sort() 是<algorithm>中
default (1)

template <class RandomAccessIterator>
  void sort (RandomAccessIterator first, RandomAccessIterator last);

custom (2)

template <class RandomAccessIterator, class Compare>
  void sort (RandomAccessIterator first, RandomAccessIterator last, Compare comp);

RandomAccessIterator
Compare：函数，接收2个参数，返回bool值，表明这2个数是否满足顺序
默认的comparison为<，接收1和2，1和2满足"<"的顺序么？ 满足，返回true
调用sort()后的序列是有序且顺序的

参考

sort - C++ Reference sort()的声明
关于C++各类排序算法与std::sort性能的比较 各种排序方法时间比较
linux - What makes the gcc std::list sort implementation so fast? - Stack Overflow