C++ syntax#Keywords
{{Short description|Set of rules defining correctly structured C++ program}}
{{Lead too short|date=April 2025}}
The syntax of C++ is the set of rules defining how a C++ program is written and compiled.
C++ syntax is largely inherited from the syntax of its ancestor language C, and has influenced the syntax of several later languages including but not limited to Java, C#, and Rust.
Basics
Much of C++'s syntax aligns with C syntax, as C++ provides backwards compatibility with C.
The C++ "Hello, World!" program program is as follows:{{Cite web|url=https://en.cppreference.com/w/cpp/io/println|title=std::println|website=cppreference.com}}
import std;
int main() {
std::println("Hello, world!");
}
= Identifier =
An identifier is the name of an element in the code. There are certain standard naming conventions to follow when selecting names for elements. Identifiers in C++ are case-sensitive.
An identifier can contain:
- Any Unicode character that is a letter (including numeric letters like Roman 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 or Boolean literal.
The identifier nullptr is not a reserved word, but is a global constant that refers to a null pointer literal. Similarly, the words true and false refer to the Boolean values true and false respectively.
= Keywords =
The following words may not be used as identifier names or redefined.
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alignasalignofandand_eqasmautobitandbitorboolbreakcasecatchcharchar8_tchar16_tchar32_tclasscomplconceptconstconstevalconstexprconstinitconst_castcontinuecontract_assertco_awaitco_returnco_yielddecltypedefaultdodoubledynamic_castelseenumexplicitexportexternfalsefloatforfriendgotoifimportinlineintlongmodulemutablenamespacenewnoexceptnotnot_eqnullptroperatororor_eqprivateprotectedpublicregisterreinterpret_castrequiresreturnshortsignedsizeofstaticstatic_assertstatic_caststructswitchtemplatethisthread_localthrowtruetrytypedeftypeidtypenameunionunsignedusingvirtualvoidvolatilewchar_twhilexorxor_eq
{{div col end}}
= Identifiers with special meaning =
The following words may be used as identifier names, but bear special meanings in certain contexts.
{{div col|colwidth=15em}}
finaloverridepreposttrivially_relocatable_if_eligiblereplaceable_if_eligible
{{div col end}}
= Preprocessor directives =
The following tokens are recognised by the preprocessor in the context of preprocessor directives.
{{div col|colwidth=15em}}
#if#elif#else#endif#ifdef#ifndef#elifdef#elifndef#define#undef#include#embed#line#error#warning#pragmadefined(follows a conditional directive)#__has_include#__has_cpp_attribute#__has_embed
{{div col end}}
= Code blocks =
The separators {{mono|{{(}}}} and {{mono|{{)}}}} signify a code block and a new scope. Class members and the body of a method 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:
void doSomething() {
int a;
{
int b;
a = 1;
}
a = 2;
b = 3; // Illegal because the variable b is declared in an inner scope.
}
= Comments =
C++ has two kinds of comments: traditional comments and end-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 external Doxygen 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
*/
= Command-line arguments =
Much like in C, the parameters given on a command line are passed to a C++ program with two predefined variables - the count of the command-line arguments in {{code|argc}} and the individual arguments as character strings in the pointer array {{code|argv}}. So the command:
myFilt p1 p2 p3
results in something like:
| class="wikitable" style="font-family: monospace,monospace;" | |||||||||||||||
| m | y | F | i | l | t | style="background:#CCC;"|\0 | p | 1 | style="background:#CCC;"|\0 | p | 2 | style="background:#CCC;"|\0 | p | 3 | style="background:#CCC;"|\0 |
| colspan="7" align="center"|argv[0] | colspan="3" align="center"|argv[1] | colspan="3" align="center"|argv[2] | colspan="3" align="center"|argv[3] |
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, {{code|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 with {{code|argv[1]}}, {{code|argv[2]}}, and {{code|argv[3]}}, as shown in the following program:
import std;
int main(int argc, char* argv[]) {
std::println("argc = {}", argc);
for (size_t i = 0; i < argc; ++i)
std::println("argv[{}] = {}", i, argv[i]);
}
== Objects ==
{{Main|C++ classes}}
C++ introduces object-oriented programming (OOP) features to C. It offers classes, which provide the four features commonly present in OOP (and some non-OOP) languages: abstraction, encapsulation, inheritance, and polymorphism. One distinguishing feature of {{nowrap|C++}} classes compared to classes in other programming languages is support for deterministic destructors, which in turn provide support for the Resource Acquisition is Initialization (RAII) concept.
