c++ - Declaring a 2D vector

Question

In some cases only the below line works.Why so?

vector< vector<int>> a(M,N);

This works in every case.

vector< vector<int>> a(M, vector<int> (N));

What's the difference?

Answer 1

std::vector has a fill constructor which creates a vector of n elements and fills with the value specified. a has the type std::vector<std::vector<int>> which means that it is a vector of a vector. Hence your default value to fill the vector is a vector itself, not an int. Therefore the second options is the correct one.

std::vector<std::vector<int>> array_2d(rows, std::vector<int>(cols, 0));

This creates a rows * cols 2D array where each element is 0. The default value is std::vector<int>(cols, 0) which means each row has a vector which has cols number of element, each being 0.