CODEWITHSUNDEEP
c++forward-declaration
feetwet
feetwet

Reputation: 3446

How do I get over this forward declaration error?

I'm trying to do a barebones example of a class A that provides callback methods to a dependent class B by passing a reference to itself. The best I've produced is the following:

a.h

#ifndef A_H
#define A_H

#include "b.h"

namespace b {
    class ClassB;
}

namespace a {

    class ClassA {
    public:
        ClassA();
        b::ClassB* m_b;
        void callback(const std::string& msg);
    };
}

#endif

a.cc

#include "a.h"

a::ClassA::ClassA() {
    m_b = new b::ClassB(this);
}

void a::ClassA::callback(const std::string& msg) {
    std::cout << msg << '\n';
}

int main(void)
{
    a::ClassA* doit = new a::ClassA();
    delete doit;
    return 0;
}

b.h

#ifndef B_H
#define B_H

#include <iostream>
#include <string>

namespace a {
    class ClassA;
}

namespace b {
    class ClassB{
    public:
        ClassB(a::ClassA* a_inst);
    };
}

#endif

b.cc

#include "b.h"

namespace b {

    ClassB::ClassB(a::ClassA* a_inst) {
        a_inst->callback("Call me maybe");
    }
}

This is the closest I can get to a compilation, but I'm left with the following errors:

b.cc: In constructor 'b::ClassB::ClassB(a::ClassA*)':
b.cc:6:9: error: invalid use of incomplete type 'struct a::ClassA'
b.h:8:8: error: forward declaration of 'struct a::ClassA'

How can I fix this? And is there a better way to construct this relationship?

Upvotes: 1

Views: 382

Answers (1)

Telokis
Telokis

Reputation: 3389

In your file b.cc, you must include a.h.
In the .h file you can't because of multiple inclusion but since .cc files are not included, they can include whatever they want.

Note :

You should not forward declare b in your a.h file because it is declared in b.h and you included it.

You could also, as stated @chris, remove the #include "b.h" in your a.h and put it in your a.cc.

Upvotes: 1

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