Object storage
As in C, C++ supports four types of memory management: static storage duration objects, thread storage duration objects, automatic storage duration objects, and dynamic storage duration objects.ISO/IEC. [https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7 Storage duration [basic.stc]
= Static storage duration objects =
Static storage duration objects are created before main() is entered (see exceptions below) and destroyed in reverse order of creation after main() 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".ISO/IEC. [https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7.1 Static Storage duration [basic.stc.static]
Static storage duration objects are initialized in two phases. First, "static initialization" is performed, and only after 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 or constexpr). 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 with constexpr, 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.
= Thread storage duration objects =
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.ISO/IEC. [https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018}} §3.7.2 Thread Storage duration [basic.stc.thread]
= Automatic storage duration objects =
The most common variable types in C++ are local variables inside a function or block, and temporary variables.ISO/IEC. [https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7.3 Automatic Storage duration [basic.stc.auto] 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 the reverse order of creation when the scope is left. This is implemented by allocation on the stack.
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 termed RAII, 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).
= Dynamic storage duration objects =
{{Main|new and delete (C++)}}
These objects have a dynamic lifespan and can be created directly with a call to {{cpp|new}} and destroyed explicitly with a call to {{cpp|delete}}.ISO/IEC. [https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7.4 Dynamic Storage duration malloc and free, from C, but these are not compatible with {{cpp|new}} and {{cpp|delete}}. Use of {{cpp|new}} returns an address to the allocated memory. The C++ Core Guidelines advise against using {{cpp|new}} directly for creating dynamic objects in favor of smart pointers through {{cpp|make_unique
Interoperability
= With C =
{{main|Compatibility of C and C++}}
C++ is often considered to be a superset of C but this is not strictly true.{{cite web |url=http://www.stroustrup.com/bs_faq.html#C-is-subset |title=Bjarne Stroustrup's FAQ – Is C a subset of C++? |access-date=5 May 2014 |archive-date=6 February 2016 |archive-url=https://web.archive.org/web/20160206214150/http://www.stroustrup.com/bs_faq.html#C-is-subset |url-status=live }} 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 from
Some incompatibilities have been removed by the 1999 revision of the C standard (C99), which now supports C++ features such as line comments (
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 an
= Inline Assembly =
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 languagesBokil, Milind A. (2021). "[https://www.researchgate.net/publication/354744729_Writing_Assembly_Routines_within_CC_and_Java_Programs Writing Assembly Routines within C/C++ and Java Programs]". ResearchGate. Retrieved April 1, 2025.{{cite journal | url=https://doi.org/10.1145/3689749 | doi=10.1145/3689749 | title=Extending the C/C++ Memory Model with Inline Assembly | date=2024 | last1=De Vilhena | first1=Paulo Emílio | last2=Lahav | first2=Ori | last3=Vafeiadis | first3=Viktor | last4=Raad | first4=Azalea | journal=Proceedings of the ACM on Programming Languages | volume=8 | pages=1081–1107 | arxiv=2408.17208 }} when optimizing for performance is essential. C++ provides support for embedding assembly language using asm declarations,cppreference.com contributors. "[https://en.cppreference.com/w/cpp/language/asm asm declaration]". cppreference.com. Retrieved April 1, 2025. but the compatibility of inline assembly varies significantly between compilers and architectures. Unlike high-level language features such as Python or Java, assembly code is highly dependent on the underlying processor and compiler implementation.
