Question

Ok… I have this struct and comparison function-

struct Edge
{
          char point1;  
          char point2;  
          int weight;   

          bool operator<( const Edge& rhs ) const
          {
              return( weight < rhs.weight );
          }
}; //end Edge

bool compareEdge( const Edge& lhs, const Edge& rhs )
{
      return( lhs.weight < rhs.weight );
}

I have a vector declared as…

vector<Edge> edges;

Finally, I try to sort using the < operator…

sort( edges.begin(), edges.end() );

and I get the following error in Visual Studio 2005…

------ Build started: Project: RadakovichLab6, Configuration: Debug Win32 ------ Compiling... graph.cpp c:\program files\microsoft visual studio 8\vc\include\algorithm(2981) : error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const Edge' (or there is no acceptable conversion)
c:\documents and settings\jake\my documents\visual studio 2005\projects\radakovichlab6\graph.h(25): could be 'Edge &Edge::operator =(const Edge &)'
while trying to match the argument list '(const Edge, Edge)'
c:\program files\microsoft visual studio 8\vc\include\algorithm(2997) : see reference to function template instantiation 'void std::_Insertion_sort1<_BidIt,Edge>(_BidIt,_BidIt,_Ty*)' being compiled
with
[
    _BidIt=std::_Vector_const_iterator<Edge,std::allocator<Edge>>,
    _Ty=Edge
]
c:\program files\microsoft visual studio 8\vc\include\algorithm(3105) : see reference to function template instantiation 'void std::_Insertion_sort<_RanIt>(_BidIt,_BidIt)' being compiled
with
[
    _RanIt=std::_Vector_const_iterator<Edge,std::allocator<Edge>>,
    _BidIt=std::_Vector_const_iterator<Edge,std::allocator<Edge>>
]
c:\program files\microsoft visual studio 8\vc\include\algorithm(3112) : see reference to function template instantiation 'void std::_Sort<std::_Vector_const_iterator<_Ty,_Alloc>,__w64 int>(_RanIt,_RanIt,_Diff)' being compiled
with
[
    _Ty=Edge,
    _Alloc=std::allocator<Edge>,
    _RanIt=std::_Vector_const_iterator<Edge,std::allocator<Edge>>,
    _Diff=__w64 int
]
c:\documents and settings\jake\my documents\visual studio 2005\projects\radakovichlab6\graph.cpp(107) : see reference to function template instantiation 'void std::sort<std::_Vector_const_iterator<_Ty,_Alloc>>(_RanIt,_RanIt)' being compiled
with
[
    _Ty=Edge,
    _Alloc=std::allocator<Edge>,
    _RanIt=std::_Vector_const_iterator<Edge,std::allocator<Edge>>
] c:\program files\microsoft visual studio 8\vc\include\algorithm(2988) : error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const Edge' (or there is no acceptable conversion)
c:\documents and settings\jake\my documents\visual studio 2005\projects\radakovichlab6\graph.h(25): could be 'Edge &Edge::operator =(const Edge &)'
while trying to match the argument list '(const Edge, const Edge)' c:\program files\microsoft visual studio 8\vc\include\algorithm(2989) : error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const Edge' (or there is no acceptable conversion)
c:\documents and settings\jake\my documents\visual studio 2005\projects\radakovichlab6\graph.h(25): could be 'Edge &Edge::operator =(const Edge &)'
while trying to match the argument list '(const Edge, Edge)' Generating Code... Compiling... main.cpp Generating Code... Build log was saved at "file://c:\Documents and Settings\Jake\My Documents\Visual Studio 2005\Projects\RadakovichLab6\Debug\BuildLog.htm" RadakovichLab6 - 3 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

The question is in the first line of the error. I get this error whether I use the overloaded < operator, or pass the comparison function to the std::sort function. The Edge structure’s default assignment operator should suffice, I would think, because there is no dynamically allocated memory. If anyone has any insight I would be grateful.

full code…

        #include <list>
        #include <map>
        #include <queue>
        #include <vector>
        #include <algorithm>
        #include <iostream>

        using namespace std;

        //the Edge and aGraph together represent the adjacency list
        //representation of a graph.
        struct Edge
        {
            char point1;  //this represents the endpoint of an edge
            char point2;  
            int weight;   //this is the weight of the edge

            bool operator<( const Edge& rhs ) const
            {
                return( weight < rhs.weight );
            }
        }; //end Edge

        class Graph
        {
        public:
            //Default constructor
            Graph();

            //This method inputs an edge into a graph.
            void inputEdge( char pointA, char pointB, int wt )
            {
                //used to input the edges of the graph
            Edge anEdge;

            //prepare for insertion into list
            anEdge.point1 = pointA;
            anEdge.point2 = pointB;
            anEdge.weight = wt;

            edges.push_back( anEdge );

            //insert edge into the adjacency list.
            aGraph[pointA].push_front( pointB );

            //insert the opposite direction into the
            //adjacency list.
            aGraph[pointB].push_front( pointA )
             }

             //This...
             void bfs();

            //This prints a graph and is used only for debugging purposes.
            void printGraph() const
            {

             list<char>::const_iterator listIter;
             map<char, list<char>>::const_iterator mapIter;

             for( mapIter = aGraph.begin(); 
              mapIter != aGraph.end();
              mapIter++ )
             {
                for( listIter = mapIter->second.begin();
                         listIter != mapIter->second.end();
                         listIter++ )
               {
                     cout << mapIter->first << " " << *listIter << endl;
               } //end for
             } //end for


             sort( edges.begin(), edges.end() );

             vector<Edge>::const_iterator vectIt;

             for( vectIt = edges.begin();
                  vectIt != edges.end();
                  vectIt++ )
             {
                cout << vectIt->point1 << " " << vectIt->point2 << " " << vectIt->weight << endl;
              } //end for
         } //end printGraph


        private:
            //This is the adjacency list
            map<char, list<char>> aGraph;

            //This is a list of edges and their weights.
            vector<Edge> edges;
        };  //end Graph

int main()
{

    Graph myGraph;

    myGraph.inputEdge( 'O', 'A', 2 );
    myGraph.inputEdge( 'O', 'B', 5 );
    myGraph.inputEdge( 'O', 'C', 4 );
    myGraph.inputEdge( 'A', 'B', 2 );
    myGraph.inputEdge( 'A', 'D', 7 );
    myGraph.inputEdge( 'B', 'D', 4 );
    myGraph.inputEdge( 'B', 'E', 3 );
    myGraph.inputEdge( 'C', 'B', 1 );
    myGraph.inputEdge( 'C', 'E', 4 );
    myGraph.inputEdge( 'E', 'D', 1 );
    myGraph.inputEdge( 'D', 'T', 5 );
    myGraph.inputEdge( 'E', 'T', 7 );
    myGraph.inputEdge( 'G', 'Z', 8 );

    myGraph.printGraph();

    cout << endl << endl;

    system("PAUSE");
    return 0;
} //end main

Here is the code…

Answer 1

You’re working with a const vector, which cannot be sorted. (Or changed).

This is because your function is const:

void printGraph() const

Remove the const so members of your class can be modified (and therefore sorted).