Initialize and allocate size for 2D vector member variable using a member function at run-time

Question

How to initialize and allocate size for 2D vector member variable using a member function at run-time?

Is there something wrong with this approach?

class A {

vector<vector<int>> m_matrix;

// trying to resize without using explicit resize
// using the constructor of 2D vectors at run time
// want the code to look simpler and avoid new/pointers
void initialize_matrix(int row, int column) {
  m_matrix = std::move(vector<vector<int>>(row, vector<int>(column, DEFAULT_VALUE)));
}
}

I would use `vector>(row, vector(column, DEFAULT_VALUE)).swap(m_matrix);` rather than std::move. — Alexei Barnes, Jul 07 '16 at 09:33
Alexei Barnes: is there something wrong if I do it the way shown? — Chetan Dugar, Jul 07 '16 at 10:03
Depends on what you mean by wrong, that's a subjective question. For all intents and purposes however, you can use whatever method you like to construct and initialize members. — Alexei Barnes, Jul 07 '16 at 10:06
both solutions seem to achieve the result in constant time. thank you for the comment — Chetan Dugar, Jul 07 '16 at 10:08

score 0 · Accepted Answer · edited May 23 '17 at 11:51

The answer to your question is yes that will work fine. It uses move to guard against the compiler mistakenly using copy-assignment which is a nice touch.

You seem to be asking for suggestions though, so I'd suggest that vector<vector<int>> is a poor choice given that all the sub-vectors are the same size. You might consider using a single vector. I looked for an example of using a single vector to replace a vector-of-vectors and the simplest thing I could find on http://stackoverflow.com was this: https://stackoverflow.com/a/37868317/2642059 There's a little bit better write up here though: http://upcoder.com/2/efficient-vectors-of-vectors

Initialize and allocate size for 2D vector member variable using a member function at run-time

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