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The Function Pointer Tutorials

Introduction to C and C++ Function Pointers, Callbacks and Functors

written by Lars Haendel

January 2002, Dortmund, Germany

http://www.newty.de

email: lore@newty.de

version 2.041b

distribute and/or modify this document under the terms of the GNU Free

Documentation License, Version 1.1 or any later version published by the

Free Software Foundation; with no Invariant Sections, with the Front-

Cover Text being the text from the title up to the table of contents, and

with no Back-Cover Texts. A copy of the license can be obtained from

http://www.gnu.org .

Be aware that there may be a newer version of this document! Check

http://www.newty.de/zip/e fpt.pdf for the latest release. If you want to

distribute this document, I suggest you to link to the URL above to prevent

spreading of outdated versions.

You may also download the source code of the examples at

http://www.newty.de/zip/source.zip . The example code is free software

under the terms of the GNU General Public License.

Contents

1 Introduction to Function Pointers 2

1.1 What is a Function Pointer ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Introductory Example or How to Replace a Switch-Statement . . . . . . . . . . . . . . . . . . . . 2

2 The Syntax of C and C++ Function Pointers 3

2.1 Deﬁne a Function Pointer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Calling Convention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3 Assign an Address to a Function Pointer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.4 Comparing Function Pointers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.5 Calling a Function using a Function Pointer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.6 How to Pass a Function Pointer as an Argument ? . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.7 How to Return a Function Pointer ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.8 How to Use Arrays of Function Pointers ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 How to Implement Callback Functions in C and C++ 7

3.1 Introduction to the Concept of Callback Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 How to Implement a Callback in C ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3 Example Code of the Usage of qsort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.4 How to Implement a Callback to a static C++ Member Function ? . . . . . . . . . . . . . . . . . 8

3.5 How to Implement a Callback to a non-static C++ Member Function ? . . . . . . . . . . . . . . 9

4 Functors to encapsulate C and C++ Function Pointers 11

4.1 What are Functors ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2 How to Implement Functors ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.3 Example of How to Use Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1 Introduction to Function Pointers

Function Pointers provide some extremely interesting, eﬃcient and elegant programming techniques. You can

use them to replace switch/if-statements, to realize your own late-binding or to implement callbacks. Unfor-

tunately – probably due to their complicated syntax – they are treated quite stepmotherly in most computer

books and documentations. If at all, they are addressed quite brieﬂy and superﬁcially. They are less error prone

than normal pointers cause you will never allocate or de-allocate memory with them. All you’ve got to do is

to understand what they are and to learn their syntax. But keep in mind: Always ask yourself if you really

need a function pointer. It’s nice to realize one’s own late-binding but to use the existing structures of C++

may make your code more readable and clear. One aspect in the case of late-binding is runtime: If you call a

virtual function, your program has got to determine which one has got to be called. It does this using a V-Table

containing all the possible functions. This c osts some time each call and maybe you can save some time using

function p ointers instead of virtual functions. Maybe not ...

1.1 What is a Function Pointer ?

Function Pointers are pointers, i.e. variables, which point to the address of a function. You must keep in mind,

that a running program gets a certain space in the main-memory. Both, the exe cutable compiled program code

and the used variables, are put inside this memory. Thus a function in the program code is, like e.g. a character

ﬁeld, nothing else than an address. It is only important how you, or better your compiler/processor, interpret

the memory a pointer points to.

1.2 Introductory Example or How to Replace a Switch-Statement

When you want to call a function DoIt() at a certain point called label in your program, you just put the call

of the function DoIt() at the point label in your source code. Then you compile your code and every time your

program comes up to the point label, your function is called. Everything is ok. But what can you do, if you

don’t know at build-time which function has got to be called? What do you do, when you want to decide it

at runtime? Maybe you want to use a so called C allback-Function or you want to select one function out of a

pool of possible functions. However you can also solve the latter problem using a switch -s tateme nt, where you

call the functions just like you want it, in the diﬀerent branches. But there’s still another way: Use a function

pointer! In the following example we regard the task to perform one of the four basic arithmetic operations.

