return statement

< cpp‎ | language

Terminates the current function and returns the specified value (if any) to its caller.


[edit] Syntax

attr(optional) return expression(optional) ; (1)
attr(optional) return braced-init-list ; (2) (since C++11)
attr(C++11) - optional sequence of any number of attributes
expression - expression, convertible to the function return type
braced-init-list - brace-enclosed list of initializers and other braced-init-lists

[edit] Explanation

1) Evaluates the expression, terminates the current function and returns the result of the expression to the caller, after implicit conversion to the function return type. The expression is optional in functions whose return type is (possibly cv-qualified) void, in constructors and in destructors.
2) Uses copy-list-initialization to construct the return value of the function.

[edit] Notes

If control reaches the end of a function with the return type void (possibly cv-qualified), end of a constructor, end of a destructor, or the end of a function-try-block for a function with the return type (possibly cv-qualified) void without encountering a return statement, return; is executed.

If control reaches the end of the main function, return 0; is executed.

Flowing off the end of a value-returning function (except main) without a return statement is undefined behavior.

In a function returning void, the return statement with expression can be used, if the expression type is void.

Returning by value may involve construction and copy/move of a temporary object, unless copy elision is used. Specifically, the conditions for copy/move are as follows:

If expression is an lvalue expression and the conditions for copy elision are met, or would be met, except that expression names a function parameter, then overload resolution to select the constructor to use for initialization of the returned value is performed twice: first as if expression were an rvalue expression (thus it may select the move constructor or a copy constructor taking reference to const), and if no suitable conversion is available, overload resolution is performed the second time, with lvalue expression (so it may select the copy constructor taking a reference to non-const).

The above rule applies even if the function return type is different from the type of expression (copy elision requires same type)

(since C++11)

The copy-initialization of the temporary that is the result of evaluating a (until C++17)the result object of the (since C++17) function call is sequenced-before the destruction of all temporaries at the end of expression, which, in turn, is sequenced-before the destruction of local variables of the block enclosing the return statement.

(since C++14)

If expression is a prvalue, the object returned by the function is initialized directly by that expression. This does not involve a copy or move constructor when the types match (see copy elision)

(since C++17)

[edit] Keywords


[edit] Example

#include <iostream>
#include <string>
#include <utility>
void fa(int i)
    if (i == 2)
    std::cout << i << '\n';
} // implied return;
int fb(int i)
    if (i > 4)
         return 4;
    std::cout << i << '\n';
    return 2;
std::pair<std::string, int> fc(const char* p, int x)
    return {p, x};
void fd()
    return fa(10); // fa(10) is a void expression
int main()
    fa(2); // returns, does nothing when i==2
    fa(1); // prints its argument, then returns
    int i = fb(5); // returns 4
    i = fb(i); // prints its argument, returns 2
    std::cout << i << '\n'
              << fc("Hello", 7).second << '\n';



[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 1579 C++11 return by converting move constructor was not allowed converting move constructor look up enabled
CWG 1885 C++14 sequencing of the destruction of atomatic variables was not explicit sequencing rules added

[edit] See also

C documentation for return statement