Forward Declaration compilation warning (g++)

[dus]

I have the following class structure for a (discrete event) simulation program:

class simulation;

class event
{
public
	event(simulation* sim_pointer) 	{p_sim = sim_pointer;};
	virtual ~event()					{};

	virtual void process() = 0;

protected:
	simulation* p_sim;

};

class simulation
{

public:
	simulation()				{};
	virtual ~simulation()			{};

	virtual void schedule(event* p_event);

	//some more functions

protected:
	list<event*> event_list;

};

class special_event;

class special_simulation : public simulation
{
	special_simulation()	: simulation()	{};
	virtual ~special_simulation()			{if (p_save_event) delete p_save_event;};

               //more functions

protected:
	special_event* p_save_event;

               //more class members
};

class special_event : public event
{
public
	special_event(special_simulation* special_sim_pointer) 	: event(special_sim_pointer)	{};
	virtual ~special_event()					{};

	virtual void process() = 0;

protected:
	//some class members
};

In this case I want to use the special_simulation class (inherited from the base simulation class). The simulation classes processes events derived from the pure virtual event class (including events inherited from special_event_class).

When compiling the code with g++ I get the following warnings:

In destructor 'virtual special_simulation::~special_simulation()':
warning: possible problem detected in invocation of delete operator:
warning: invalid use of undefined type `struct special_event'
warning: forward declaration of `struct special_event'
note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined.

Is this warning serious, what are possible implications (memory leaks?)?
How can I fix this problem?

[treuss]

Is this warning serious, what are possible implications (memory leaks?)?

It sounds serious and its never a good idea to ignore warnings.

How can I fix this problem?

Just move the definition of your destructor out of your class declaration, e.g. move the following to the end of the file:

inline
special_simulation::~special_simulation()
{
  if (p_save_event) delete p_save_event;
};

[Graham]

It's basically giving you the warning because it can't "see" the class's destructor, so there's a possibility that you may be doing a polymorphic delete on a class without a virtual destructor. Yes, pay attention to it; yes, do as treuss suggests, but I don't think it's serious as long as you are convinced that the delete is OK.

[Kheun]

You need to split definition and declaration, preferrably into .h and .cpp files because in order to delete an object, you require to have the complete class declaration being declared ahead of the code. In addition, in the constructor of simulation and special_simulation class, you should intialize the p_sim and p_save_event pointer to 0. If not, your program may crash in the destructors because they are pointing to some invalid address. Beside, in both destructors, it is valid to delete a NULL pointer without checking statement.

[dus]

Thanks all!

Treuss solution removed the warning!