Why I get the following error: "returning reference to temporary [-Werror=return-local-addr]"

given the following code:

#include <set>

using std::set;

class Pool {
    set<int> s;
public:   
    typedef typename set<int>::iterator Iterator;
    const Iterator& begin() const {
      return s.begin(); //** error
    }
};    

Why I get the following error (I understand the meaning of the error, but, I don't understand why I get it in this case)?

returning reference to temporary [-Werror=return-local-addr]

How can I fix it?

The set<...>::begin function returns its iterator by value. Since you do not store that value anywhere, it's a temporary value, and you can't really have references to temporary values.

The simple solution is for your function to also return by (non-const) value.