The Function Pointer Tutorials
Introduction to C and C++ Function Pointers, Callbacks and Functors
written by Lars Haendel
January 2005, Bochum, Germany
http://www.newty.de
email: Have a look at the web page please
version 2.07

Copyright (c) 2000-2005 by Lars Haendel. Permission is granted to copy,
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Be aware that there may be a newer version of this document!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/fpt/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prevent spreading of outdated versions.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/fpt/zip/source.zip . The example code is free software
under the terms of the GNU General Public License.

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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. Unfortunately
– 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 costs some time each call and maybe you can save some time using
function pointers instead of virtual functions. Maybe not ... 1
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 executable 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 Callback-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-statement, 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.2
//------------------------------------------------------------------------------------
// 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 ’/’.
// The four arithmetic operations ... one of these functions is selected
// at runtime with a swicth 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 - specifies which operation to execute
void Switch(float a, float b, char opCode)
{
float result;
// execute operation
switch(opCode){
case ’+’ : result = Plus (a, b); break;
case ’-’ : result = Minus (a, b); break;
case ’*’ : result = Multiply (a, b); break;
case ’/’ : result = Divide (a, b); break; }
1Modern 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.
2It’s only an example and the task is so easy that I suppose nobody will ever use a function pointer for it ;-)
2
cout << "Switch: 2+5=" <<> 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 << const =" NULL;" pt2function =" DoIt;" pt2function =" &DoMore;" pt2constmember =" &TMyClass::DoMore;" pt2member =" &TMyClass::DoIt;"> may also legally point to &DoMore
2.4 Comparing Function Pointers
You can use the comparison-operators (==, !=) the same way as usual. In the following example it is checked,
whether pt2Function and pt2Member actually contain the address of the functions DoIt and TMyClass::DoMore.
A text is shown in case of equality.
// 2.4 comparing function pointers
// C
if(pt2Function >0){ // check if initialized
if(pt2Function == &DoIt)
printf("Pointer points to DoIt\n"); }
else
printf("Pointer not initialized!!\n");
// C++
if(pt2ConstMember == &TMyClass::DoMore)
cout << "Pointer points to TMyClass::DoMore" <<>* are used together with an instance of a class in order to call one of their (non-static) member functions. If
the call takes place within another member function you may use the this-pointer.
// 2.5 calling a function using a function pointer
int result1 = pt2Function (12, ’a’, ’b’); // C short way
int result2 = (*pt2Function) (12, ’a’, ’b’); // C
TMyClass instance1;
int result3 = (instance1.*pt2Member)(12, ’a’, ’b’); // C++
int result4 = (*this.*pt2Member)(12, ’a’, ’b’); // C++ if this-pointer can be used
TMyClass* instance2 = new TMyClass;
int result4 = (instance2->*pt2Member)(12, ’a’, ’b’); // C++, instance2 is a pointer
delete instance2;
2.6 How to Pass a Function Pointer as an Argument ?
You can pass a function pointer as a function’s calling argument. You need this for example if you want to pass
a pointer to a callback function. The following code shows how to pass a pointer to a function which returns
an int and takes a float and two char:
//------------------------------------------------------------------------------------
// 2.6 How to Pass a Function Pointer
// is a pointer to a function which returns an int and takes a float and two char
void PassPtr(int (*pt2Func)(float, char, char))
{
int result = (*pt2Func)(12, ’a’, ’b’); // call using function pointer
cout <<>
// specifies which function to return
float (*GetPtr1(const char opCode))(float, float){
if(opCode == ’+’)
return &Plus;
else
return &Minus;} // default if invalid operator was passed
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// Solution using a typedef: Define a pointer to a function which is taking
// two floats and returns a float
typedef float(*pt2Func)(float, float);
// Function takes a char and returns a function pointer which is defined
// with the typedef above. specifies which function to return
