Problem with function pointers

**~~vNvNation~~** · 06-12-2004

I have this setup:

I have a class, and one of the members is a function pointer. I want to be able to implement various possible versions of the function it points to in the same class the function pointer resides in. However, I get various errors when I try to initialize the function pointer to a function that also is a member of the same class.

I've shown the first part, then the error it generates, then in the second part I try to remedy it by declaring the function pointer to be a member of the Vec class, but it still doesn't work (I show the error codes of visual C++ 6). Of course, I couldn't then put this same function pointer in the baseclass, but it could at least point to a member function (sadly I can't even get this much to work properly).

Code:

#include	<iostream>
using namespace std;

//pFunc is of data type function pointer which returns void, accepts void as formal parameter
typedef	void(*pFunc)(void);

void	SomeFunc()
{
	cout	<<	"This is just some func" << endl;
}

typedef	struct	
{
	Vec()
	{
		x = y = z = w = 0;
		funcPtr	=	0;
	}
	void	VectorFunc()
	{
		cout	<<	"This is the vector func" << endl;
	}
	pFunc	funcPtr;
	float	x;
	float	y;
	float	z;
	float	w;	//complex plane
}Vec,*VectorPTR;

typedef	 void(Vec::*VecFuncPtr)(void);

int	main(void)
{
	Vec a;
	a.funcPtr	=	SomeFunc; //works
	a.funcPtr();
	
	a.funcPtr	=	Vec::VectorFunc;	//error:
	/*
	error C2440: '=' : cannot convert from 'void (__thiscall Vec::*)(void)' to 'void (__cdecl *)(void)'
	*/

	VecFuncPtr b;
	b = Vec::VectorFunc;
	b();	//error:
	/*
		error C2064: term does not evaluate to a function
	*/

	return 0;
}

**Codeplug** · 06-12-2004

There's a big difference between regular function pointers and member function pointers. There's also a little extra syntax that's required for declaring and invoking a pointer to member function.

Hopefully this site will fill in the gaps for ya:
http://www.function-pointer.org/

gg

**~~vNvNation~~** · 06-12-2004

Hi, that is a very good site, and subsequently I am in the process of answering my questions.

I appreciate the help, I shall throw some reputation your way

Have a nice day.

edit

I take that back, it is an EXCELLENT site. I am rather enjoying myself reading the site and going through the examples (I'm going through with functors right now).

**bennyandthejets** · 06-13-2004

> float (*GetPtr1(const char opCode)) (float, float);
how can this return a pointer to a function, this prototpye is a little messy?

The return type surrounds the function declaration. So, the function GetPtr1 returns a pointer to a function that takes two floats as parameters and returns a float. GetPtr1 itself takes a single const char as a parameter. I would recommend typedefing your function pointers first, it looks a lot neater.

>typedef void(Vec::*VecFuncPtr)(void);
i know how typedef works, but what synonim does this typedef creates and to what?

It is a pointer to a member function of class Vec that takes no parameters and has no return value.

**Codeplug** · 06-13-2004

Originally Posted by http://www.function-pointer.org

2.7 How to Return a Function Pointer ?
It's a little bit tricky but a function pointer can be a function's return value. In the following example there are two solutions of how to return a pointer to a function which is taking two float arguments and returns a float. If you want to return a pointer to a member function you have just got to change the definitions/declarations of all function pointers.

Code:

   //-----------------------------------------------------------------------------------------
   // 2.7 How to Return a Function Pointer
   //     'Plus' and 'Minus' are defined above in 2.1-4. They return a float and take two float


   // direct solution
   // function takes a char and returns a pointer to a function which is taking two
   // floats and returns a float. <opCode> specifies which function to return
   // Yes it's messy, that's why you should use a typedef
   float (*GetPtr1(const char opCode))(float, float)
   {
      if(opCode == '+')  return &Plus;
      if(opCode == '-')  return &Minus;
   }


   // solution using a typedef
   // define a pointer to a function which is taking two floats and returns a float
   // "pt2Func" is now a synonym type name for a pointer to function with that signature
   typedef float(*pt2Func)(float, float);

   // function takes a char and returns a function pointer which is defined as a
   // type above. <opCode> specifies which function to return
    // as you can see, "pt2Func" is the return type and is alot easier to read
   pt2Func GetPtr2(const char opCode)
   {
      if(opCode == '+')  return &Plus;
      if(opCode == '-')  return &Minus;
   }


   // execute example code
   void Return_A_Function_Pointer()
   {
      cout << endl << "Executing 'Return_A_Function_Pointer'" << endl;

      float (*pt2Function)(float, float);     // define a function pointer

      pt2Function=GetPtr1('+');               // get function pointer from function 'GetPtr1'
      cout << pt2Function(2, 4) << endl;      // call function using the pointer


      pt2Function=GetPtr2('-');               // get function pointer from function 'GetPtr2'
      cout << pt2Function(2, 4) << endl;      // call function using the pointer
   }

gggg

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