I m getting an error invalid use of incomplete type class map

Question

I am making a basic text based RPG sorry if my question is stupid because im new to c    So basically I have a small combat class that i have to link back and forth to from the map class so I had to forward declare my map class and it comes up with the error  BTW sorry there aren t any comments   Here is the error  invalid use of incomplete type class Map   include  lt iostream gt   include  lt string gt   include  lt cstdlib gt  using namespace std  class Map  class Player   public      int health      int damage      int defense      int gems 0      string race      string name      string location       class Enemy       public       int ehealth      int edamage      int edefense      int echoice      class combat   public      Map  mapobj      int damagedealt      Player playerobj      Enemy enemeyobj      string cchoice      string retry       void initial                 cout  lt  lt  A wild orc has appeared n           cout  lt  lt  What do you do  n           cout  lt  lt  --------------------------- n           cout  lt  lt   -------------------------  n           cout  lt  lt   ----Attack-----Defend----  n           cout  lt  lt   -------------------------  n           cout  lt  lt  --------------------------- n           cin  gt  gt cchoice          this- gt battle              void newturn                 cout  lt  lt  The orc is still alive            cout  lt  lt  What do you do            cout  lt  lt   n--------------------------- n           cout  lt  lt   -------------------------  n           cout  lt  lt   ----Attack-----Defend----  n           cout  lt  lt   -------------------------  n           cout  lt  lt  --------------------------- n           cin  gt  gt cchoice          this- gt battle               void battle                  enemeyobj echoice   rand     2          if  enemeyobj echoice  1                        if  cchoice   Attack                             playerobj damage              enemeyobj ehealth enemeyobj ehealth-playerobj damage              cout  lt  lt  You did   lt  lt playerobj damage lt  lt   points of damge to the enemey  n               if  enemeyobj ehealth gt 0                                playerobj health enemeyobj edamage-playerobj health                  cout  lt  lt  The enemyattacked you  You now have   lt  lt playerobj health lt  lt   health                   if  playerobj health gt 0                                        this- gt newturn                                      else if  playerobj health lt  0                                        cout  lt  lt playerobj name lt  lt  was killed n                       cout  lt  lt  Game Over                                                 else if  enemeyobj ehealth lt  0                                    cout  lt  lt  You have defeated the orc                        if  playerobj location   a                                                 mapobj- gt relaypointa                                                                    else if  cchoice   Defend                                 damagedealt enemeyobj edamage-playerobj defense                  playerobj health damagedealt-playerobj health                  cout  lt  lt  You defend but the enemey was able to deal n                   cout  lt  lt damagedealt lt  lt   points of damage your health is n                   cout  lt  lt playerobj health                  if  playerobj health gt 0                                        this- gt newturn                                      else if  playerobj health lt  0                                        cout  lt  lt playerobj name lt  lt  was killed n                       cout  lt  lt  Game Over                                                     else if  enemeyobj echoice 2                        if  cchoice   Attack                              damagedealt enemeyobj edefense-playerobj damage               enemeyobj ehealth enemeyobj ehealth-damagedealt               cout  lt  lt  You did   lt  lt damagedealt lt  lt   points of damage to the enemey                if  enemeyobj ehealth gt 0                                  this- gt newturn                                else if  enemeyobj ehealth lt  0                                  cout  lt  lt  You have defeated the orc                     mapobj- gt relaypointa                                             else if  cchoice   Defend                                 cout  lt  lt  Both parties defended                   this- gt newturn                                              class Map       public      combat combatobj      string mchoice      int espawn      Player playerobj      Enemy enemeyobj      void relaypointaespawn             playerobj location   relaypointa       enemeyobj ehealth   rand     50   100      enemeyobj edamage   rand     50   75      enemeyobj edefense   rand     50   50      combatobj initial              void relaypointa                 cout  lt  lt  You have found yourself at the n           cout  lt  lt  mouth of a mighty river to the north n           cout  lt  lt  What do you want to do  n                 void relaypointb             playerobj location   relaypointb       cout  lt  lt    n n                     n       cout  lt  lt                        n       cout  lt  lt        Wild North      n       cout  lt  lt                        n       cout  lt  lt                        n n       cout  lt  lt  You have entered the wild north this is where your journey starts n       cout  lt  lt  What would you like to do n n       cin  gt  gt mchoice      if  mchoice   Travel                 cout  lt  lt  Where would you like to travel  n           cin  gt  gt mchoice          if  mchoice   North                                else if  mchoice   East                                else if  mchoice   South                                else if  mchoice   West                         this- gt relaypointaespawn                      else                       cout  lt  lt  Invalid command n n               this- gt relaypointb                                       void relaypointcespawn                 playerobj location   a           enemeyobj ehealth   rand     50   100          enemeyobj edamage   rand     50   75          enemeyobj edefense   rand     50   50          espawn   rand     2

User · Answer

I am just providing another case where you can get this error message. The solution will be the same as Adam has mentioned above. This is from a real code and I renamed the class name.

class FooReader {
  public:
     /** Constructor */
     FooReader() : d(new FooReaderPrivate(this)) { }  // will not compile here
     .......
  private:
     FooReaderPrivate* d;
};

====== In a separate file =====
class FooReaderPrivate {
  public:
     FooReaderPrivate(FooReader*) : parent(p) { }
  private:
     FooReader* parent;
};

The above will no pass the compiler and get error: invalid use of incomplete type FooReaderPrivate. You basically have to put the inline portion into the *.cpp implementation file. This is OK. What I am trying to say here is that you may have a design issue. Cross reference of two classes may be necessary some cases, but I would say it is better to avoid them at the start of the design. I would be wrong, but please comment then I will update my posting.

User · Answer

Your first usage of Map is inside a function in the combat class  That happens before Map is defined  hence the error   A forward declaration only says that a particular class will be defined later  so it s ok to reference it or have pointers to objects  etc  However a forward declaration does not say what members a class has  so as far as the compiler is concerned you can t use any of them until Map is fully declared   The solution is to follow the C   pattern of the class declaration in a  h file and the function bodies in a  cpp  That way all the declarations appear before the first definitions  and the compiler knows what it s working with

[c++] I'm getting an error "invalid use of incomplete type 'class map'

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