pt2Func GetPtr2(const char opCode)
{
if(opCode == ’+’)
return &Plus;
else
return &Minus; // default if invalid operator was passed
}
// Execute example code
void Return_A_Function_Pointer()
{
cout << pt2function="GetPtr1(’+’);" pt2function="GetPtr2(’-’);"> and are arrays
// with 10 pointers to functions which return an int and take a float and two char
// first way using the typedef
pt2Function funcArr1[10] = {NULL};
// 2nd way directly defining the array
int (*funcArr2[10])(float, char, char) = {NULL};
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// assign the function’s address - ’DoIt’ and ’DoMore’ are suitable functions
// like defined above in 2.1-4
funcArr1[0] = funcArr2[1] = &DoIt;
funcArr1[1] = funcArr2[0] = &DoMore;
/* more assignments */
// calling a function using an index to address the function pointer
printf("%d\n", funcArr1[1](12, ’a’, ’b’)); // short form
printf("%d\n", (*funcArr1[0])(12, ’a’, ’b’)); // "correct" way of calling
printf("%d\n", (*funcArr2[1])(56, ’a’, ’b’));
printf("%d\n", (*funcArr2[0])(34, ’a’, ’b’));
}
// C++ -------------------------------------------------------------------------------
// type-definition: ’pt2Member’ now can be used as type
typedef int (TMyClass::*pt2Member)(float, char, char);
// illustrate how to work with an array of member function pointers
void Array_Of_Member_Function_Pointers()
{
cout <<> and are
// arrays with 10 pointers to member functions which return an int and take
// a float and two char
// first way using the typedef
pt2Member funcArr1[10] = {NULL};
// 2nd way of directly defining the array
int (TMyClass::*funcArr2[10])(float, char, char) = {NULL};
// assign the function’s address - ’DoIt’ and ’DoMore’ are suitable member
// functions of class TMyClass like defined above in 2.1-4
funcArr1[0] = funcArr2[1] = &TMyClass::DoIt;
funcArr1[1] = funcArr2[0] = &TMyClass::DoMore;
/* more assignments */
// calling a function using an index to address the member function pointer
// note: an instance of TMyClass is needed to call the member functions
TMyClass instance;
cout << (instance.*funcArr1[1])(12, ’a’, ’b’) << bigger="cmpFunc(item1,"> // due to: qsort
#include // randomize
#include // printf
// comparison-function for the sort-algorithm
// two items are taken by void-pointer, converted and compared
int CmpFunc(const void* _a, const void* _b)
{
// you’ve got to explicitly cast to the correct type
const float* a = (const float*) _a;
const float* b = (const float*) _b;
if(*a > *b) return 1; // first item is bigger than the second one -> return 1
else
if(*a == *b) return 0; // equality -> return 0
else return -1; // second item is bigger than the first one -> return -1
}
// example for the use of qsort()
void QSortExample()
{
float field[100];
::randomize(); // initialize random-number-generator
for(int c=0;c<100;c++) c="0;c<10;c++)"> // due to: cout
class TClassA
{
public:
void Display(const char* text) { cout << myself =" (TClassA*)">Display(string);
}
// function does something which implies a callback
// note: of course this function can also be a member function
void DoItA(void* pt2Object, void (*pt2Function)(void* pt2Object, char* text))
{
/* do something */
pt2Function(pt2Object, "hi, i’m calling back using a argument ;-)"); // make callback
}
// execute example code
void Callback_Using_Argument()
{
// 1. instantiate object of TClassA
TClassA objA;
// 2. call ’DoItA’ for
DoItA((void*) &objA, TClassA::Wrapper_To_Call_Display);
}
Example B: Pointer to a class instance is stored in a global variable The function DoItB does
something with objects of the class TClassB which implies a callback. A pointer to the static wrapper function
TClassB::Wrapper To Call Display is passed to DoItB. This wrapper is the callback-function. The wrapper
uses the global variable void* pt2Object and explicitly casts it to an instance of TClassB. It is very important,
that you always initialize the global variable to point to the correct class instance. You can write arbitrary other
classes like TClassB and use them with DoItB as long as these other classes provide the necessary functions.
Note: This solution may be useful if you have an existing callback interface which cannot be changed. It is
not a good solution because the use of a global variable is very dangerous and could cause serious errors.
//-----------------------------------------------------------------------------------------
// 3.5 Example B: Callback to member function using a global variable
// Task: The function ’DoItB’ makes something which implies a callback to
// the member function ’Display’. Therefore the wrapper function
// ’Wrapper_To_Call_Display is used.
