I get CE. Why?

#include <iostream>
#include <vector>
#include <sstream>
#include <set>
using namespace std;

template < class TypeTree > class less_of_tree
{
public :
________bool operator () (TypeTree a, TypeTree b)
________{
________________return a.name < b.name;
________}
};

class tree
{
public :
________string name;
________set < tree, less_of_tree < tree > > child;

________tree ();
________tree (string s);
________void add (int x, vector < string > way);
________void print (string row);
};
...

e:\judge\Judge\Vc7\include\xtree(988): error: no instance of function "less_of_tree<TypeTree>::operator() [with TypeTree=tree]" matches the argument list and object (the object has cv-qualifiers that prevent a match)
            argument types are: (const tree, const std::_Tree<std::_Tset_traits<tree, less_of_tree<tree>, std::allocator<tree>, false>>::key_type)
            object type is: const less_of_tree<tree>
     if (this->comp(_Key(_Pnode), _Keyval))

         ^
          detected during:
            instantiation of "std::_Tree<_Traits>::_Nodeptr std::_Tree<_Traits>::_Lbound(const std::_Tree<_Traits>::key_type &) const [with _Traits=std::_Tset_traits<tree, less_of_tree<tree>, std::allocator<tree>, false>]" at line 801
            instantiation of "std::_Tree<_Traits>::iterator std::_Tree<_Traits>::lower_bound(const std::_Tree<_Traits>::key_type &) [with _Traits=std::_Tset_traits<tree, less_of_tree<tree>, std::allocator<tree>, false>]" at line 779
            instantiation of "std::_Tree<_Traits>::iterator std::_Tree<_Traits>::find(const std::_Tree<_Traits>::key_type &) [with _Traits=std::_Tset_traits<tree, less_of_tree<tree>, std::allocator<tree>, false>]" at line 50 of "43d08e8e-4606-476a-85b3-38e5b7230ae9"

Edited by author 10.05.2008 18:42

try to use 'const':

template < class TypeTree > class less_of_tree
{
public :
________bool operator () (const TypeTree a, const TypeTree b) const // <-- here
________{
________________return a.name < b.name;
________}
};

Edited by author 10.05.2008 19:00