Dear reader, The following minimal program compiles and runs properly (note the commented out lines concerning the namespaces): Code: #include <iostream> #include <map> using namespace std; //namespace k { class TKey { public: TKey(int a ) : fA(a) {}; int GetA() const { return fA; }; private: int fA; }; //}; //using namespace k; bool operator<(const TKey & k1, const TKey & k2) { return k1.GetA() < k2.GetA(); }; int main () { // Construct TKey TKey k1(3); // Declare my map and its iterator map<TKey,int> myMap; map<TKey,int>::iterator it; // Insert k1 into the map myMap.insert( pair<TKey,int>(k1,k1.GetA())); // Put out content of the map showing all is fine: cout << "myMap contains:\n"; for ( it=myMap.begin() ; it != myMap.end(); it++ ) cout << it->first.GetA() << " => " << it->second << endl; // Done return 0; }; The output is, as you can check yourself easily: myMap contains: 3 => 3 However, when I now comment in the commented-out lines, I get the following error: /usr/include/c++/4.3/bits/stl_function.h: In member function ‘bool std::less<_Tp>:perator()(const _Tp&, const _Tp&) const [with _Tp = k::TKey]’: /usr/include/c++/4.3/bits/stl_tree.h:1141: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = k::TKey, _Val = std::pair<const k::TKey, int>, _KeyOfValue = std::_Select1st<std::pair<const k::TKey, int> >, _Compare = std::less<k::TKey>, _Alloc = std::allocator<std::pair<const k::TKey, int> >]’ /usr/include/c++/4.3/bits/stl_map.h:469: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >:ther>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair<const _Key, _Tp>&) [with _Key = k::TKey, _Tp = int, _Compare = std::less<k::TKey>, _Alloc = std::allocator<std::pair<const k::TKey, int> >]’ map2.C:37: instantiated from here /usr/include/c++/4.3/bits/stl_function.h:230: error: no match for ‘operator<’ in ‘__x < __y’ Here is get a bit puzzled, can anyone help me understand what is going on here? Many thanks in advance, Ytsen.
Problem solved thanks to Icon: forums devarticles com slesh member php ? u=23135 "Include the operator< inside the namespace. That is where it semantically belongs anyway, I guess. I think the issue is that the point where operator< is needed is inside the stl somewhere (<functional> in my case). This stl file does not have 'using namespace k' of course. That is why it cannot find a suitable operator<, if you add operator< to the namespace then 'argument-dependent-lookp' will make sure the operator< is found. Google for ADL or Koenig-lookup to find out more about that."
