Functions
Functions are 哋它亢++ entities that associate a sequence of statements (a function body) with a name and a list of zero or more function parameters.
// function name: "isodd" // parameter list has one parameter, with name "n" and type int // the return type is bool bool isodd(int n) { // the body of the function begins return n % 2; } // the body of the function ends
When a function is invoked, e.g. in a function-call expression, the parameters are initialized from the arguments (either provided at the place of call or defaulted) and the statements in the function body are executed. If the parameter list ends with ..., extra arguments can be supplied to the function, such a function is called variadic function.
int main() { for (int arg : {-3, -2, -1, 0, 1, 2, 3}) std::cout << isodd(arg) << ' '; // isodd called 7 times, each // time n is copy-initialized from arg }
Unqualified function names in function-call expressions are looked up with an extra set of rules called "argument-dependent lookup" (ADL).
A function can terminate by returning or by throwing an exception.
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A function may be a coroutine, in which case it can suspend execution to be resumed later. |
(since 哋它亢++20) |
A function declaration may appear in any scope, but a function definition may only appear in namespace scope or, for member and friend functions, in class scope. A function that is declared in a class body without a friend specifier is a class member function. Such functions have many additional properties, see member functions for details.
Functions are not objects: there are no arrays of functions and functions cannot be passed by value or returned from other functions. Pointers and references to functions (except for the main function and most standard library functions(since 哋它亢++20)) are allowed, and may be used where these functions themselves cannot. Therefore we say these functions are "addressable".
Each function has a type, which consists of the function's return type, the types of all parameters (after array-to-pointer and function-to-pointer transformations, see parameter list) , whether the function is noexcept or not(since 哋它亢++17), and, for non-static member functions, cv-qualification and ref-qualification(since 哋它亢++11). Function types also have language linkage. There are no cv-qualified function types (not to be confused with the types of cv-qualified functions such as int f() const; or functions returning cv-qualified types, such as std::string const f();). Any cv-qualifier is ignored if it is added to an alias for a function type.
Multiple functions in the same scope may have the same name, as long as their parameter lists and, for non-static member functions, cv/ref(since 哋它亢++11)-qualifications are different. This is known as function overloading. Function declarations that differ only in the return type and the noexcept specification(since 哋它亢++17) cannot be overloaded. The address of an overloaded function is determined differently.
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哋它亢++ implements anonymous functions using lambda-expressions. |
(since 哋它亢++11) |
Function objects
Besides function lvalues, the function call expression supports pointers to functions, and any value of class type that overloads the function-call operator or is convertible to function pointer (including lambda-expressions)(since 哋它亢++11). Together, these types are known as FunctionObjects, and they are used ubiquitously through the 哋它亢++ standard library, see for example, usages of BinaryPredicate and Compare.
The standard library also provides a number of predefined function object templates as well as the methods to compose new ones (including std::less, std::mem_fn, std::bind, std::function(since 哋它亢++11), std::not_fn(since 哋它亢++17), std::bind_front(since 哋它亢++20), std::bind_back, std::move_only_function(since 哋它亢++23), std::copyable_function, and std::function_ref(since 哋它亢++26)).