C++ syntax

A snippet of C++ code

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:

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.

• alignas
• alignof
• and
• and_eq
• asm
• auto
• bitand
• bitor
• bool
• break
• case
• catch
• char
• char8_t
• char16_t
• char32_t
• class
• compl
• concept
• const
• consteval
• constexpr
• constinit
• const_cast
• continue
• contract_assert
• co_await
• co_return
• co_yield
• decltype
• default
• default
• do
• double
• dynamic_cast
• else
• enum
• explicit
• export
• extern
• false
• float
• for
• friend
• goto
• if
• import
• inline
• int
• long
• module
• mutable
• namespace
• new
• noexcept
• not
• not_eq
• nullptr
• operator
• or
• or_eq
• private
• protected
• public
• register
• reinterpret_cast
• requires
• return
• short
• signed
• sizeof
• static
• static_assert
• static_cast
• struct
• switch
• template
• this
• thread_local
• throw
• true
• try
• typedef
• typeid
• typename
• union
• unsigned
• using
• virtual
• void
• volatile
• wchar_t
• while
• xor
• xor_eq

Identifiers with special meaning

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

• final
• override
• pre
• post
• trivially_relocatable_if_eligible
• replaceable_if_eligible

Preprocessor directives

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

• #if
• #elif
• #else
• #endif
• #ifdef
• #ifndef
• #elifdef
• #elifndef
• #define
• #undef
• #include
• #embed
• #line
• #error
• #warning
• #pragma
• #defined
• #__has_include
• #__has_cpp_attribute
• #__has_embed

Code blocks

The separators { and } 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 argc and the individual arguments as character strings in the pointer array argv. So the command:

myFilt p1 p2 p3

results in something like:

m

y

F

i

l

t

\0

p

1

\0

p

2

\0

p

3

\0

argv[0]

argv[1]

argv[2]

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, 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 argv[1], argv[2], and argv[3], as shown in the following program:

import std;

int main(int argc, char* argv[]) {
    std::println("{}", argc);
    for (size_t i = 0; i < argc; ++i)
        std::println("argv[{}] = {}", i, argv[i]);
}

Objects

Main article: 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 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.

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".

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.

Automatic storage duration objects

The most common variable types in C++ are local variables inside a function or block, and temporary variables. 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 article: new and delete (C++)

These objects have a dynamic lifespan and can be created directly with a call to new and destroyed explicitly with a call to delete. C++ also supports malloc and free, from C, but these are not compatible with new and delete. Use of new returns an address to the allocated memory. The C++ Core Guidelines advise against using new directly for creating dynamic objects in favor of smart pointers through make_unique<T> for single ownership and make_shared<T> for reference-counted multiple ownership, which were introduced in C++11.

Interoperability

With C

Main article: Compatibility of C and C++

C++ is often considered to be a superset of C but this is not strictly true. 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 void* to other pointer types but C++ does not (for type safety reasons). Also, C++ defines many new keywords, such as new and class, 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 as variable-length arrays, native complex-number types (however, the std::complex class in the C++ standard library provides similar functionality, although not code-compatible), designated initializers, compound literals, and the restrict keyword. 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). 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 an extern "C" {/*...*/} block. Such a function may not rely on features depending on name mangling (i.e., function overloading).

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 languages when optimizing for performance is essential. C++ provides support for embedding assembly language using asm declarations, 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 : Use the GCC extended inline assembly syntax. Using __asm__ keyword instead of asm 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 and IBM 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.

• TI ARM Clang and Embedded Compilers : 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.

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 .global in 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.

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 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

Operators that cannot be overloaded

Operator	Symbol
Scope resolution	::
Conditional	?:
dot	.
Member selection	.*
"sizeof"	sizeof
"typeid"	typeid

Main article: 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 dynamic_cast operator, which allows code to safely attempt conversion of an object, via a base reference/pointer, to a more derived type: downcasting. The attempt 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 via static_cast, as ancestral classes are specified in the derived class's interface, visible to all callers.) dynamic_cast relies on run-time type information (RTTI), metadata in the program that enables differentiating types and their relationships. If a dynamic_cast to a pointer fails, the result is the nullptr constant, whereas if the destination is a reference (which cannot be null), the cast throws an exception. Objects known to be of a certain derived type can be cast to that with static_cast, bypassing RTTI and the safe runtime type-checking of dynamic_cast, so this should be used only if the programmer is very confident the cast is, and will always be, valid.

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 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 clone() (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 an abstract 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.

Static polymorphism

See also: Parametric polymorphism and 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 article: Template (C++)

See also: Template metaprogramming and Generic programming

C++ templates enable generic programming. 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.

