class function pointer

This is a discussion on class function pointer within the C++ Programming forums, part of the General Programming Boards category; hello Is there a known way to assign a class method to a function pointer? I tried this, which doesn't ...

  1. #1
    coder
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    class function pointer

    hello
    Is there a known way to assign a class method to a function pointer?

    I tried this, which doesn't work:
    Code:
    class MyClass {
    	void func ()
    	{
    		...
    	}
    }
    
    int main ()
    {
    	void (*fptr) ();	//function pointer
    	MyClass foo;		//instance
    
    	fptr = foo.func;
    }
    compiler (g++) says:
    error: argument of type ‘void (Class:: )()’ does not match ‘void (*)()’

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    Class functions has a special syntax:
    void (MyClass::*)()
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  3. #3
    Kernel hacker
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    But why would you actually want to do that? The whole point of having classes is that they have member functions. With virtual member functions, you can create the same effect, but avoid the actual function pointer mess - yes, internally the compiler generates roughly the same code, but it's much easier to read and use.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  4. #4
    coder
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    ok thanks
    but, what if I wish to use the pointer with different classes?

  5. #5
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    Quote Originally Posted by matsp
    But why would you actually want to do that?
    The main use the fpointer to call the function I need
    eg:
    STATE = game -> call Game::main ()
    STATE = menu -> call Menu::main ()

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    You're thinking in a C way. You probably want something like this in C++.
    Code:
    class base {
    public:
        virtual void main() = 0;  // no really, Elysia, this void main is okay :)
    };
    
    class game : public base {
    public:
        virtual void main() {  // "virtual" not technically required
            // ...
        }
    };
    
    class menu : public base {
    public:
        virtual void main() {  // "virtual" not technically required
            // ...
        }
    };
    
    // and so on
    
    base *p = new menu;
    p->main();
    dwk

    Seek and ye shall find. quaere et invenies.

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  7. #7
    Malum in se abachler's Avatar
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    Code:
     
    fptr = MyClass::foo.func;
    should work, but i woudlnt recommend it. Why exactly do you need a function pointer to a member function?

    Is thewre some reason you cant make it a friend function to all the clsses its associated with?
    Last edited by abachler; 02-13-2008 at 02:54 PM.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

  8. #8
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    Well, I'm not used with class stuff like virtual functions, inheritance and friendship...
    so I'm trying to learn more about at the moment, since I'm a bit confused now

    However I've encountered another problem using a function-pointer stack:
    Code:
    #include <iostream>
    using namespace std;
    
    #include <stack>
    
    void menu ()
    {
    	cout << "hello\n";
    }
    
    typedef void (*fptr) ();
    
    int main ()
    {
    	stack<fptr> State;
    
    	fptr temp;
    	temp = menu;
    
    	State.push (temp);
    	State.top ();
    
    	return 0;
    }
    this code didn't print "hello": what's wrong?

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    Well, you're not calling the function, are you?
    It should be something like
    State.top()();
    or maybe
    ( State.top() )();
    Seeing as top is a function that returns a reference to the object at the top of the stack.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #10
    coder
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    Aaargh you're right
    I was missing that detail indeed
    thanks

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