forward declaration and template function error

[bayuaj]

I am a new member of cplusplus forum and this is my first post here.
Currently I have a frustrating problem with forward declaration and template function. Below is the snippet of the code:

class TaskScheduler; --> //forward declaration of ‘struct TaskScheduler’
//
//

class TaskEvent {
//
//
};

class HostTask {
//
//
};

template<class T> inline HostTask*
findT(TaskScheduler* tss, T* e)
{
map<int, HostTask*>::iterator it;
bool bEq = false;
for(it = tss->tasks_.begin(); it != tss->tasks_.end(); it++) { --> //error: invalid use of incomplete type ‘struct TaskScheduler’
if(dynamic_cast<TaskEvent*>(e))
bEq = dynamic_cast<TaskEvent*>(e)->equal(it->second->ev_);
else if(dynamic_cast<HostTask*>(e))
bEq = dynamic_cast<HostTask*>(e)->equal(it->second);
if(bEq) {
return it->second;
}
}
return NULL;
}
//

//class TaskScheduler definition
class TaskScheduler : virtual public HCIEventsHandler {
friend HostTask* findT<TaskEvent>(TaskScheduler* tss, TaskEvent* e); //findT function is used here
//
//
};

Here is the error message that I've got which is shown in the code as well:
./bt-taskscheduler.h:159: error: forward declaration of ‘struct TaskScheduler’
./bt-taskscheduler.h:229: error: invalid use of incomplete type ‘struct TaskScheduler’

Could anybody show me what is going wrong in this code? Any help is appreciated..

[Bazzy]

Why don't you move findT after TaskScheduler?
You can't use members of a class which wasn't declared properly

Also, use [code][/code] tags

[kbw]

If you use templates with RTTI, you get the worst of both worlds, code bloat and the run-time cost of using RTTI.

You'd be better off having a templated version that is specific to your familt of types or going for a more traditional OO hierarchy.

[bayuaj]

Thank you all for your reply :)
@Bazzy: did you mean to move the findT after the complete TaskScheduler class? If it's so I got another error which says: ‘findT’ is neither function nor member function; cannot be declared friend.
@kbw: could you please explain me what you mean about "templated version that is specific to your familt of types or going for a more traditional OO hierarchy"?
Thank you..

[Bazzy]

Forward declare findT

[bayuaj]

ok, I'll try it..

[kbw]

As Bazzy suggested, it's the order of declaration that matters. I've filled in some minimal code to make it compile.

Looking further at what you have now, I think you need a static way to determine what kind of T is passed to findT. Taken from Alexandrescu's Loki, IsSameType will do it for you. I've added a findK to demonstrate. At least, this way you won't have the RTTI runtime overhead.

I still think the overall design needs to be revised.

#include <map>

class HCIEventsHandler {};

class TaskScheduler;

class TaskEvent
{
	int ev_;

public:
	bool equal(int);
};

class HostTask
{
	bool equal(HostTask*);
};

template <class T>
HostTask* findT(TaskScheduler* tss, T* e);

template <class T>
HostTask* findK(TaskScheduler* tss, T* e);

class TaskScheduler : public HCIEventsHandler
{
	template <>
	friend HostTask* findT(TaskScheduler* tss, TaskEvent* e);

	std::map<int, HostTask*> tasks_;
};

template<class T>
HostTask* findT(TaskScheduler* tss, T* e)
{
	for (std::map<int, HostTask*>::iterator it = tss->tasks_.begin(); it != tss->tasks_.end(); ++it)
	{
		bool bEq = false;

		if (dynamic_cast<TaskEvent*>(e))
			bEq = dynamic_cast<TaskEvent*>(e)->equal(it->second->ev_);
		else if(dynamic_cast<HostTask*>(e))
			bEq = dynamic_cast<HostTask*>(e)->equal(it->second);

		if (bEq)
		{
			return it->second;
		}
	}

	return NULL;
}


template <typename T, typename U>
struct IsSameType
{
    enum { value = false };
};

template <typename T>
struct IsSameType<T, T>
{
    enum { value = true };
};

template<class T>
HostTask* findK(TaskScheduler* tss, T* e)
{
	for (std::map<int, HostTask*>::iterator it = tss->tasks_.begin(); it != tss->tasks_.end(); ++it)
	{
		bool bEq = IsSameType<HostTask, T>::value
						? e->equal(it->second)
						: e->equal(it->second->ev_);

		if (bEq)
		{
			return it->second;
		}
	}

	return NULL;
}

[bayuaj]

Hi kbw.. thank u a lot for your suggestion.. that really works!