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

The Answer

It's a single contiguous storage (a 1d array). Each time it runs out of capacity it gets reallocated and stored objects are moved to the new larger place — this is why you observe addresses of the stored objects changing.

It has always been this way, not since C++17.

TL; DR

The storage is growing geometrically to ensure the requirement of the amortized O(1) push_back(). The growth factor is 2 (Cap_n+1 = Cap_n + Cap_n) in most implementations of the C++ Standard Library (GCC, Clang, STLPort) and 1.5 (Cap_n+1 = Cap_n + Cap_n / 2) in the MSVC variant.

If you pre-allocate it with vector::reserve(N) and sufficiently large N, then addresses of the stored objects won't be changing when you add new ones.

In most practical applications is usually worth pre-allocating it to at least 32 elements to skip the first few reallocations shortly following one other (0→1→2→4→8→16).

It is also sometimes practical to slow it down, switch to the arithmetic growth policy (Cap_n+1 = Cap_n + Const), or stop entirely after some reasonably large size to ensure the application does not waste or grow out of memory.

Lastly, in some practical applications, like column-based object storages, it may be worth giving up the idea of contiguous storage completely in favor of a segmented one (same as what std::deque does but with much larger chunks). This way the data may be stored reasonably well localized for both per-column and per-row queries (though this may need some help from the memory allocator as well).