Is std::vector::data() preserved by moving?

Question

Possible Duplicate:
Does moving a vector invalidate iterators?

Consider the following codes:

std::vector<T> prepare(T*& data) {
    std::vector<T> buffer;
    // Fill in buffer.
    data = buffer.data();
    return buffer;
}

...

T* data;
auto vec = prepare(data);
// line 12

Is it possible that vec.data() != data in line 12? Similarly,

std::vector<T> buffer;
// ... Fill in buffer ...
T* data = buffer.data();
auto vec = std::move(buffer);
// line 5

Is it possible that vec.data() != data in line 5?

Practically both are not possible in the implementation of libstdc++ and libc++, because the move constructors are implemented as simple pointer assignments, but it seems the standard does not specify anything on it (similar to Is the capacity required to be preserved when moving a std::vector?). Can the "constant complexity" guarantee that vec.data() == data?

Things that invalidate iterators/pointers are pretty well enumerated. — Ben Voigt, Dec 28 '12 at 15:46
Also related: http://stackoverflow.com/questions/4124989/does-stdvectorswap-invalidate-iterators — Ben Voigt, Dec 28 '12 at 15:48

Jonathan Wakely · Answer 1 · 2012-12-28T16:26:01.037

0

Constant complexity means the container isn't allowed to copy/move individual elements, so it must transfer ownership of the existing storage to the new object, so the pointer returned by data() must be the same.

For move assignment (rather than move construction) that is only true if propagate_on_container_move_assignment is true for the vector's allocator type or the allocators compare equal.

edited Dec 28 '12 at 16:26

answered Dec 28 '12 at 15:09

Jonathan Wakely

166,810
27
341
521

Some crazy implementation could move/copy constant number of individual elements. – zch Dec 28 '12 at 15:12
@zch: That's forbidden by the contiguity requirement. – Ben Voigt Dec 28 '12 at 15:45
@BenVoigt, what about `if(<10) copyAll(); else moveOwnership();`? – zch Dec 28 '12 at 16:00
@zch: Check the two questions I linked in comments on this question. – Ben Voigt Dec 28 '12 at 16:02
1

@zch, that's known as the _small object optimization_ but `vector::swap()` must be nothrow, which disallows the small object optimization – Jonathan Wakely Dec 28 '12 at 16:15

Is std::vector::data() preserved by moving?

1 Answers1