Question

can’t understand this: g++ compiler is angry on:

lengths.insert(pair<Deux,long>(d,one));

where

struct Deux {long big; long small};
map<Deux, long> lengths;
Deux d;
long one;

so, g++ said, that i miss operator<. after making overloaded operator< for struct Deux, i saw new interesting, but the same error:

map <long, Node*>ArrayOfNodes;
map <long, Node*>::iterator it;  
  for (it=ArrayOfNodes[Root]->nodes.begin();it<ArrayOfNodes[Root]->nodes.end();++it)
      cout<<it->first<<endl;

also used structure Node:

struct Node {
   long name;
   long guest;
   map <long,Node*>nodes;
/*bool operator<(const Node& node)const{
 if ((*this).name<node.name) return true;
 if ((*this).name>node.name) return false;
  return (*this).guest<(*this).guest;
}*/

and error is:

    no match for operator< in it < ((Path*)this)->Path::ArrayOfNodes.
 std::map<_Key, _Tp, _Compare, _Alloc>::operator[] [with _Key = long int, _Tp = Node*,
 _Compare = std::less<long int>, _Alloc = std::allocator<std::pair<const long int, Node*> >]
 (((const long int&)((const long int*)(&((Path*)this)->Path::Root))))->Node::nodes.std::map<_Key, _Tp, _Compare, _Alloc>::end 
 [with _Key = long int, _Tp = Node*, _Compare = std::less<long int>, _Alloc = std::allocator<std::pair<const long int, Node*> >]()

Answer 1

The compiler complains that there is no operator < for Deux (I guess). The key must be a comparable class with operator < or you must pass a third template parameter to map – the comparator.

You see, the map keeps its keys in an ordered way. In order to order them, it needs a predicate. By default it tries to use operator <

try writing something like this:

bool operator < (Deux const & d1, Deux const & d2)
{
   if(d1.big > d2.big)
      return false;
   if(d1.big < d2.big)
     return true;
   return d1.small < d2.small;  
}