I'm not really sure why references are being used here, if you're just making a copy straight-away. vector<int> square(vector<int> vcopy) { for

I'm not really sure why references are being used here, if you're just making a copy straight-away. vector<int> square(vector<int> vcopy) { for (int &i : vcopy) i *= i; return vcopy; } is shorter, (IMO) much more readable, and on my tests, measurably faster than the reference version. If you really want the std::accumulate version, vector<int> square(vector<int> vcopy) { accumulate(begin(vcopy), end(vcopy), begin(vcopy), [](int i) { return i * i; }); return vcopy; } works just as well. Still, I guess it's beside the point.