== Variations across compilers ==
Different C++ compilers implement inline assembly in distinct ways.
- GCC (GNU Compiler Collection) and Clang:{{Cite web |title=Extended Asm (Using the GNU Compiler Collection) |url=https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html |website=GCC Online Documentation |publisher=GNU Project |access-date=April 1, 2025}} 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 IntelIntel Corporation. "[https://www.intel.com/content/www/us/en/docs/cpp-compiler/developer-guide-reference/2021-9/inline-assembly.html Inline Assembly]". Intel® C++ Compiler Classic Developer Guide and Reference, Version 2021.9. Retrieved April 1, 2025. and IBMIBM. "[https://www.ibm.com/docs/en/xl-c-aix/13.1.3?topic=statements-inline-assembly-extension Inline assembly statements (IBM extension)]". IBM Documentation. Retrieved April 1, 2025. 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.{{Cite web |title=Inline Assembler Overview |url=https://learn.microsoft.com/en-us/cpp/assembler/inline/inline-assembler-overview?view=msvc-170 |website=Microsoft Learn |publisher=Microsoft |access-date=1 April 2025}} - TI ARM Clang and Embedded Compilers:{{Cite web |title=Interfacing C and C++ With Assembly Language |url=https://software-dl.ti.com/codegen/docs/tiarmclang/compiler_tools_user_guide/compiler_manual/runtime_environment/interfacing-c-and-c-with-assembly-language-stdz0544217.html#interfacing-c-and-c-with-assembly-language |website=Texas Instruments |publisher=Texas Instruments Incorporated |date=February 23, 2025 |access-date=April 1, 2025}} 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 Assembly ==
C++ provides two primary methods of integrating ASM code.
1. Standalone assembly files – Assembly code is written separately and linked with C++ code.{{cite web |url=https://wiki.osdev.org/C%2B%2B_to_ASM_linkage_in_GCC |title=C++ to ASM linkage in GCC |website=OSDev Wiki |access-date=1 April 2025}}
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++, use
extern "C" to prevent C++ name mangling.
//main.cpp
import std;
extern "C" int add_asm(int, int); // Declare the assembly function
int main() {
int result = add_asm(5, 7);
std::println("Result from ASM: {}", result);
return 0;
}
- asm code using RISC-V architecture
.section .text
.global add_asm
add_asm:
add a0, a0, a1 # Add first argument (a0) and second argument (a1), store in a0
ret # Return (a0 holds return value)
- Global variables in assembly must be declared as
extern in C++ and marked.globalin assembly.
// main.cpp
import std;
extern "C" int global_var; // Declare global variable from assembly
int main() {
std::println("Global variable from ASM: {}", global_var);
return 0;
}
- asm using RISC-V architecture
.section .data
.global global_var
.align 4
global_var:
.word 42 # Define integer value
- Inline assembly allows embedding ASM directly in C++ using the
asm keyword.
//main.cpp (using GCC/CLANG compiler)
import std;
int main() {
int x = 10, y = 20, sum;
asm volatile (
"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);
return 0;
}
Encapsulation
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 as modular programming.
It is generally considered good practice to make all data 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.{{Cite book |first1=Herb |last1=Sutter |first2=Andrei |last2=Alexandrescu |author-link1=Herb Sutter |author-link2=Andrei Alexandrescu |year=2004 |title=C++ Coding Standards: 101 Rules, Guidelines, and Best Practices |publisher=Addison-Wesley}}{{Cite book |last1=Henricson |first1=Mats |last2=Nyquist |first2=Erik |title=Industrial Strength C++ |publisher=Prentice Hall |year=1997 |isbn=0-13-120965-5 |url=https://archive.org/details/industrialstreng0000henr}}
= Inheritance =
Inheritance allows one data type to acquire properties of other data types. Inheritance from a base 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 called virtual 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 as C# or Java, accomplish something similar (although more limited) by allowing inheritance of multiple interfaces 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 in {{nowrap|C++}} as a class containing only pure virtual functions, often known as an abstract 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 called dominance.
