How can I resize a 2D C++ vector?

Question

I have a 2D char vector:

vector< vector > matrix;

I will read in a matrix as an input and store it in that v

Answer 1

While construction with sized vector as default value, given by @leemes, is quite an answer, there is an alternative without using an additional vector and copy ctor:

assert(COL >= 0);
assert(ROW >= 0);
vector> matrix;
for (size_t i{0}; i != COL; ++i)
{
    matrix.emplace_back(ROW);
}