Effective STL： Item 21：永远让比较函数对相同元素返回false[5]

                           const string*,            // associative containers

                           bool> {

bool operator()(const string *ps1, const string *ps2) const

{

return *ps2 < *ps1;                          // return whether *ps2

}                                                // precedes *ps1 (i.e., swap

                                                     // the order of the