std::vector<int> v(1000000);
/* or instead
std::vector<int> v;
v.resize(1000000); */
for(int i = 0; i < 1000000; ++i)
    v[i] = i;

std::vector<int> v;
v.reserve(1000000);
for(int i = 0; i < 1000000; ++i)
    v.push_back(i);