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Calling member functions through pointers

Started by SpaceRogue May 22, 2003 at 12:09 PM 3 replies 700+ views

Original Post

May 22, 2003 at 12:09 PM

I'm having a little trouble with pointers to member functions. I have this class with two member functions and a data member which is a pointer to a function like this:

class Thingie
{
public:
	bool (Thingie::*funcpointer)();
	bool funcA();
	bool funcB();
	Thingie();
	virtual ~Thingie();
};

I know I've set funcpointer=funcA successfully as debugging reveals the correct address is stored in funcpointer, but I can't seem to call the function from outside the class using something like:

Thingie A;
(A.*funcpointer)();

That will not compile, giving me an error that funcpointer is an undeclared identifier. If I change it to:

(A.funcpointer)();

*(A.funcpointer)();

I get an error that the expression does not evaluate to a function. What am I doing wrong here?

[edited by - SpaceRogue on May 22, 2003 1:11:19 PM]

Spacecat

May 22, 2003 at 12:28 PM

I recommend you look up this site:
http://www.newty.de/CCPP/callback/callback.html

They explain why you -can''t- cleanly use a member function of a class with a callback pointer, and a workaround.

SpaceRogue

Author

SpaceRogue

May 22, 2003 at 12:46 PM

Actually I''ve already been using that site as a guide, but still couldn''t get the syntax right until just a few minutes ago.

I finally figured out it has to be:

(A.*(A.funcpointer))()

This works, but certainly is ugly.

A wrapper might look better.

daerid

May 22, 2003 at 1:20 PM

I like member function wrappers

  
template< typename _Type, typename _Result, typename _Arg >
class MemberFunction
{
public:
  MemberFunction( _Result (_Type::*pfn)( const _Arg& ) ) :
    mPfn( pfn ) {}

  _Result operator()( _Type& obj, const _Arg& a ) {
  }
private:
  _Result (_Type::*mPfn)( const _Arg& );
};



class test
{
public:
  void DoSomething( const std::string& out ) {
    std::cout << "test::DoSomething( " << out << " );" << std::endl;
    std::cout << "test::i = " << this->i << std::endl;
  }
  int i;
};

void main() {
  test t;
  t.i = 4;
  
  MemberFunction< test, void, std::string > pfn( test::DoSomething );
  pfn( t, "test" );

}

Kind of contrived, and also, there's stdlib headers that allow for the same thing. Take a look at . Also, boost::function is pretty neat too.

[edited by - daerid on May 22, 2003 2:20:46 PM]

SpaceRogue

Author

SpaceRogue

May 22, 2003 at 3:57 PM

Thanks, although I think your example is a bit beyond what I'm capable of understanding at the moment.

I've only been programming for three months, so I'm just happy to have made it as far as I have. Templates still give me headaches from just looking at them. Although I understand the concept just fine, I still find all but the simplest ones illegible.

[edited by - SpaceRogue on May 22, 2003 4:59:15 PM]

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