How can I forward declare a class and use its member funcions later in c++?

Question

Is it possible to forward declare a class and then use its member funcions? I'm trying to do this:

class Second;

class First{
private:
  int x=0;
public:
  void move(Second* s,int i){
   s->setLine(i);
   s->called(true);
  }
  int getX(){return x;}
}

class Second{
private:
 int line=2;
 bool cal=false;
public:
 void setLine(int l){line = l;}
 void called(bool b){cal=b}
 bool interact(First* f){
  if ((f->getX())>3)
     return true;
  else
     return false;
 }
}

My actual problem is a bit more complex and the funcions do more stuff but what I'm trying to do is have these two classes use eachother funcions and have them interact this way. Does anyone know if there is a way to do this?

I tried that but the compiler gave me an error "forward declaration of class Second" so i thought maybe if they were together it wouldnt be a problem — helpme, May 23 '16 at 22:41
@helpme Check http://stackoverflow.com/questions/625799/resolve-header-include-circular-dependencies-in-c — πάντα ῥεῖ, May 23 '16 at 22:49

R Sahu · Answer 1 · 2016-05-23T22:54:13.173

Is it possible to forward declare a class and then use its member functions?

No, it is not. You cannot access any member of a forward declared class, variables, functions, enums, nested types, etc., until the class is defined.

You need to move the implementation of the function that calls the member function of the forward declared class after the forward declared class is defined.

class Second;

class First{
   private:
      int x=0;
   public:
      void move(Second* s,int i); // Don't define it here.
      int getX(){return x;}
};

class Second{

   ...

};

// Define the function now.
void First::move(Second* s,int i){
   s->setLine(i);
   s->called(true);
}

yes thank you I separated everything into header and cpp files so that I declare the funcions first and then define them and it works now . Thanks for the help! — helpme, May 23 '16 at 22:54

score 1 · Accepted Answer · answered May 23 '16 at 22:32

1

You can put the definition of First::move outside the class, after the definition of Second. Only the declaration needs to appear inside the definition of First.

You could in fact put the definition of First::move in a .cpp file and not in any header.

answered May 23 '16 at 22:32

Alan Stokes

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I just did this with header files declaring the funcions there but then when I make the cpp files and define them I still get an error from the compiler. Is it because both classes use funcions from each other? – helpme May 23 '16 at 22:39
Probably not; more likely to be missing includes. But without seeing your code I can't tell. – Alan Stokes May 23 '16 at 22:41
Yes I was missing an include sorry. Thank you for your help it works now! – helpme May 23 '16 at 22:52

score 1 · Answer 3 · answered May 23 '16 at 22:35

The following will do the trick for you, but it's better to separate declaration and implementation.

class Second;

class First{
private:
  int x=0;
public:
  void move(Second* s,int i); //<- move implementation after Second's declaration
  int getX(){return x;}
}

class Second{
private:
 int line=2;
 bool cal=false;
public:
 void setLine(int l){line = l;}
 void called(bool b){cal=b}
 bool interact(First* f){
  if ((f->getX())>3)
     return true;
  else
     return false;
 }
};

void First::move(Second* s,int i){
s->setLine(i);
s->called(true);
}

thank you for the help I separated the declaratio and implementation as suggested and it works now! — helpme, May 23 '16 at 22:56

How can I forward declare a class and use its member funcions later in c++?

3 Answers3