template <class RandomAccessIterator, class Compare> void sort (RandomAccessIterator first, RandomAccessIterator last, Compare comp);
手册中对于第三个参数的说明是这样（http://www.cplusplus.com/reference/algorithm/sort/?kw=sort ）：
comp Binary function that accepts two elements in the range as arguments, and returns a value convertible tobool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines. The function shall not modify any of its arguments. This can either be a function pointer or a function object.
原文：http://www.cnblogs.com/kinga/p/4896562.html