Scope of a return value of a function

Question

class obj {
};

obj func() {
    return obj();
}

int main() {
   obj& o = func(); // possible?
}

What is the scope the return value of a function that returns objects by value? in other word, inside the scope that this function is called, is it possible to take the reference of the returned by value object?

Possible duplicate of [How come a non-const reference cannot bind to a temporary object?](http://stackoverflow.com/questions/1565600/how-come-a-non-const-reference-cannot-bind-to-a-temporary-object) — LogicStuff, May 06 '16 at 08:35
[*GotW #88: A Candidate For the “Most Important `const`”*](https://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/) — BoBTFish, May 06 '16 at 08:36

Jarod42 · Answer 1 · 2016-05-06T09:26:13.883

2

You cannot take non-const reference of temporary.

You may do

obj o = func(); // new obj created but copy constructor may be ellided
const obj& o = func(); // lifetime extended
obj&& o = func(); // lifetime extended

edited May 06 '16 at 09:26

answered May 06 '16 at 08:34

Jarod42

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could you maybe explain why "non-const" is invalid? – mkmostafa May 06 '16 at 08:35
@mkmostafa: That is well-covered on thousands of existing questions, not to mention your C++ textbook. – Lightness Races in Orbit May 06 '16 at 10:50

score 2 · Accepted Answer · answered May 06 '16 at 08:36

Yes, you can bind a reference to the return value of a function. This extends the lifetime of the returned object to that of the reference (if it's global or function-local), or to the end of the current function (if the reference is a data member).

However, you cannot bind a temporary (prvalue) to a non-const lvalue reference. So it's like this:

obj& o = func(); // illegal, because non-const lvalue ref
const obj& o = func(); // fine, lifetime extended
obj&& o = func(); // fine, lifetime extended

Andreas DM · Answer 3 · 2016-05-06T09:52:02.223

1

It's not allowed by the language to take a non-const reference of temporary, you can however:

const obj& o = func();
// or
obj&& o = func();
// or simply
obj o = func();

edited May 06 '16 at 09:52

answered May 06 '16 at 09:20

Andreas DM

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Scope of a return value of a function

3 Answers3