CODEWITHSUNDEEP
c++c++14language-lawyer
hl037_
hl037_

Reputation: 3867

Lifetime of temporary rvalue referenced object

Suppose you have this code (in c++14) :

auto && a = a_function_returning_a_temp_rvalue();

Where is stored the returned value, when is its destructor called, is a const (considering the function doesn't return a const ), how is it specified in the standard ? (is it even legal)

Also, do you confirm, the temporary is really bound to a, and no operator= neither constructors will be called ?

It compiles on g++ and avoid typing long template classes, but before using it, I would like to check if it's clearly stated in c++14

Upvotes: 0

Views: 91

Answers (2)

Barry
Barry

Reputation: 302767

The rule is in [class.temporary]:

There are three contexts in which temporaries are destroyed at a different point than the end of the full-expression. [...] The third context is when a reference is bound to a temporary.116 The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except:
— A temporary object bound to a reference parameter in a function call (5.2.2) [...]
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is [...]
— A temporary bound to a reference in a new-initializer (5.3.4) persists until [...]

In this example:

auto && a = a_function_returning_a_temp_rvalue();

I'm assuming the function returns something of type T (as opposed to T& or T&&). In this case, we do have a temporary bound to a reference, and none of those exceptions apply. Hence, the lifetime of that temporary is extended for the lifetime of a.

Upvotes: 2

aschepler
aschepler

Reputation: 72271

Yes, this is guaranteed to work.

[12.2/4-5]

There are two contexts in which temporaries are destroyed at a different point than the end of the full-expression. ...

The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except:

  • A temporary object bound to a reference parameter in a function call persists until the completion of the full-expression containing the call.

  • The lifetime of a temporary bound to the returned value in a function return statement is not extended; the temporary is destroyed at the end of the full-expression in the return statement.

  • A temporary bound to a reference in a new-initializer persists until the completion of the full-expression containing the new-initializer.

No constructor or operator= is called to initialize a, and a is not const if it doesn't need to be.

Upvotes: 1

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