How to create a vector of objects efficiently in C++？

Question

vector<vector<int>> v(100, vector<int>(100)); // 101 vector<int> are created

I need to created a vector of vectors (or objects) like above. But then there will be 101 vector<int> created, which is one more I needed. If i understand it correctly, first a vector<int>(100) is created and then it's copied 100 times.

So I want to know if there's a way to avoid the reduncdant construction? Just create 100 elements.

Answer 1

I guess the std::vector could handle this case with move semantics emplacing the first vector(100) at index 0 and doing 99 copies. But it doesn't.

But you can easily do that yourself:

vector<vector<int>> v;
v.reserve(100);
v.emplace_back(100);
for (int i = 1; i < 100; ++i) {
    v[i] = v[0];
}

You can do the same for the inner vector if you have a Matrix of complex objects where one extra copy is bad.