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return statement - cppreference.com

From cppreference.com

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

Syntax

attr (optional) return expression (optional) ; (1)
attr (optional) return braced-init-list ; (2) (since C++11)
attr (optional) co_return expression (optional) ; (3) (since C++20)
attr (optional) co_return braced-init-list ; (4) (since C++20)

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, and disallowed in constructors and in destructors.

3,4) In a coroutine, the keyword co_return must be used instead of return for the final suspension point (see coroutines for details).

The expression or braced-init-list(since C++11) (if any) is known as the operand of the return statement.

There is a sequence point between the copy-initialization of the result of the function call and the destruction of all temporaries at the end of expression.

(until C++11)

The copy-initialization of the result of the 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++11)

If the return type of the function is a reference type and a return statement (1,2) binds the returned reference to the result of a temporary expression, the program is ill-formed.

(since C++26)

If control reaches the end of

  • a function with the return type (possibly cv-qualified) void,
  • a constructor,
  • a destructor, or
  • 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 the main function and specific coroutines(since C++20), without a return statement is undefined behavior.

In a function returning (possibly cv-qualified) void, the return statement with expression can be used, if the expression type is (possibly cv-qualified) void.

If the return type of a function is specified as a placeholder type, it will be deduced from the return value. If decltype(auto) is used, the type deduction treats an expression that can be an entity as an entity.

(since C++14)

Notes

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:

Automatic move from local variables and parameters

The expression is move-eligible if it is a (possibly parenthesized) identifier expression that names a variable of automatic storage duration whose type is

  • a non-volatile object type
 (since C++11)
  • or a non-volatile rvalue reference to object type
 (since C++20)

and that variable is declared

  • in the body
  • or as a parameter

of the innermost enclosing function or lambda expression.

(since C++11)

If the expression is move-eligible, overload resolution to select the constructor to use for initialization of the returned value or, for co_return, to select the overload of promise.return_value()(since C++20) is performed twice :

  • first as if expression were an rvalue expression (thus it may select the move constructor), and
  • if the first overload resolution failed
 (since C++11)
(until C++23)
  • or it succeeded, but did not select the move constructor (formally, the first parameter of the selected constructor was not an rvalue reference to the (possibly cv-qualified) type of expression)
 (since C++11)
(until C++20)
  • then overload resolution is performed as usual, with expression considered as an lvalue (so it may select the copy constructor).
 (since C++11)
(until C++23)

If the expression is move-eligible, it is treated as an xvalue (thus overload resolution may select the move constructor).

(since C++23)

Guaranteed copy elision

If expression is a prvalue, the result object 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)
Feature-test macro Value Std Feature
__cpp_implicit_move 202207L (C++23) Simpler implicit move

Keywords

return, co_return

Example

#include <iostream>
#include <string>
#include <utility>

void fa(int i)
{
    if (i == 2)
        return;
    std::cout << "fa("<< i << ")\n";
} // implied return;

int fb(int i)
{
    if (i > 4)
        return 4;
    std::cout << "fb(" << 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(1); // prints its argument, then returns
    fa(2); // does nothing when i == 2, just returns
    
    int i = fb(5); // returns 4
    i = fb(i);     // prints its argument, returns 2
    std::cout << "i = " << i << '\n'
              << "fc(~).second = " << fc("Hello", 7).second << '\n';
    
    fd();
}

struct MoveOnly
{
    MoveOnly() = default;
    MoveOnly(MoveOnly&&) = default;
};

MoveOnly move_11(MoveOnly arg)
{
    return arg; // OK. implicit move
}

MoveOnly move_11(MoveOnly&& arg)
{
    return arg; // OK since C++20. implicit move
}

MoveOnly&& move_23(MoveOnly&& arg)
{
    return arg; // OK since C++23. implicit move
}

Output: 

fa(1)
fb(4)
i = 2
fc(~).second = 7
fa(10)

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 1541 C++98 expression could not be omitted if the return type is cv-qualified void it can be omitted
CWG 1579 C++11 return by converting move constructor was not allowed converting move
constructor lookup enabled
CWG 1885 C++98 sequencing of the destruction of automatic variables was not explicit sequencing rules added

See also