Why is std::vector contiguous?

Question

Besides the fact that the standard defines it to be contiguous, why is std::vector contiguous?

If it runs out of space, it needs to reallocate a new block and copy t

Answer 1

There are a few reasons for this:

First, iteration over a contiguous container is a lot faster than over a non-contiguous one due to two factors: the first is branch prediction - the processor doesn't need to throw away its pipeline every time you finish reading one of the sub-containers, and less pipeline resets means faster code. The second is that it's much easier to fully cache a contiguous block of memory than a bunch of assorted small blocks, making it much more likely that your array is cached on its entirety.

Second, there's a lot of C++ code being written out there that has to interact with C code, and a lot of that code will expect a contiguous space of memory when receiving an array/buffer, because that's the least data structure implementation-dependent way to do it. When you're interacting with code that expects buffers/arrays constantly, the overhead of converting your std::deque into an array takes its toll compared to the practically instantaneous passage of an std::vector to an array (which can be basically just giving a pointer to the internal array).

All of this justifies the existence of a contiguous container. As others have said, when you don't need either fast iteration or contiguousness of memory, you can always use an std::deque.