Tuesday, August 31, 2010

Introduction to Pointer to function in c/cpp

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 deallocate 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.

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.

Introductory Example or How to Replace a Switch-Statement

Switch case uses jump-tables, therefore we keep ourself to int as cases, viewing it from efficiency point of view. We will see how it works here.
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. It's only an example and the task is so easy that I suppose nobody will ever use a function pointer for it ;-)

// 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
case '+' : result = Plus (a, b); break;
case '-' : result = Minus (a, b); break;
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 - 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);

 mportant 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!

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