What happens to a pointer that points to an element in a vector when I shuffle it?

Question

I have a std::vector<int> and a pointer int* that points to an element in the vector. Let’s say the pointer points to the third element: pointer=&vector.at(2). If I now shuffle the vector, will it still point to the same element (the third) or will it point the the new location where the element which used to be the third has now moved?

After that, I’d like to make the question a little bit more general: How do pointers and iterators to elements in a vector behave when the vector is expanded or reduced?

Answer 1

The pointer will continue to point to the same location, so when you shuffle, it'll point to whatever element has been moved into the location you specified.

When you expand the size of a vector, all existing pointers and iterators into the vector can become invalid. When you shuffle, they continue to refer to the same location, which will (usually) contain a different value than it did before shuffling.

Reducing the size of a vector will depend on exactly how you do that. One way is to create a temporary vector as a copy of the current vector, swap the two, then destroy the temporary (usually implicitly, by letting it go out of scope). If you do this, the pointers will be into the temporary, and be invalidated when it's destroyed.

If you use shrink_to_fit that (probably) won't invalidate iterators/pointers, but may not have any effect (the standard specifies that it's a non-binding request, and doesn't say anything about it invalidating iterators/pointers).