Is it safe to bind an unsigned int to a signed int reference?
References can't bind to objects with different type directly. Given const int& s = u;, u is implicitly converted to int firstly, which is a temporary and then s binds to the temporary int. (Lvalue-references to const (and rvalue-references) could bind to temporaries.) The lifetime of the temporary is prolonged to the lifetime of s, i.e. it'll be destroyed when get out of main.