Clang and Embedded Compilers[111]: 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.
Interoperability Between C++ and ASMC++ provides two primary methods of integrating ASM code.
1. Standalone assembly files – Assembly code is written separately and linked with C++ code.[112]
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.cpp#include<iostream>extern"C"intadd_asm(int,int);// Declare the assembly functionintmain(){intresult=add_asm(5,7);std::cout<<"Result from ASM: "<<result<<std::endl;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.cpp#include<iostream>extern"C"intglobal_var;// Declare global variable from assemblyintmain(){std::cout<<"Global variable from ASM: "<<global_var<<std::endl;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)#include<iostream>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::cout<<"Sum using inline ASM: "<<sum<<std::endl;return0;}
^This code is copied directly from Bjarne Stroustrup's errata page (p. 633). He addresses the use of'\n' rather thanstd::endl. Also seeCan I write "void main()"?Archived 2 July 2020 at theWayback Machine for an explanation of the implicitreturn 0; in themain function. This implicit return isnot available in other functions.
^Naugler, David (May 2007). "C# 2.0 for C++ and Java programmer: conference workshop".Journal of Computing Sciences in Colleges.22 (5).Although C# has been strongly influenced by Java it has also been strongly influenced by C++ and is best viewed as a descendant of both C++ and Java.
^ab"Nobody Understands C++: Part 5: Template Code Bloat". articles.emptycrate.com/: EmptyCrate Software. Travel. Stuff. 6 May 2008.Archived from the original on 25 April 2016. Retrieved8 March 2010.On occasion you will read or hear someone talking about C++ templates causing code bloat. I was thinking about it the other day and thought to myself, "self, if the code does exactly the same thing then the compiled code cannot really be any bigger, can it?" [...] And what about compiled code size? Each were compiled with the command g++ <filename>.cpp -O3. Non-template version: 8140 bytes, template version: 8028 bytes!
^Stroustrup, Bjarne (2000).The C++ Programming Language (Special ed.). Addison-Wesley. p. 310.ISBN0-201-70073-5.A virtual member function is sometimes called amethod.
^Mycroft, Alan (2013)."C and C++ Exceptions | Templates"(PDF).Cambridge Computer Laboratory - Course Materials 2013-14.Archived(PDF) from the original on 13 May 2016. Retrieved30 August 2016.
^Stroustrup, Bjarne (2013).The C++ Programming Language. Addison Wesley. p. 345.ISBN9780321563842.
^Stroustrup, Bjarne (2013).The C++ Programming Language. Addison Wesley. pp. 363–365.ISBN9780321563842.
^Stroustrup, Bjarne (2013).The C++ Programming Language. Addison Wesley. pp. 345, 363.ISBN9780321563842.
^Vilhena, Paulo Emílio de; Lahav, Ori; Vafeiadis, Viktor; Raad, Azalea (2024). "Extending the C/C++ Memory Model with Inline Assembly". Proceedings of the ACM on Programming Languages, Vol. 8, OOPSLA2, Article 309. doi:10.1145/3689749.
^cppreference.com contributors. "asm declaration".cppreference.com. Retrieved April 1, 2025.
Standard C++ Foundation – a non-profit organization that promotes the use and understanding of standard C++.Bjarne Stroustrup is a director of the organization.
