c++ - Error is generated with the sort method while sorting the values inside a vector -

when compiling below code using g++ in debian machine, following errors generated...can pls me why error are? tried commenting sort line error dissappears our task requires sorting done can possible solution

code:

#include <iostream> #include <vector> #include <algorithm>  using namespace std;  // here simple struct struct mystruct {    int num;    // define operator <    bool operator <(const mystruct& rhs)    {       return (num < rhs.num);    } };  int main() {    vector<mystruct> myvector;    // let size 5.    myvector.resize(5);    // push 5 instances of mystruct num ranging    // 5 1    mystruct teststruct;    int = 0;    (i = 0; < 5; ++i)    {       teststruct.num = 5 - i;       myvector[i] = teststruct;    }    // sort vector    sort(myvector.begin(), myvector.end());    // try display num each element. sorted    (i = 0; < 5; ++i)    {       cout << myvector[i].num << '\n';    }    return 0; 

}

output:

in file included /usr/include/c++/4.7/algorithm:63:0, testvect.cpp:3: /usr/include/c++/4.7/bits/stl_algo.h: in instantiation of ‘_randomaccessiterator std::__unguarded_partition(_randomaccessiterator, _randomaccessiterator, const _tp&) [with _randomaccessiterator = __gnu_cxx::__normal_iterator >; _tp = mystruct]’: /usr/include/c++/4.7/bits/stl_algo.h:2309:70: required ‘_randomaccessiterator std::__unguarded_partition_pivot(_randomaccessiterator, _randomaccessiterator) [with _randomaccessiterator = __gnu_cxx::__normal_iterator >]’ /usr/include/c++/4.7/bits/stl_algo.h:2340:54: required ‘void std::__introsort_loop(_randomaccessiterator, _randomaccessiterator, _size) [with _randomaccessiterator = __gnu_cxx::__normal_iterator >; _size = int]’ /usr/include/c++/4.7/bits/stl_algo.h:5476:4: required ‘void std::sort(_raiter, _raiter) [with _raiter = __gnu_cxx::__normal_iterator >]’ testvect.cpp:33:41: required here /usr/include/c++/4.7/bits/stl_algo.h:2271:4: error: passing ‘const mystruct’ ‘this’ argument of ‘bool mystruct::operator<(const mystruct&)’ discards qualifiers [-fpermissive]

you use quite dated compiler stl used const& parameters, in more modern versions passed rvalue references , not require const operator<, fix it:

change:

  bool operator <(const mystruct& rhs) 

to

  bool operator <(const mystruct& rhs) const                                        ^^^^^ 

alternately, use later version of compiler supports more modern versions of c++ , enable more modern versions '-std=c++11' or '-std=c++14'.