Operators and operator overloading
| class="wikitable plainrowheaders floatright"
|+ Operators that cannot be overloaded ! Operator ! Symbol |
| Scope resolution
| {{cpp| ::}} |
| Conditional
| {{cpp| ?:}} |
| dot
| {{cpp| .}} |
| Member selection
| {{cpp| .*}} |
| "sizeof"
| {{cpp| sizeof}} |
| "typeid"
| {{cpp| typeid}} |
{{Main|Operators in C and C++}}
C++ provides more than 35 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and others. Almost all operators can be overloaded 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 as smart 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 their short-circuit evaluation property.
Polymorphism
{{See also|Polymorphism (computer science)}}
Polymorphism enables one common interface for many implementations, and for objects to act differently under different circumstances.
C++ supports several kinds of static (resolved at compile-time) and dynamic (resolved at run-time) polymorphisms, supported by the language features described above. Compile-time polymorphism does not allow for certain run-time decisions, while runtime polymorphism typically incurs a performance penalty.
= Dynamic polymorphism =
== Inheritance ==
{{See also|Subtyping}}
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 the
== Virtual member functions ==
Ordinarily, when a function in a derived class overrides 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 called dynamic dispatch. Virtual member functions or methods{{Cite book |quote=A virtual member function is sometimes called a method. |first=Bjarne |last=Stroustrup |year=2000 |page=310 |title=The C++ Programming Language |edition=Special |publisher=Addison-Wesley |isbn=0-201-70073-5}} 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 using virtual function tables. If the object type is known, this may be bypassed by prepending a fully 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 a
A member function can also be made "pure virtual" by appending it with
Static polymorphism
{{See also|Parametric polymorphism|ad hoc polymorphism}}
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 their formal 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 a default 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 =
{{main|Template (C++)}}
{{See also|Template metaprogramming|Generic programming}}
C++ templates enable generic programming. {{nowrap|C++}} supports function, class, alias, and variable templates. Templates may be parameterized by types, compile-time constants, and other templates. Templates are implemented by instantiation 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 for generic programming, template metaprogramming, and code optimization, but this power implies a cost. Template use may increase object 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. 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 from macros: 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 static polymorphism (see below) and generic programming.
In addition, templates are a compile-time mechanism in C++ that is Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template 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 the curiously 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 and Turing-complete, they can also be used to let the compiler resolve recursive conditionals and generate substantial programs through template metaprogramming. Contrary to some opinion, template code will not generate a bulk code after compilation with the proper compiler settings.{{cite web |access-date=8 March 2010 |publisher=EmptyCrate Software. Travel. Stuff. |location=articles.emptycrate.com/ |title=Nobody Understands C++: Part 5: Template Code Bloat |date=6 May 2008 |url=https://articles.emptycrate.com/2008/05/06/nobody_understands_c_part_5_template_code_bloat.html |quote=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++
Lambda expressions
C++ provides support for anonymous functions, also known as lambda expressions, with the following form:
[capture](parameters) -> return_type { function_body }
Since C++20, the keyword {{code|2=cpp|1=template}} is optional for template parameters of lambda expressions:
[capture]
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.:
[](int x, int y) { return x + y; } // inferred
[](int x, int y) -> int { return x + y; } // explicit
The
== Exception handling==
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.{{Cite web |url=http://www.cl.cam.ac.uk/teaching/1314/CandC++/lecture7.pdf |title=
Some C++ style guides, such as Google's,{{cite web |title=Google C++ Style Guide |url=https://google.github.io/styleguide/cppguide.html#Exceptions |access-date=25 June 2019 |archive-date=16 March 2019 |archive-url=https://web.archive.org/web/20190316065327/http://google.github.io/styleguide/cppguide.html#Exceptions |url-status=live}} LLVM's,{{cite web |title=LLVM Coding Standards |url=https://llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions |website=LLVM 9 documentation |access-date=25 June 2019 |archive-date=27 June 2019 |archive-url=https://web.archive.org/web/20190627023217/http://llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions |url-status=live}} and Qt's,{{cite web |title=Coding Conventions |url=https://wiki.qt.io/Coding_Conventions |website=Qt Wiki |access-date=26 June 2019 |archive-date=26 June 2019 |archive-url=https://web.archive.org/web/20190626231458/https://wiki.qt.io/Coding_Conventions |url-status=live}} forbid the usage of exceptions.
