I have a function
struct foo { std::vector<int> v; }; foo func(); will the vector inside foo be moved or copied when returning from the function?
1 Answer
It will be moved.(*)
Since you do not provide an explicit move constructor for your foo class, the compiler will implicitly generate one for you that invokes the move constructor (if available) for all the members of your class. Since std::vector defines a move constructor, it will be invoked.
Per Paragraph 12.8/15 of the C++11 Standard:
The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members. [...]
Also notice, that the compiler is allowed to elide the call to the copy/move constructor of your class when returning an object by value. This optimization is called (Named) Return Value Optimization.
(*) I am assuming here that your use case is to create a local object with automatic storage inside foo() and return it.
3 Comments
Joseph Mansfield
It won't be moved if the
foo it returns is perhaps a foo with static storage duration. There's probably some other cases too. But yes, I agree with you for the case that the asker is probably asking about.
Andy Prowl
@sftrabbit: OK, that's correct. I assumed that he wanted to returned a local object with automatic storage. Edited, thank you.
Andy Prowl
@BoPersson: I did mention that, although not RVO explicitly. I will do it.
func.vector, while this is about returning a struct with the default move constructor and avectormember.