1

I have two classes that reference each other. Class B has a list of pointers to A objects and each A object requires a pointer to the B object that created it. Why does this compile when written in a single file but not when split into multiple files?

namespace.h

#ifndef NAMESPACES_H
#define NAMESPACES_H

#include "A.h"
#include "B.h"
#include "C.h"

namespace NAMESPACE
{
    class A;
    class B;
    class C;
}
#endif

A.h

#ifndef A_H
#define A_H
#include "namespaces.h"
#include <iostream>

namespace NAMESPACE{
class A
{
private:
    int x;
    B* ptrB;
public:
    void print();
    A(int g_x, B* g_ptrB);
};
}

#endif

A.cpp

#include "A.h"

namespace NAMESPACE{


A::print()
    {
        std::cout<<"I am class A, my x = "<<x<<std::endl;
    }
    A::A(int g_x, B* g_ptrB)
    {
        x = g_x;
        ptrB = g_ptrB;
    }
}

B.h

#ifndef B_H
#define B_H

#include "namespaces.h"
#include <vector>
#include <iostream>
namespace NAMESPACE{

class B
{
private:
    std::vector<A*> vA;
public:
    B(int g_x);
    void print();
};
}

#endif

B.cpp

#include "B.h"

namespace NAMESPACE{

B::B(int g_x)
    {
        vA.push_back(new A(g_x, this));
    }
    void B::print()
    {
        std::cout<<"I am class B\n";
        vA[0]->print();
    }
}

main.cpp

#include <iostream>
#include "namespaces.h"

int main()
{
    B *b = new B(3);
    b->print();
    return 0;
}

/* By contrast, this single .cpp file compiles just fine */

#include <iostream>
#include <vector>
class B;
class A
{
private:
    int x;
    B* ptrB;
public:
    void print()
    {
        std::cout<<"I am class A, my x = "<<x<<std::endl;
    }
    A(int g_x, B* g_ptrB)
    {
        x = g_x;
        ptrB = g_ptrB;
    }
};

class B
{
private:
    std::vector<A*> vA;
public:
    B(int g_x)
    {
        vA.push_back(new A(g_x, this));
    }
    void print()
    {
        std::cout<<"I am class B\n";
        vA[0]->print();
    }
};

class C:public A
{
public:
    C(int g_x, B* g_ptrB):A(g_x, g_ptrB){}
};

int main()
{
    B *b = new B(3);
    b->print();
    return 0;
}

When compiling the separate files, I get a lot of errors like 

In file included from namespaces.h:4:0,
                 from B.h:4,
                 from B.cpp:1:
A.h:11:2: error: ‘B’ does not name a type
  B* ptrB;
  ^
A.h:14:13: error: ‘B’ has not been declared
  A(int g_x, B* g_ptrB);
             ^
In file included from namespaces.h:6:0,
                 from B.h:4,
                 from B.cpp:1:
C.h:10:13: error: ‘B’ has not been declared
  C(int g_x, B* g_ptrB):A(g_x, g_ptrB){}

But when compiling a single file, it all runs smoothly and I get

I am class B
I am class A, my x = 3

How come?

McMillian64
  • 11
  • 2
  • in A.h you have not declared B – M.M Apr 26 '19 at 05:03
  • isn't the B declaration in namespaces.h and included in A.h? – McMillian64 Apr 26 '19 at 05:07
  • 3
    no, you have circular includes there. `namespaces.h` should not include any of the other headers – M.M Apr 26 '19 at 05:08
  • You need to *first* declare the classes in the header, *then* include the other files (if intended). The other includes can’t see the declarations that happen after them. – Sami Kuhmonen Apr 26 '19 at 05:12
  • @M.M is right, more in main you call B instead of NAMESPACE::B. You can look at this : https://www.youtube.com/watch?v=su9ittf-ozk&list=PLHTh1InhhwT55y4fRRTBIelxnRSZ8G5yg&index=53 (CPPCON) it may interest you – Martin Morterol Apr 26 '19 at 05:26

0 Answers0