Function pointer undeclared identifier

[dwdude]

Hello there. Finally decided to make an account here :)

Anyways, I just started trying to implement function pointers in my code, and I seem to be having a few issues. (this is, of course, test code):

#include <iostream>

using namespace std;

class MyClass;

class SomeClass
{
public:
	SomeClass();
	MyClass* test;
	int SomeFunc(int x, char* y);
};

class MyClass
{
public:
	int (SomeClass::*funcptr)(int, char*);
};

int SomeClass::SomeFunc(int x, char* y)
{
	cout << "int x: " << x << endl;
	cout << "char* y: " << y << endl;
	return 0;
}

int main(int argc, char* argv[])
{
	SomeClass* someclass = new SomeClass();

	someclass->test->funcptr = &SomeClass::SomeFunc;
	(someclass->test->*funcptr)(2, "Lol"); // C2065: 'funcptr': undeclared identifier

	return 0;
}

Something tells me that this is functionality that I cannot do? And if so, I seem to be doing it wrong.

Basically, I have two classes: SomeClass and MyClass. MyClass right now just has a function pointer. SomeClass has a member function that I want MyClass to point to. However, trying to call that function seems to not be working the way I wanted it to. In fact, it's not working.

Any suggestions?

[Salem]

> (someclass->test->*funcptr)(2, "Lol");

Remove the * (someclass->test->funcptr)(2, "Lol");

[dwdude]

> (someclass->test->*funcptr)(2, "Lol");

Remove the * (someclass->test->funcptr)(2, "Lol");

Thank you for your reply.

However, upon removal of of the *, I get another error on that same line: error C2064: term does not evaluate to a function taking 2 arguments

This is all being compiled in Visual Studio 2008 Pro, by the way.

In g++, the error is: error: must use .* or ->* to call pointer-to-member function in `someclass->SomeClass::test->MyClass::funcptr (...)'

[MosaicFuneral]

Are you trying to declare a SomeClass member from inside of MyClass?

[dwdude]

Are you trying to declare a SomeClass member from inside of MyClass?

What I wanted was for MyClass to create a function pointer to SomeClass::SomeFunc(int x, char* y) .

[MosaicFuneral]

You never declare 'funcptr', you just have it randomly appearing in main() referencing something. It also can't be declared in main() and then used as a type in your outside defined class.

[dwdude]

You never declare 'funcptr', you just have it randomly appearing in main() referencing something. It also can't be declared in main() and then used as a type in your outside defined class.

18. int (SomeClass::*funcptr)(int, char*);

From what I've read, that is how you declare a function pointer. A function pointer to a member function in SomeClass named funcptr that has a return value of type int with 2 parameters of type int and char*.

Then, to define what that function pointer actually points to:

32. someclass->test->funcptr = &SomeClass::SomeFunc;

But my assumptions must be wrong and I'm doing something wrong somewhere.

[MosaicFuneral]

SomeClass::*funcptr doesn't exist, its not a SomeClass member. I believe it would be something more like:

typedef int (*fp)(int, char*);
class MyClass{
public:
    fp *funcptr;
}

I cannot confirm that is correct, I need to oil up the rusting gears in my head. I generally don't use pointer functions unless I'm using a raw code catalyst of sorts. I'm sure someone will sort this out.

[mitrmkar]

I think this is what you were after, note that you need an instance of the class through which you make the call ( pB = new B; below)

#include <iostream>
struct B;
struct A
{
  B * pB;

  A();
  ~A();

  void func(int i, const char * p) 
  {
    std::cout << i << std::endl << p << std::endl;
  }
};

struct B
{
  void (A::* funcptr)(int, const char *);
};

A::A()
{
  // a B is needed
  pB = new B;
}

A::~A()
{
  delete pB;
}

int main()
{
  A * pA = new A;

  pA->pB->funcptr = &A::func;

  (pA->*(pA->pB->funcptr))(2, "Lol");

  delete pA;

  return 0;
}

[dwdude]

I think this is what you were after, note that you need an instance of the class through which you make the call ( pB = new B; below)

#include <iostream>
struct B;
struct A
{
  B * pB;

  A();
  ~A();

  void func(int i, const char * p) 
  {
    std::cout << i << std::endl << p << std::endl;
  }
};

struct B
{
  void (A::* funcptr)(int, const char *);
};

A::A()
{
  // a B is needed
  pB = new B;
}

A::~A()
{
  delete pB;
}

int main()
{
  A * pA = new A;

  pA->pB->funcptr = &A::func;

  (pA->*(pA->pB->funcptr))(2, "Lol");

  delete pA;

  return 0;
}

That was just what I was looking for. Although, line 38 threw me a bit off, I must admit. I had to read that a few times before I got it straight, the second pA seemed almost redundant, but now I get it. Thanks a lot!