'.*' : cannot dereference a 'callbackf' on a 'Base'

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when i am executing following code:

class Derived;
typedef void (Derived::*callbackf)() ;

class Base;
typedef map<Base*, callbackf*> MYLIST;
class Base
{
public:
	Base(void){};
	MYLIST mylist;
	void Attach(Base* p, callbackf*){mylist[p] = voidpt;};
	~Base(void){};
};

class Derived :	public Base
{
public:
	Derived(void)
            {
                callbackf ponLocoMoving = &Derived::onLocoMoving;
        	Attach(this, &ponLocoMoving);};
            }
	void onMoving(){};
	~Derived(void){};
};
int _tmain(int argc, _TCHAR* argv[])
{
	
	Base *b;
	b= new Derived();

	MYLIST::iterator itr;
	itr = b->mylist.begin();

	for (itr = b->mylist.begin(); itr != b->mylist.end(); itr++)
	{
		Base* clsOb = (*itr).first;
		callbackf fn = b->mylist[(*itr).first];
		(*clsOb.*fn)();
	}
	return 0;
}

I am getting two errors: 'initializing' : cannot convert from 'void *' to 'callbackf' 'newline' : cannot convert from 'Base *__w64 ' to 'Derived *__w64 ' '.*' : cannot dereference a 'callbackf' on a 'Base'

Posted 20-Nov-09 22:37pm

Chanchalgaud

Updated 20-Nov-09 23:08pm

Nuri Ismail

v2

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Richard MacCutchan · Answer 1 · 2009-11-22T00:35:00

Solution 1

See http://www.codeproject.com/Messages/3279872/map-saves-class-object-and-function-pointer-gives-.aspx for further comments.

Posted 22-Nov-09 0:35am

Richard MacCutchan

Lim Bio Liong · Answer 2 · 2009-11-22T04:33:00

I am getting two errors: 'initializing' : cannot convert from 'void *' to 'callbackf' 'newline' : cannot convert from 'Base *__w64 ' to 'Derived *__w64 ' '.*' : cannot dereference a 'callbackf' on a 'Base'

The first problem :

>> I am getting two errors: 'initializing' : cannot convert from 'void *' to 'callbackf'

... is related to your type definition of MYLIST :

typedef map<Base*, callbackf*> MYLIST;

You need not declare the mapped object type as "callbackf*". callbackf is already a pointer and so MYLIST can simply be stated as :

typedef map<Base*, callbackf> MYLIST;

You have to make other modifications to your overall code if you do decide to make the above change.

The second problem :

>> 'newline' : cannot convert from 'Base *__w64 ' to 'Derived *__w64 ' '.*' : cannot dereference a 'callbackf' on a 'Base'

...is known as contravariance. There is a predefined conversion from a pointer to a member function of a base class to a pointer to a member function of a derived class but not the reverse.

This is because a member function of a base class will only be able to access member data and/or member functions of the base class only. But the member function of a derived class may access the member data/functions of the derived class which will not be defined in the base class.

Because of this, C++ will not allow the following (found in your code) to be compiled :

(*clsOb.*fn)();

where clsOb is a pointer to Base. This is because fn is typed as a member function of class "Derived" and not "Base".

- Bio.