struct class member in C++

Question

Im new and learning C++, is this code a valid C++ implementation, please see code below,

// FoomImpl.h
#include "Foo.h"

namespace FooFoo { namespace Impl {

class FooImpl
{
   public:
      FooImpl( FooFoo::CFoo::stBar bar )
      {
         m_bar = bar;
      }

   private:
      FooFoo::CFoo::stBar m_bar;
};

} }

// Foo.h
#include "FoomImpl.h"

namespace FooFoo {

class CFoo
{
 public:
    struct stBar
    {
       int aBar;
    };

 public:
    CFoo( stBar bar ) : impl(NULL)
    {
       impl = new FooFoo::Impl::FoomImpl( bar );
    }

    CFoo( ) : impl(NULL){}
    ~CFoo( )
    {
       if(impl)
         delete impl;
    }

 private:
    FooFoo::Impl::FoomImpl *impl;
};
}

Many thanks.

Flinsch · Accepted Answer · 2010-11-04 11:40:25Z

1

No, there is need for forward declaration as you have a circular dependency.

answered Nov 4, 2010 at 11:40

Flinsch

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2 Comments

domlao Over a year ago

Where can I put the forward declaration? Do I need to forward declare the two class?

Flinsch Over a year ago

You can forward both classes but than you wouldn't have access to FooFoo::stBar. So, just forward FooImpl by removing the #include "FooImpl.h" directive and replace it with the forward declaration class FooImpl;. As you access FooImpl within class FooFoo only via pointers, FooFoo doesn't need to know the size and the concrete definition of class FooImpl. But, as FooFoo cannot know FooImpl's implementation, you have to move every functionality regarding FooImpl (like c'tor and d'tor calls via new and delete) to the corresponding .cpp file.

score 1 · Accepted Answer · 2010-11-04 11:56:27Z

You cannot have two different objects holding each other on the stack, because when the compiler goes through the header file of an object it needs to now the size of all it's data members in order to know the size of the object itself. Obviously in your case that is not possible.

The way you can do this is move your CFoo constructor in a cpp file, and then in Foo.h above the class just write class Impl::FoomImpl;. This is called a forward declaration. It just tells the compiler that this is a class name. This is enough to use a pointer to that class. Now Foo.h no longer needs to include FoomImpl.h, because it only uses a pointer to Impl::FoomImpl, which is of a known size (most likely 4 bytes).

If you are inside a certain namespace, no need to specify the scope...

namespace FooFoo
{
   //no need to use "FooFoo::" here
}

In order to avoid further confusion I would suggest you keep the number of namespaces down. Unless you are developing a library that will be used by other developers, you risk little even by using none at all, as long as your class names are unambiguous. Personally, in an end user software I only use them to scope enums and I give really clear and specific names to my classes.

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