std :: vector如何复制其数据以进行重新分配？

Question

I wrote a template array class similar to std::vector except it reuses empty indices or 'holes' when an element is erased, instead of moving all the following elements back to fill it.

I ran into a bug though when I tried to store a nontrivial object as I was using memcpy to copy elements during reallocation. Does std::vector loop through all its elements and call the copy constructor, or does it use some other trick that I'm unaware of?

Answer 1

Yes, copying the elements then destroying the originals is the basic idea.

You can optimize with template metaprogramming, of course:

• If the type has a trivial copy constructor, you can use memcpy
• If the type has a trivial destructor, you can skip that step

C++11 adds a new dimension in that you can move the objects too.

Answer 2

No, you should not use memcpy() for non-pods. Classic vanilla C++03 implementation would be to use copy constructor, but this is not very productive. In C++11, you can use whole arsenal of meta-info, such as is_trivially_copyable to perform memcpy() when applicable, check for presence of move constructor, etc.