What does std::vector look like in memory?

Question

I read that std::vector should be contiguous. My understanding is, that its elements should be stored together, not spread out across the memory. I have simply acce

Answer 1

It roughly looks like this (excuse my MS Paint masterpiece):

The std::vector instance you have on the stack is a small object containing a pointer to a heap-allocated buffer, plus some extra variables to keep track of the size and and capacity of the vector.

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So it seems as though when I push_back() to the numbers vector, its older elements change their location.

The heap-allocated buffer has a fixed capacity. When you reach the end of the buffer, a new buffer will be allocated somewhere else on the heap and all the previous elements will be moved into the new one. Their addresses will therefore change.

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Does it maybe store them together, but moves them all together, when more space is needed?

Roughly, yes. Iterator and address stability of elements is guaranteed with std::vector only if no reallocation takes place.

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I am aware, that std::vector is a contiguous container only since C++17

The memory layout of std::vector hasn't changed since its first appearance in the Standard. ContiguousContainer is just a "concept" that was added to differentiate contiguous containers from others at compile-time.