STL之iter_swap & swap

template <class ForwardIterator1, class ForwardIterator2, class T>                
inline void __iter_swap(ForwardIterator1 a, ForwardIterator2 b, T*) {             
    T tmp = *a;                                                                   
    *a = *b;                                                                      
    *b = tmp;                                                                     
}                                                                                 
                                                                                  
template <class ForwardIterator1, class ForwardIterator2>                         
inline void iter_swap(ForwardIterator1 a, ForwardIterator2 b) {                   
    __iter_swap(a, b, value_type(a));                                             
}