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

Question

When we are compiling the below code using g++ in debian machine, then following errors are generated...can anyone pls help me why the error are? I tried by commenting sort line then error dissappears however our task requires sorting to be done then what can be the possible solution

Code:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

// Here is a simple struct
struct MyStruct
{
   int Num;
   // Define the operator <
   bool operator <(const MyStruct& Rhs)
   {
      return (Num < Rhs.Num);
   }
};

int main()
{
   vector<MyStruct> MyVector;
   // Let the size be 5.
   MyVector.resize(5);
   // Push 5 instances of MyStruct with Num ranging
   // from 5 to 1
   MyStruct TestStruct;
   int i = 0;
   for (i = 0; i < 5; ++i)
   {
      TestStruct.Num = 5 - i;
      MyVector[i] = TestStruct;
   }
   // Now sort the vector
   sort(MyVector.begin(), MyVector.end());
   // Try to display Num for each element. It is sorted
   for (i = 0; i < 5; ++i)
   {
      cout << MyVector[i].Num << '\n';
   }
   return 0;

}

Output:

In file included from /usr/include/c++/4.7/algorithm:63:0, from 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 from ‘_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 from ‘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 from ‘void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator >]’ testvect.cpp:33:41: required from here /usr/include/c++/4.7/bits/stl_algo.h:2271:4: error: passing ‘const MyStruct’ as ‘this’ argument of ‘bool MyStruct::operator<(const MyStruct&)’ discards qualifiers [-fpermissive]

Answer 1

You use quite dated compiler where stl used const& parameters, in more modern versions those are passed by rvalue references and does not require const operator<, so to fix it:

Change:

  bool operator <(const MyStruct& Rhs)

to

  bool operator <(const MyStruct& Rhs) const
                                       ^^^^^

Alternately, use a later version of the compiler which supports more modern versions of C++ and then enable the more modern versions with '-std=c++11' or '-std=c++14'.

Answer 2

Corrected Code:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

// Here is a simple struct
struct MyStruct
{
   int Num;
   // Define the operator <
   bool operator <(const MyStruct& Rhs)const
   {
      return (Num < Rhs.Num);
   }
};

int main()
{
   vector<MyStruct> MyVector;
   // Let the size be 5.
   MyVector.resize(5);
   // Push 5 instances of MyStruct with Num ranging
   // from 5 to 1
   MyStruct TestStruct;
   int i = 0;
   for (i = 0; i < 5; ++i)
   {
      TestStruct.Num = 5 - i;
      MyVector[i] = TestStruct;
   }
   // Now sort the vector
   sort(MyVector.begin(), MyVector.end());
   // Try to display Num for each element. It is sorted
   for (i = 0; i < 5; ++i)
   {
      cout << MyVector[i].Num << '\n';
   }
   return 0;