pointer-to-function in C++

jacob · #1 (**permalink**) October 20th, 2004, 04:28 PM

Have got a problem wit pointer-to-funtion in C++, so here it goes...

I want to define some functions in some class A that I can give as arguments to a function from some other class B. However I cannot make this work. First I have made this small program that actually works...

Code:

#include <iostream>

using std::cout; 
using std::endl;

bool testMethod()
{
    return false;
}

bool test(bool (*method)())
{
    return (*method)();
}

int main()
{
    cout << test(testMethod) << endl;    

    return 0;
}

And then I moved on to doing something like this, which does not work...

Code:

class Test
{

public:

    bool someMethod(void)
    {
        return false;
    };

    bool testMethod(bool (*method)(void))
    {
        return (*method)();
    };

    void test()
    {
        bool t = this->testMethod(someMethod);
    };

};

The error message I get when I do this is like the following...

Code:

error C2664: 'Test::testMethod' : cannot convert parameter 1 from 'bool (void)' to 'bool (__cdecl *)(void)'

Is it some kind of typecasting problem? And how to get around it? It is the same error message I get if I set up the two class that I want.

Thanks in advance...

Jacob.

mehdi62b · #2 (**permalink**) October 22nd, 2004, 05:47 AM

Hi jacob,
the following link might help you,
http://www.functionx.com/cpp/Level2/Lesson07.htm

--------------------------------------------
Mehdi.:)

klep · #3 (**permalink**) October 23rd, 2004, 05:20 AM

Hi Jacob,

Pointers to methods are a little trickier than pointers to functions. They need to be referred to in the context of an object (or a class if it's a pointer to a static method). In your example, the correct code for the test() method is:

  bool testMethod(bool (Test::*methodPtr)(void))
  {
    return (this->*methodPtr)();
  };

Note that the parameter type refers to the method in the context of its class and the call uses an actual object. This syntax is dense, but it's flexible -- it means you can pass a method on a class and call it on any object of that class, not just on "this". Kind of neat.

To pass a method in a function or method, you also need to refer to it in the context of its class. So the correct code for Test::test() is:

void Test::test()
{
  bool t = this->testMethod(&Test::someMethod);
}

----
Scott J. Kleper
Author, "Professional C++"
(Wrox, 2005)