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 template 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.:

[](int x, int y) { return x + y; } // inferred
[](int x, int y) -> int { return x + y; } // explicit

The [capture] list supports the definition of closures. Such lambda expressions are defined in the standard as syntactic sugar for an unnamed function object.

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. It permits this to be done in a uniform manner and separately from the main code, while detecting all errors. 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. At the same time, an exception is presented as an object carrying the data about the detected problem.

Some C++ style guides, such as Google's, LLVM's, and Qt's, forbid the usage of exceptions.

The exception-causing code is placed inside a try block. The exceptions are handled in separate catch blocks (the handlers); each try block can have multiple exception handlers, as it is visible in the example below.

import std;

int main() {
    try {
        std::vector<int> vec{3, 4, 3, 1};
        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 throw 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.

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

This section is an excerpt from Enumerated type § C++.[edit]

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 a typedef, so that instead of naming the type enum name, simply name it name. This can be simulated in C using a typedef: typedef enum {Value1, Value2} name;

C++11 also provides a second kind of enumeration, called a scoped 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 a using enum declaration (introduced in C++20) has been used to bring the enumerators into the current scope. A scoped enumeration is specified by the phrase enum class (or enum struct). For example:

enum class Color {Red, Green, Blue};

The underlying 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:

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

Concepts

Main article: 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):

template<C1 T>
requires C2<T>
C3 auto Fun(C4 auto param) requires C5<T>;

• C1: A type-constraint. This kind replaces class or typename for 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. auto variable. C++20 added abbreviated function templates which use auto as a placeholder type in the parameter declaration. 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 to C2 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 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 .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. Even with the introduction of modules, headers continue to play a role in modern C++, as existing codebases have not completely migrated to modules.

Modules

This section is an excerpt from Precompiled header § Modules.[edit]

Since C++20, C++ has offered modules as a modern alternative to precompiled headers, however they differ from precompiled headers in that they do not require the preprocessor directive #include, but rather are accessed using the word import. A module must be declared using the word module to indicate that a file is a module. A module, once compiled, is stored as a .pcm (precompiled module) file which acts very similar to a .pch (precompiled header) file.

Modules provide the benefits of precompiled headers in that they compile much faster than traditional headers which are #included and are processed much faster during the linking phase, but also greatly reduce boilerplate code, allowing code to be implemented in a single file, rather than being separated across an header file and source implementation file which was typical prior to the introduction of modules (however, this separation of "interface file" and "implementation file" is still possible with modules, but less common due to the increased boilerplate). Furthermore, modules eliminate the necessity to use #include guards or #pragma once, as modules do not directly modify the source code, unlike #includes, which during the preprocessing step must include source code from the specified header. Thus, importing a module is not handled by the preprocessor, but is rather handled during the compilation phase. Modules, unlike headers, do not have to be processed multiple times during compilation. However, similar to headers, any change in a module necessitates the recompilation of not only the module itself but also all its dependencies — and the dependencies of those dependencies, et cetera.

C++ modules most commonly have the extension .cppm, though some alternative extensions include .ixx and .mxx. All symbols within a module that the programmer wishes to be accessible outside of the module must be marked export.

Modules do not allow for granular imports of specific namespaces, classes, or symbols within a module, unlike Java or Rust which do allow for the aforementioned. Importing a module imports all symbols marked with export, making it akin to a wildcard import in Java or Rust. Importing links the file and makes all exported symbols accessible to the importing translation unit, and thus if a module is never imported, it will never be linked.

Since C++23, the C++ standard library has been exported as a module as well, though as of currently it must be imported in its entirety (using import std;). However, this may change in the future, with proposals to separate the standard library into more modules such as std.core, std.math, and std.io. The module names std and std.* are reserved by the C++ standard, however most compilers allow a flag to override this.

Modules may not export or leak macros, and because of this the order of modules does not matter (however convention is typically to begin with standard library imports, then all project imports, then external dependency imports in alphabetical order). If a module must re-export an imported module, it can do so using export import, meaning that the module is first imported and then exported out of the importing module. Also, because using statements will not be included into importing files (unless explicitly marked export), it is much less likely that using a using statement to bring symbols into the global namespace will cause name clashes within module translation units.

Though standard C has not included modules into the standard, dialects of C allow for modules, for example Clang supports modules for the C language, though the syntax and semantics of Clang C modules differ from C++ modules significantly.

Example

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

Hello.cppm

export module myproject.Hello;

import std;

export namespace hello {
    void printHello() {
        std::println("Hello world!");
    }
}

Main.cpp

import myproject.Hello;

int main() {
    hello::printHello();
}

See also

• C syntax
• Java syntax
• C Sharp syntax