The exception-causing code is placed inside a
import std;
int main() {
try {
std::vector
int i{vec.at(4)}; // Throws an exception, std::out_of_range (indexing for vec is from 0-3 not 1-4)
} catch (const std::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 (const std::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 the Throwable (whose subclasses are Error and Exception), C++ allows anything, both primitive types and objects, to be thrown and caught. C++ does not have an Error class like those languages, but has an Exception class (std::exception). In the aforementioned languages, the distinction between Error and Exception is made in that Errors usually represent irrecoverable states, while Exceptions are more acceptable to catch and represent circumstances that are normal to occur throughout the execution of a program.
Unlike signal 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.
Enumerated types
{{excerpt|Enumerated type|C++}}
Concepts
{{main|Concepts (C++)}}
Concepts are an extension to the templates feature provided by the C++ programming language. Concepts are named Boolean predicates on template parameters, evaluated at compile time. A concept may be associated with a template (class template, function template, member function of a class template, variable template, or alias template), in which case it serves as a constraint: 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):{{cite book |last=Fertig |first=Andreas |author-link=Andreas Fertig |date=2021 |title=Programming with C++20 |url=https://andreasfertig.com/books/programming-with-cpp20/ |location= |publisher=Fertig Publications |page=23 |isbn=978-3-949323-01-0}}
template
requires C2
C3 auto Fun(C4 auto param) requires C5
C1: A type-constraint. This kind replacesclassortypenamefor declaring a type 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 for placeholder variable aka.autovariable. C++20 added abbreviated function templates which useautoas a placeholder type in the parameter declaration.{{ cite web | title = ISO/IEC 14882:2020 | publisher = ISO | date = December 2020 | url = http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=79358 | access-date =14 July 2022 }} 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 toC2with 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 forms C1 and C2 can be used in all kinds of templates.
Code inclusion
= Headers =
{{see also|Include directive}}
Traditionally (prior to C++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 {{mono|.h}}, {{mono|.hpp}} or {{mono|.hh}}) and a source file (typically with extension {{mono|.cpp}} or {{mono|.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 guard 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. Even with the introduction of modules, headers continue to play a role in modern C++, as existing codebases have not completely migrated to modules.
== Header units ==
Headers are traditionally included via textual inclusion by the preprocessor using #include, while modules are included during compilation through import. However, headers may also be imported using import, even if they are not declared as modules – these are called "header units", and they are designed to allow existing codebases to migrate from headers to modules more gradually.{{cite web|url=https://learn.microsoft.com/en-us/cpp/build/walkthrough-header-units?view=msvc-170|title=Walkthrough: Build and import header units in Microsoft Visual C++|date=12 April 2022 |publisher=Microsoft}} The syntax is similar to including a header, with the difference being that #include is replaced with import and a semicolon is placed at the end of the statement. Header units automatically export all symbols, and differ from proper modules in that they allow the emittance of macros, meaning all who import the header unit will obtain its contained macros. This offers minimal breakage between migration to modules. The semantics of searching for the file depending on whether quotation marks or angle brackets are used apply here as well. For instance, one may write "MyHeader.h" as a header unit. Most build systems, such as CMake, do not support this feature yet.
