Question

It seems that I can sort a std::vector<std::pair<int, std::string>>, and it will sort based on the int value. Is this a well defined thing to do?

Does std::pair have a default ordering based on its elements?

Answer 1

std::pair uses lexicographic comparison: It will compare based on the first element. If the values of the first elements are equal, it will then compare based on the second element.

The definition in the C++03 standard (section 20.2.2) is:

template <class T1, class T2>
bool operator<(const pair<T1, T2>& x, const pair<T1, T2>& y);

Returns: x.first < y.first || (!(y.first < x.first) && x.second < y.second).