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manannan · May 30th, 2006, 8:09 PM

When using the STL, I'd often want to instantiate an anonymous functor -- something like this:

#include <set>
#include <algorithm>

int main()
{
  std::set<int> s;
  s.insert(1); s.insert(2);

  class
  {
  public:
    bool operator () (int a, int b) { return a < b; }
  } comparer;

  bool test = comparer(1, 2); // Works
  return *( std::max_element(s.begin(), s.end(), comparer) ); // Fails!
}

The problem is, under gcc, this fails with the error

(Toggle Plain Text)

test.cpp:17: error: no matching function for call to �max_element(std::_Rb_tree_const_iterator<int>, std::_Rb_tree_const_iterator<int>, main()::<anonymous class>&)�

Looking at the <algorithm> header, I say that the error message makes no sense, and the program should compile just fine.
Now it seems that the only way for me to use a functor is to make its class global. But isn't there a way to somehow avoid cluttering the global namespace and keep the code beautiful? Or should I report a bug in the gcc? :-)

May 30th, 2006, 8:09 PM	#1
manannan Newbie Join Date: May 2006 Posts: 2 Rep Power: 0	Using internal classes in STL functions When using the STL, I'd often want to instantiate an anonymous functor -- something like this: (Toggle Plain Text) #include <set> #include <algorithm> int main() { std::set<int> s; s.insert(1); s.insert(2); class { public: bool operator () (int a, int b) { return a < b; } } comparer; bool test = comparer(1, 2); // Works return ( std::max_element(s.begin(), s.end(), comparer) ); // Fails! } #include <set> #include <algorithm> int main() { std::set<int> s; s.insert(1); s.insert(2); class { public: bool operator () (int a, int b) { return a < b; } } comparer; bool test = comparer(1, 2); // Works return ( std::max_element(s.begin(), s.end(), comparer) ); // Fails! } The problem is, under gcc, this fails with the error (Toggle Plain Text) test.cpp:17: error: no matching function for call to �max_element(std::_Rb_tree_const_iterator<int>, std::_Rb_tree_const_iterator<int>, main()::<anonymous class>&)� test.cpp:17: error: no matching function for call to �max_element(std::_Rb_tree_const_iterator<int>, std::_Rb_tree_const_iterator<int>, main()::<anonymous class>&)� Looking at the <algorithm> header, I say that the error message makes no sense, and the program should compile just fine. Now it seems that the only way for me to use a functor is to make its class global. But isn't there a way to somehow avoid cluttering the global namespace and keep the code beautiful? Or should I report a bug in the gcc? :-) Last edited by manannan; May 30th, 2006 at 8:34 PM.