CODEWITHSUNDEEP
c++compiler-errorsforward-declarationcircular-reference
SegFault
SegFault

Reputation: 1044

C++ circular reference, error when using methods, even after forward declaring

Suppose I have:

class B;
class A{
    private:
        B *b;
    public:
        bar(){ b->foo()};
        foo();
}

class B{
    private:
        A *a;
    public:
        bar(){ a->foo();}
        foo();
}

When compiled this file gives an

error "invalid use of incomplete type struct B",

even after I have forward declared the class B. As far as I understand it is because when I am calling the function foo() on b, the compiler still doesn't know that such a function exists. How can I solve this problem?

Upvotes: 1

Views: 273

Answers (3)

Some programmer dude
Some programmer dude

Reputation: 409216

The error is because you try to use a method from B in A::bar. While class B has been declared, it has not been defined.

Like the others say, you should separate the definition and the implementation, and it will work.

Upvotes: 1

stijn
stijn

Reputation: 35901

Put the implementation in a source file instead of in the header.

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258618

The forward declaration provides no implementation details. B is not known to A, other than the fact that it exists.

To solve this, separate your declaration from the implementation.

You're also missing return types for the methods.

File A.h:

class B;
class A{
    private:
        B *b;
    public:
        void bar();
        void foo();
};

File A.cpp:

#include "A.h"
#include "B.h"
void A::bar(){ 
   b->foo();
}

File B.h:

class A;
class B{
    private:
        A *a;
    public:
        void bar();
        void foo();
};

File B.cpp:

#include "B.h"
#include "A.h"
void B::bar(){ 
   a->foo();
}

Upvotes: 10

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