Question

I am throwing myself in the cold blizzard of learning C++. I already know Java but C++ seems odd on certain thing.

Here is the thing : I have a class A and a class B. Class A have an instance of class B inside of itself. Class B needs to be able to call some class A methods so, I put a pointer to class A inside class B’s constructor, and store it inside a variable so I can retrieve these methods with this variable.

Here is my code (simplified) :

ClassA.h

#ifndef CLASSA
#define CLASSA

#include "ParentClass.h"
#include "ClassB.h"

class ClassA : public ParentClass {

private:
ClassB *classB;

public:
ClassA(void);
virtual ~ClassA(void);

};
#endif

ClassA.cpp

   #include "ClassA.h"

   //-----------------------------------------------------
   ClassA::ClassA(void){
    classB= new ClassB(this);
   }
   //-----------------------------------------------------
   ClassA::~ClassA(void)
   {
   }
   //-----------------------------------------------------

ClassB.h

#ifndef CLASSB
#define CLASSB

#include "ClassA.h"

class ClassB{

public:
ClassB(ClassA &pClassA){ classA = pClassA; };
ClassA *getClassAInstance(){ return classA; };

private:
ClassA *classA;
};
#endif

ClassB.cpp

#include "ClassA.h"

/*ClassB::ClassB(void){
classA= pClassA;
}*/

Visual Express doesn’t give errors but when I compile, it says that class B doesn’t recognize what is a class A. What the hell ?

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(8): error C2061: syntax
error : identifier ‘ClassA’

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(10): error C2143: syntax
error : missing ‘;’ before ‘*’

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(10): error C4430:
missing type specifier – int assumed. Note: C++ does not support
default-int

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(10): error C4430:
missing type specifier – int assumed. Note: C++ does not support
default-int

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(10): warning C4183:
‘getClassAInstance’: missing return type; assumed to be a member
function returning ‘int’

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(13): error C2143: syntax
error : missing ‘;’ before ‘*’

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(13): error C4430:
missing type specifier – int assumed. Note: C++ does not support
default-int

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(13): error C4430:
missing type specifier – int assumed. Note: C++ does not support
default-int

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(8): error C2065:
‘classA’ : undeclared identifier

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(8): error C2065:
‘pClassA’ : undeclared identifier

1>c:\xxx\xxx\xxx\xxx\xxx\xxx\xxx\xxx\ClassB.h(10): error C2065:
‘classA’ : undeclared identifier

Undeclared what ? Returning int ?

undeclared identifier

Why? I included everything, right ? Any ideas what is wrong, guys ?

Answer 1

This is because of a circular reference between your header files. You can break it by forward-declaring one of the classes inside the header of the other, like this:

#ifndef CLASSA
#define CLASSA

#include "ParentClass.h"
//#include "ClassB.h" <<<<< Circular reference is now broken

class ClassB; // Added a forward declaration for use below.

class ClassA : public ParentClass {

private:
ClassB *classB;

public:
ClassA(void);
virtual ~ClassA(void);

};
#endif