= Modules =
{{excerpt|Precompiled header|Modules}}
== Example ==
A simple example of using C++ modules is as follows:
{{mono|MyClass.cppm}}
export module myproject.MyClass;
import std;
using String = std::string;
export namespace myproject {
class MyClass {
private:
int x;
String name;
public:
MyClass(int x, const String& name):
x{x}, name{name} {}
int getX() const noexcept {
return x;
}
void setX(int newX) noexcept {
x = newX;
};
String getName() const noexcept {
return name;
}
void setName(const String& newName) noexcept {
name = newName;
}
};
}
{{mono|Main.cpp}}
import std;
import myproject.MyClass;
using myproject::MyClass;
int main() {
MyClass me(10, "MyName");
me.setX(15);
std::println("Hello, {}!", me);
}
Attributes
Since C++11, C++ has supported attribute specifier sequences{{cite web|title=Attribute specifier sequence (since C++11)|url=https://cppreference.com/w/cpp/language/attributes.html|website=cppreference.com|access-date=6 June 2025}}. Attributes can be applied to any symbol that supports them, including classes, functions/methods, and variables, and any symbol marked with an attribute will be specifically treated by the compiler as necessary. These can be thought of as similar to Java annotations for providing additional information to the compiler, however they differ in that attributes in C++ are not metadata that is meant to be accessed using reflection. Furthermore, one cannot create custom attributes in C++, unlike in Java where one may define custom annotations in addition to the standard ones. However, C++ does have implementation/vendor-specific attributes which are non-standard. These typically have a namespace associated with them. For instance, GCC and Clang have attributes under the gnu:: namespace, and all such attributes are of the form {{cpp|gnu::*}}.
One may apply multiple attributes as a list, for instance {{cpp|A, B, C}} (where A, B, and C are attributes). Furthermore, attributes may also accept arguments, like {{cpp|A("This is a parameter")}}.
= Standard attributes =
The C++ standard defines the following attributes:
Legend:
{{color box|#E6E6B7}}: Deprecated
{{color box|#F99}}: Removed
| class="wikitable"
! Name !! Description | |
| {{cpp|noreturn}} | Indicates that the specified function will not return to its caller. |
| style="background:#F99" | {{cpp|carries_dependency}}
| style="background:#F99" | Indicates that the dependency chain in release-consume | |
| {{cpp|deprecated}} {{cpp|deprecated("reason")}} | Indicates that the use of the marked symbol is allowed but discouraged/deprecated for the reason specified (if given). |
| {{cpp|fallthrough}} | Indicates that the fall through from the previous case label is intentional. |
| {{cpp|maybe_unused}} | Suppresses compiler warnings on an unused entity. |
| {{cpp|nodiscard}} {{cpp|nodiscard("reason")}} | Issues a compiler warning if the return value of the marked symbol is discarded or ignored for the reason specified (if given). |
| {{cpp|likely}} {{cpp|unlikely}} | Indicates that the compiler should optimise for the case where a path of execution through a statement is more or less likely to occur than the other(s). |
| {{cpp|no_unique_address}} | Indicates that a non-static data member need not have an address distinct from all other non-static data members of its class. |
| {{cpp|assume(expression)}} | Indicates that the given expression always evaluates to {{cpp|true}} at a given point, allowing the compiler to make optimisations based on that information. |
| {{cpp|indeterminate}} | Indicates that an object bears an indeterminate value if it is not initialised. |
| {{cpp|unsequenced}} | Indicates that a function is stateless, effectless, idempotent and independent. |
| {{cpp|reproducible}} | Indicates that a function is effectless and idempotent. |
= Scoped attributes =
As mentioned previously, GCC and Clang have scoped (namespaced) attributes, such as {{cpp|gnu::always_inline}}, {{cpp|gnu::hot}}, and {{cpp|gnu::const}}. To apply multiple scoped attributes, one may write:
gnu::always_inline gnu::hot gnu::const nodiscard
inline int f(); // declare f with four attributes
gnu::always_inline, gnu::const, gnu::hot, nodiscard
int f(); // same as above, but uses a single attr specifier that contains four attributes
// C++17:
using gnu : const, always_inline, hot nodiscard
int fgnu::always_inline(); // an attribute may appear in multiple specifiers
See also
Notes
{{reflist|group=lower-alpha}}