Question

I’m having trouble putting together an object model that involves nested classes, using MinGW C++. Here is an example that exposes my issue:

foo.h:

/*
 * foo.h
 *
 *  Created on: Sep 25, 2011
 *      Author: AutoBot
 */

#ifndef FOO_H_
#define FOO_H_

class Foo
{
public:
    class Bar;
    Bar bar;
} extern foo;


#endif /* FOO_H_ */

bar.h:

/*
 * bar.h
 *
 *  Created on: Sep 25, 2011
 *      Author: AutoBot
 */

#ifndef BAR_H_
#define BAR_H_

#include <iostream>
using namespace std;

#include "foo.h"

class Foo::Bar
{
public:
    void Test() {cout <<"Test!";}
};


#endif /* BAR_H_ */

main.cpp:

/*
 * main.cpp
 *
 *  Created on: Sep 25, 2011
 *      Author: AutoBot
 */



#include "foo.h"

Foo foo;

int main (int argc, char *argv[])
{
    foo.bar.Test();
    return 0;
}

Eclipse CDT build log:

**** Build of configuration Debug for project NestedClassTest ****

**** Internal Builder is used for build               ****
g++ -O0 -g3 -Wall -c -fmessage-length=0 -o src\main.o ..\src\main.cpp
In file included from ..\src\main.cpp:10:0:
..\src\/foo.h:15:6: error: field 'bar' has incomplete type
..\src\main.cpp: In function 'int main(int, char**)':
..\src\main.cpp:16:6: error: 'class Foo' has no member named 'bar'
Build error occurred, build is stopped
Time consumed: 171  ms.

So essentially it’s failing to recognize the definition of bar I made in bar.h. Is this normal? Why shouldn’t I be able to define nested classes this way? Is this simply a limitation of the MinGW toolchain? Any help/advice is appreciated.

Side note: in my actual program, I intend for the “foo” of it to serve as, theoretically, a singleton class. It would represent the application as a whole, and it’s subsystems (or “bars”) would be defined and instantiated once inside the class. I’m doing this to try to make the code more manageable. If anyone has a more feasible design pattern in mind, please tell me!

Answer 1

The problem is here:

class Foo
{
public:
    class Bar;
    Bar bar;
};

At this point Foo::Bar is an incomplete type. And you cannot declare a variable of an incomplete type. It would be no different from trying to do this:

class Foo;
Foo foo;

They’re both not allowed and for the same reason.

You could turn bar into some kind of (smart) pointer. But that’s the only way to solve this.