Difference between revisions of "cpp/language/functions"

Revision as of 01:28, 10 November 2021

Functions are C++ 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.

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(until C++20)non standard library functions and a few standard library functions(since C++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 C++17), and, for non-static member functions, cv-qualification and ref-qualification(since C++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 C++11)-qualifications are different. This is known as function overloading. Function declarations that differ only in the return type and the noexcept specification(since C++17) cannot be overloaded. The address of an overloaded function is determined differently.

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 C++11). Together, these types are known as FunctionObjects, and they are used ubiquitously through the C++ standard library, see for example, usages of BinaryPredicate and Compare.

The standard library also provides a number of pre-defined function object templates as well as the methods to compose new ones (including std::less, std::mem_fn, std::bind, std::function(since C++11), and std::bind_front(since C++20)).