Can I define a map whose key is a structure?

Question

and how can I do it in C++?

Answer 1

You can use any type as a map key, as long as it implements an operator< (plus the usual copy-and-assign requirements for values stored in containers).

For instance:

struct example { int x; }

bool operator < (const example &l, const example &r) { return l.x < r.x; }

std::map<example, int> values;

Alternatively, you may provide a comparison function as the third argument of the map template instead of defining operator<. More details here (parameter Compare).