Allow vector to be assigned to itself

[Timmmm]

This sounds like an odd thing to do but some implementations of std::random_shuffle() do this because they call std::swap(x, x) (and cppreference implies that they are allowed to). Old versions of libstdc++ used to do this but now they don't. The current as-of-now version of libc++ does it for one variant of std::random_shuffle() but not others. I have opened a bug for them to fix that, but swap(a, a) should still be allowed anyway.

Quoting cppreference:

C++ named requirements: Swappable Any lvalue or rvalue of this type can be swapped with any lvalue or rvalue of some other type

Also relevant SO question

[igaztanaga]

I can't see why std::swap should fail, as it would use a temporary vector to do the move assignment.

Edit: I see the assertion would fire. But I don't like the idea of pessimizing the vector move operation with a branch. Maybe I should simply allow it with empty vectors.

[igaztanaga]

I've added a commit with some changes:

https://github.com/boostorg/container/commit/2bc6f4d1a030661b3242244cd5f90260f4293620

-> Assertion is changed to allow empty self-moving, which is typical when self swap is made through std::swap. -> priv_swap allows explicit self-swapping.

Let me know if this change avoids assertions in your use case.

[Timmmm]

This might fix std::swap(x, x) however I think you misunderstood the bug report - this comment is wrong:

   //x.size() == 0 is allowed for buggy std libraries.

This bug report is not about buggy standard libraries. This is a bug in boost that is exposed by an unusual (but perfectly allowed) standard library. The following should work, but I don't think it will with your change.

boost::container::vector<int> x;
x.push_back(5);
x = std::move(x);

Hope that's clear!

[igaztanaga]

I was about to reply that self-movement need not to be supported at all, but this seems to have changed in C++17 (where MoveAssignable requirement has a note about self-assignment), but in C++11 any self move-assignment is undefined behavior. This change is reflected also in a SO question and Howard Hinnant's reply and update:

https://stackoverflow.com/questions/13129031/on-implementing-stdswap-in-terms-of-move-assignment-and-move-constructor

Boost.Container implemented C++11 guarantees, maybe it's time to update to C++17 requirements.

[igaztanaga]

Opened #88 for a general review of self move assignments.

[igaztanaga]

Closing this as #88 was fixed allowing vector to assign to itself. Thanks for the report.