c++ - "No known conversion" from const when passing "this" as a parameter -

i making game utilizes state stack manager keeps track of different game states, main menu.

however, have encountered problem can't seem solve.

this state class, stripped down contain offending code:

class statemanager;  namespace first { namespace second {  class state {   friend class statemanager;   protected:     /**      * associates state specified state manager instance.      * @param statemanager instance associate state      */     void associatewithmanager(statemanager *statemanager) {       mstatemanager = statemanager;     }   private:     statemanager *mstatemanager; };  } // second } // first 

the following state manager, stripped down:

namespace first { namespace second {  class statemanager {   public:     /**      * adds state stack.      * @param state state add      */     state &add(state &state) {       state.associatewithmanager(this);       return state;     } };  } // second } // first 

when try compile this, following error (line numbers bit off, since have include guards, etc.):

src/statemanager.cc: in member function 'state& statemanager::add(state&)': src/statemanager.cc:7:34: error: no matching function call 'state::associatewithmanager(statemanager* const)' src/statemanager.cc:7:34: note: candidate is: in file included ./include/statemanager.h:4:0,                  src/statemanager.cc:1: ./include/state.h:29:10: note: void state::associatewithmanager(statemanager*) ./include/state.h:29:10: note:   no known conversion argument 1 'statemanager* const' 'statemanager*' 

apparently, this pointer treated const pointer, though don't use const keyword on add method. i'm not sure going on here. this pointer const? i'm pretty sure i've used way in past no problems though.

also, way i'm going 'correct' way? or there better solution when comes letting state know manager? perhaps using singleton, i'm not big fan of that.

edit: realize forward declaration outside namespaces reason this. should accept mike's answer, since helped me come conclusion? or should post own?

is error? when compile it, error first:

test.cpp:8:31: error: ‘statemanager’ has not been declared 

when declaring class friend, class must have been declared, otherwise declaration ignored. need declare class statemanager; in surrounding namespace before definition of class state. change, code compiles me.