c++ class重载比较运算符的写法

本地能编译通过，但是在一个脑残oj上RE了。。不知道是不是哪里写的不规范

/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h: In function ‘const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&) [with _Tp = S]’:
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:2642:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size) [with _RandomAccessIterator = S*, _Size = int]’
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:2713:   instantiated from ‘void std::sort(_RandomAccessIterator, _RandomAccessIterator) [with _RandomAccessIterator = S*]’
/main.cxx:34:   instantiated from here
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:91: error: passing ‘const S’ as ‘this’ argument of ‘bool S::operator<(const S&)’ discards qualifiers
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:92: error: passing ‘const S’ as ‘this’ argument of ‘bool S::operator<(const S&)’ discards qualifiers
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:94: error: passing ‘const S’ as ‘this’ argument of ‘bool S::operator<(const S&)’ discards qualifiers
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:98: error: passing ‘const S’ as ‘this’ argument of ‘bool S::operator<(const S&)’ discards qualifiers
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_algo.h:100: error: passing ‘const S’ as ‘this’ argument of ‘bool S::operator<(const S&)’ discards qualifiers

以上是错误提示。。

class S{
    public:
        int l,r;
        S* nxt;
        S(int x,int y){
            l=x;
            r=y;
            nxt=NULL;
        }
        S(){
            l=0; r=0; nxt=NULL;
        }
        int con(){
            if(nxt->l<=r) return 1;
            else return 0;
        }
        bool operator <(const S& next){
            if(l<next.l) return 1;
            else return 0;
        }
};

后面用到了sort

    sort(seg+1,seg+n+1);

把operator <的参数中的const去掉，有可能是常引用的问题

bool operator <(const S& next) const{
if(l<next.l) return 1;
else return 0;
}