circular dependency(?) in C++

Question

Hi all,

My initial suspicion was that there was a circular dependency in my code and went through http://stackoverflow.com/questions/625799/resolve-circular-dependencies-in-c. But this hasn't resolved my compilation errors. Here is the code with 3 classes - A, G & N.

//A.h

#ifndef A_H_
#define A_H_

class com::xxxx::test::G;

namespace com { namespace xxxx { namespace test {

class A {

public:
 A();
 ~A();
 void method1(void);

private:
 G* g;
};

} } }

#endif /* A_H_ */


//A.cpp

#include "A.h"
#include "G.h"

namespace com { namespace xxxx { namespace test {

A::A() {
 g = new com::xxxx::test::G();
}

A::~A() {
 delete g;
}

void A::method1() {
 g->method2(*this);
}

} } }


//G.h

#ifndef G_H_
#define G_H_

class com::xxxx::test::A;

namespace com { namespace xxxx { namespace test {

class G {
public:
 void method2(const A&);
};

} } }

#endif /* G_H_ */


//G.cpp

#include "N.h"

namespace com { namespace xxxx { namespace test {

void G::method2(const A& a) {
 N n(a, *this);
}

} } }


//N.h

#ifndef N_H_
#define N_H_

#include "A.h"
#include "G.h"

namespace com { namespace xxxx { namespace test {

class N {
public:
 N(const A& obj1, const G& obj2) : a(obj1), g(obj2) {}
 void method3(void);

private:
 A a;
 G g;
};

} } }

#endif /* N_H_ */

I am getting about 10 compilation errors in A.h and A.cpp I am listing the compilation errors below:

./src/A.h:11: error: 'com' has not been declared
../src/A.h:25: error: ISO C++ forbids declaration of 'G' with no type
../src/A.h:25: error: invalid use of '::'
../src/A.h:25: error: expected ';' before '*' token
../src/A.cpp: In constructor 'com::xxxx::test::A::A()':
../src/A.cpp:16: error: 'g' was not declared in this scope
../src/A.cpp: In destructor 'com::xxxx::test::A::~A()':
../src/A.cpp:20: error: 'g' was not declared in this scope
../src/A.cpp: In member function 'void com::xxxx::test::A::method1()':
../src/A.cpp:24: error: 'g' was not declared in this scope

What could be the mistake in the above code?

Thank you in advance,
Regards,
Raghava.

Answer 1

The forward declaration

 class com::xxxx::test::G;

is illegal. Members of a namespace must be declared within it.

namespace com { namespace xxxx { namespace test {
    class G;

Also, as Kenny says, namespaces aren't used like this in C++. Unless your project is distributed as a library or is of reasonably large size (dozens of files minimum), you probably don't need your own namespace.

Answer 2

Is class com::xxxx::test::G; legal in C++ ? I would have written:

namespace com {
   namespace xxxx {
       namespace test {
          class G;
       }
   }
}

Answer 3

When the compiler tries to compile a.cop, the first thing it encounters (included from a.h) is this:

class com::xxxx::test::G;

At this point there is nothing to tell the compiler what exactly com, xxxx and test are. Each of these can be either a namespace or a class. This also means that G is unclear, which leads to all other errors.

Answer 4

As others have pointed out, using class com::xxxx::test::G; is illegal.

The simpler conversion is (preserving inline-ness):

namespace com { namespace xxxx { namespace test { class G; } } }

I prefer this way of forward declaring because "grepping" does not show scope, whereas this syntax immediately lay it out for all to see.