#include // due to: cout
void* pt2Object; // global variable which points to an arbitrary object
class TClassB
{
public:
void Display(const char* text) { cout <<> to a pointer to TClassB
// warning: MUST point to an appropriate object!
TClassB* mySelf = (TClassB*) pt2Object;
// call member
mySelf->Display(string);
}
// function does something which implies a callback
// note: of course this function can also be a member function
void DoItB(void (*pt2Function)(char* text))
{
/* do something */
pt2Function("hi, i’m calling back using a global ;-)"); // make callback
}
// execute example code
void Callback_Using_Global()
{
// 1. instantiate object of TClassB
TClassB objB;
// 2. assign global variable which is used in the static wrapper function
// important: never forget to do this!!
pt2Object = (void*) &objB;
// 3. call ’DoItB’ for
DoItB(TClassB::Wrapper_To_Call_Display);
}
4 Functors to encapsulate C and C++ Function Pointers
4.1 What are Functors ?
Functors are functions with a state. In C++ you can realize them as a class with one or more private members
to store the state and with an overloaded operator6 () to execute the function. Functors can encapsulate C and
C++ function pointers employing the concepts templates and polymorphism. You can build up a list of pointers
to member functions of arbitrary classes and call them all through the same interface without bothering about
their class or the need of a pointer to an instance. All the functions just have got to have the same returntype
and calling parameters. Sometimes Functors are also known as Closures. You can also use Functors to
implement callbacks.
4.2 How to Implement Functors ?
First you need a base class TFunctor which provides a virtual function named Call or a virtually overloaded
operator () with which you will be able to call the member function. It’s up to you if you prefer the overloaded
operator or a function like Call. From the base class you derive a template class TSpecificFunctor which
is initialized with a pointer to an object and a pointer to a member function in its constructor. The derived
class overrides the function Call and/or the operator () of the base class: In the overrided versions
it calls the member function using the stored pointers to the object and to the member function.
6If you prefer you can also use a function called Execute or something like that.
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//-----------------------------------------------------------------------------------------
// 4.2 How to Implement Functors
// abstract base class
class TFunctor
{
public:
// two possible functions to call member function. virtual cause derived
// classes will use a pointer to an object and a pointer to a member function
// to make the function call
virtual void operator()(const char* string)=0; // call using operator
virtual void Call(const char* string)=0; // call using function
};
// derived template class
template class TSpecificFunctor : public TFunctor
{
private:
void (TClass::*fpt)(const char*); // pointer to member function
TClass* pt2Object; // pointer to object
public:
// constructor - takes pointer to an object and pointer to a member and stores
// them in two private variables
TSpecificFunctor(TClass* _pt2Object, void(TClass::*_fpt)(const char*))
{ pt2Object = _pt2Object; fpt=_fpt; };
// override operator "()"
virtual void operator()(const char* string)
{ (*pt2Object.*fpt)(string);}; // execute member function
// override function "Call"
virtual void Call(const char* string)
{ (*pt2Object.*fpt)(string);}; // execute member function
};
4.3 Example of How to Use Functors
In the following example we have two dummy classes which provide a function called Display which returns
nothing (void) and needs a string (const char*) to be passed. We create an array with two pointers to TFunctor
and initialize the array entries with two pointers to TSpecificFunctor which encapsulate the pointer to an
object and the pointer to a member of TClassA respectively TClassB. Then we use the functor-array to call
the respective member functions. No pointer to an object is needed to make the function calls and
you do not have to bother about the classes anymore!
//-----------------------------------------------------------------------------------------
// 4.3 Example of How to Use Functors
// dummy class A
class TClassA{
public:
TClassA(){};
void Display(const char* text) { cout <<> specFuncA(&objA, TClassA::Display);
// b) functor which encapsulates pointer to object and to member of TClassB
TSpecificFunctor specFuncB(&objB, &TClassB::Display);
// 3. make array with pointers to TFunctor, the base class, and initialize it
TFunctor* vTable[] = { &specFuncA, &specFuncB };
// 4. use array to call member functions without the need of an object
vTable[0]->Call("TClassA::Display called!"); // via function "Call"
(*vTable[1]) ("TClassB::Display called!"); // via operator "()"
cout << endl << "Hit Enter to terminate!" << endl;
cin.get();
return 0;
}
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