std :: vector（n，std :: vector（n））或std :: vector（n * n）？

Question

1. What way to store nm matrix is more efficient? To create a vector of n vectors of size m or to a create big vector of size nm?

2. How is a vector of vectors stored in the memory? Does it contains only references/pointers or whole vectors?

Answer 1

1. It's more efficient to use a single vector of size N x M. This way all the memory is contiguous, and there is only a single pointer to all of it, rather than N pointers.

2. A vector of vectors is stored as a pointer to an array of pointers, each of which points to an array of values.