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An object-oriented wrapper around function pointers

, 9 Sep 2004
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Function pointers provide a handy mechanism for 1<sup>st</sup> order functions but are rather complex in usage. This article gives a wrapper around those function pointers making the use of them easier.

Introduction

Sometimes functions only differ in a few things you cannot express by using variables, what you really love to do is passing a function that can handle the 'special' case and keep your abstraction nice. In some languages like Scheme, you can pass functions as arguments; in C++, you have to use function pointers.

Function pointers are a good system for C, but not for C++; you have to define a non-class member or static class member... So, we cannot access non-static data members of a class without using global variables.

The solution is creating a wrapper around the function pointers that contains the function pointer and a pointer to the class.

Note: an alternative used by STL are function objects; I don't like this approach since it causes too much type-work Wink | ;) .

Usage

I'll describe a short guide on how to incorporate/use the function pointer wrappers.

  1. Add the following statements to your header file:
    ...
    #include "functor.h"
    ...
  2. (optional) Do a typedef of the templated function pointer, this makes it much easier to read Smile | :) .

    It is recommended to make this typedef inside your class definition, to prevent polluting your global namespace. I usually place them in the private declarations of my class.

    typedef Functor<Test, void, int> Function;

    In this example, Test is the class that contains the function, void is the return value of the function, and int is the only argument of the function.

  3. Define your class, I added a quick sample of a class here:
    /* 
     * header file
     */
    class Test
    {
    public:
      void TestFunc();
    
    private:
      void Add(int x);
      void Mul(int x);
      void DoSomething(int x, Function f);
    
      int value_;
    };
    
    /* 
     * source file
     */
    void Test::Add(int x)
    {
      value_ += x;
    }
    
    void Test::Mul(int x)
    {
      value_ *= x;
    }
    
    void Test::DoSomething(int x, Function f)
    {
      // some very complicated code
      ...
      f(x);
      ...
      // lots of other code
    }

    In the example, you should note f(x), this is the calling of the function pointer wrapper. Looks exactly like a regular function call, but the f is an argument of the function.

  4. Create the function pointer wrapper:
    void Test::TestFunc()
    {
      // create the function pointer wrapper (functor)
      Function myadd(this, &Test::Add);
      Function mymul(this, &Test::Mul);
    
      // now do the action!
      DoSomething(5, myadd);
    }

    You create a wrapper object just by passing two arguments to the constructor: the host class of the functions, usually a this, and the name of the method that the wrapper should use.

    You can now pass the function to any method and call it as it was a normal function Smile | :)

Technical Stuff

The system of the wrapper is based on the usage of templates and operator overloading. Templates are used to make it more generic (i.e., to be able to create a wrapper for any function with a specified number of arguments). The operator overloading is used to simulate a regular procedure call, we overload the operator() to achieve this.

Known Limitations

The source code only contains a wrapper for 1 and 2 arguments. If you need more, you'll need to extend the classes or pass a struct/class that contains the data as single argument.

Todo

  • Compatibility with const member functions
  • Create an example project
  • Create an inline version for compatibility with VC++ 6 or lower.

History

  • Version 1.0.1 - September 12, 2004

    Some corrections based on suggestions of Roland Pibinger and KevinHall. Thanks!

  • Version 1.0.0 - September 9, 2004

    Initial release.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Kenny Moens
Web Developer
Belgium Belgium
Kenny Moens is a student computer science at Vrije Universiteit Brussel (VUB), a university located in Brussels, Belgium.
 
He's been programming for about 10 years with experience in Pascal, Basic, Perl, PHP, C, C++, Java, Scheme, Lisp and Smalltalk.

Comments and Discussions

 
GeneralCompile Error Pinmemberhaiwenliao11-Mar-09 17:24 
GeneralSome Ideas PinmemberRoland Pibinger13-Sep-04 11:33 
GeneralSome Remarks PinmemberRoland Pibinger11-Sep-04 9:08 
GeneralSome small corrections to the article PinmemberKevinHall10-Sep-04 6:16 
GeneralRe: I don't agree with everything you say. PinmemberKevinHall9-Sep-04 14:41 
GeneralRe: I don't agree with everything you say. PinmemberWREY10-Sep-04 4:21 
GeneralRe: I don't agree with everything you say. PinmemberKevinHall10-Sep-04 6:06 
GeneralI didn't know that. PinmemberWREY10-Sep-04 6:46 
GeneralRe: I didn't know that. PinmemberKevinHall10-Sep-04 6:59 
GeneralThis might help. PinmemberWREY10-Sep-04 9:10 
GeneralRe: I don't agree with everything you say. PinmemberKenny Moens9-Sep-04 23:58 
GeneralNice topic -- a lot of room for improvement though PinmemberKevinHall9-Sep-04 11:57 
GeneralRe: Nice topic -- a lot of room for improvement though Pinmemberemilio_grv9-Sep-04 22:32 
GeneralRe: Nice topic -- a lot of room for improvement though PinmemberKevinHall10-Sep-04 3:36 
GeneralRe: Nice topic -- a lot of room for improvement though Pinmemberemilio_grv10-Sep-04 5:10 
GeneralRe: Nice topic -- a lot of room for improvement though PinmemberKevinHall10-Sep-04 5:50 
GeneralRe: Nice topic -- a lot of room for improvement though PinmemberKenny Moens10-Sep-04 0:02 
GeneralRe: Nice topic -- a lot of room for improvement though PinmemberKevinHall10-Sep-04 6:20 

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