c++ - Defining private class in .cpp while mentioning it in .h -

.h

class monotonepieces {     ... private:     class compedges;     set<int, compedges> _edgescrossingline; }; 

.cpp

class monotonepieces::compedges {     ... };  monotonepieces::monotonepieces(const vector<vector2>& vertices) :      _edgescrossingline((compedges(_vertices, &this->_sweepinglineposition))), {} 

with error:

'monotonepieces::compedges' : undefined class not allowed argument compiler intrinsic type trait '__is_empty'

class usage is:

monotonepieces mp(poly); 

the definition std::set<int, compedges> _edgescrossingline needs know size of compedges complete. must define compedges prior line. however, still free place implementation of compedges's methods in .cpp file.