how c++ vector<vector<int>> manages memory

Question

vector container holds object in continuous memory. it is easy to understand for cases like vector. but what if it is a vector of vectors, like vector>, each vector in this vector of vectors can have different length. how does it manage the memory? Does it allocate a fixed length vector every time we push in a new vector? if so, what will happen if the first vector grows out of size during push_back. would it trigger a full vector of vector reallocate and copy/move?

Answer 1

A vector is a pointer to a dynamic array. If you push_back and find you're out of space in the array you have, you allocate a new, bigger array, copy over everything from the old array, and then stick the new value in.

If you have a vector of vectors, the same holds true for each of the inner vectors.

What you need to understand here is that a vector of vectors (unlike a 2D array), is not contiguous in memory. Each of the inner vectors' arrays can be stored anywhere in memory. Or in other words, " each vector in a vector of vectors is a completely different vector. Each with their own, completely separate and separately managed buffer. ¹ "

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^{1. Thanks to user4581301 for this!}

Answer 2

A vector contains a pointer to a contiguous memory block. When it runs out of memory, it allocates a new memory block. A vector of vectors is just a vector of pointers to memory blocks. Although each memory block is a contiguous block , they are not necessarily contiguous to each other, that is, not necessarily when one vector ends, the next one starts, there is almost always a gap.

Why the not necessarily and almost always semantics? Because it depends on the memory allocator you're using and on the operating system internals. Ultimately, it's (one of) the job(s) of the OS to allocate and serve memory blocks to user-space programs.