map of vectors C++

Question

Hi if I C++ I created an STL map of the type map<int, vec<double>> are the vectors stored under the different keys placed consecutively in memory?

Answer 1

No.^*

A map dynamically allocates elements as required. So each vector could be placed anywhere in memory (and may move over time).

Remember also that the element storage within the vector itself is also dynamically allocated. So even if you just created a straightfoward std::vector<double> my_vectors[10], the elements themsleves would not be contiguous between the individual vectors.

_{* An implementation could conceivably allocate from an array-backed store, but you wouldn't be able to rely on this behaviour. So it's best to assume "no".}

Answer 2

Technically, that would be dependent on implementation of map, but in general, a map is implemented as a Tree, so the stores are not consecutive in memory, due to the nature of the tree datastructure.