The task is ﬁrst solved using a switch-statement. Then it is shown, how the same can be done using a function

pointer.

//-----------------------------------------------------------------------------------------

// 1.2 Introductory Example or How to Replace a Switch-Statement

// Task: Perform one of the four basic arithmetic operations specified by the

// characters ’+’, ’-’, ’*’ or ’/’.

#include <iostream.h> // due to: cout

// the four arithmetic operations

// one of these functions is selected at runtime with a switch or a function pointer

float Plus (float a, float b) { return a+b; }

float Minus (float a, float b) { return a-b; }

float Multiply (float a, float b) { return a*b; }

float Divide (float a, float b) { return a/b; }

// solution with a switch-statement - <opCode> specifies which operation to execute

void Switch(float a, float b, char opCode)

{

float result;

switch(opCode) // execute operation

{

case ’+’ : result = Plus (a, b); break;

case ’-’ : result = Minus (a, b); break;

Modern compilers are very good! With my Borland Compiler the time I was able to save calling a virtual function which

multiplies two ﬂoats was about 2 percent.

It’s only an example and the task is so easy that I suppose nobody wil l ever use a function pointer for it ;-)

case ’*’ : result = Multiply(a, b); break;

case ’/’ : result = Divide (a, b); break;

}

cout << "switch: 2+5=" << result << endl; // display result

}

// solution with a function pointer - <pt2Func> is a function pointer and points to

// a function which takes two floats and returns a float. The function pointer

// "specifies" which operation shall be executed.

void Switch_With_Function_Pointer(float a, float b, float (*pt2Func)(float, float))

{

float result = pt2Func(a, b); // call using function pointer

cout << "switch replaced by function pointer: 2-5="; // display result

cout << result << endl;

}

// execute example code

void Replace_A_Switch()

{

cout << endl << "Executing function ’Replace_A_Switch’" << endl;

Switch(2, 5, /* ’+’ specifies function ’Plus’ to be executed */ ’+’);

Switch_With_Function_Pointer(2, 5, /* pointer to function ’Minus’ */ &Minus);

}

Important note: A function pointer always points to a function with a speciﬁc signature! Thus all functions,

you want to use with the same function pointer, must have the same parameters and return-type!

2 The Syntax of C and C++ Function Pointers

Regarding their syntax, there are two diﬀerent types of function pointers: On the one hand there are pointers

to ordinary C functions or static C++ member functions, on the other hand there are pointers to non-static

C++ member functions. The basic diﬀerence is that all pointers to non-static member functions need a hidden

argument: The this-pointer to an instance of the class. Always keep in mind: These two types of function

pointers are incompatible with each other.

2.1 Deﬁne a Function Pointer

Since a function pointer is nothing else than a variable, it must be deﬁned as usual. In the following example

we deﬁne two function pointers named pt2Function and pt2Member. They point to functions, which take one

ﬂoat and two char and return an int. In the C++ example it is assumed, that the function, our pointer points

to, is a member function of TMyClass.

// 2.1 define a function pointer

int (*pt2Function) (float, char, char); // C

int (TMyClass::*pt2Member)(float, char, char); // C++

2.2 Calling Convention

Normally you don’t have to think about a function’s calling convention: The compiler assumes cdecl as

default if you don’t specify another convention. However if you want to know more, keep on reading ... The

calling convention tells the compiler things like how to pass the arguments or how to generate the name of a

function. Examples for other calling conventions are stdcall, pascal, fastcall. The calling convention belongs

to a functions signature: Thus functions and function pointers with diﬀerent calling convention

are incompatible with each other! For Borland and Microsoft compilers you specify a speciﬁc calling

convention between the return type and the function’s or function pointer’s name. For the GNU GCC you use

剩余12页未读，继续阅读

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