The Syntax of C and C++ Function Pointers

1  Define a Function Pointer

Regarding their syntax, there are two different types of function pointers:
On the one hand there are pointers to ordinary C functions or to 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.
Since a function pointer is nothing else than a variable, it must be defined as usual. In the following example we define three function pointers named pt2Function, pt2Member and pt2ConstMember. They point to functions, which take one float and two char and return an int. In the C++ example it is assumed, that the functions, our pointers point to, are (non-static) member functions of TMyClass.

// 2.1 define a function pointer and initialize to NULL
int (*pt2Function)(float, char, char) = NULL;                        // C
int (TMyClass::*pt2Member)(float, char, char) = NULL;                // C++
int (TMyClass::*pt2ConstMember)(float, char, char) const = NULL;     // 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. Some examples for other calling conventions are __stdcall, __pascal and __fastcall. The calling convention belongs to a function's 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 the __attribute__ keyword: Write the function definition followed by the keyword __attribute__ and then state the calling convention in double parentheses. And if you want to know how function calls work under the hood you should take a look at the chapter Subprograms in Paul Carter's PC Assembly Tutorial.

// 2.2 define the calling convention
void __cdecl DoIt(float a, char b, char c);                             // Borland and Microsoft
void         DoIt(float a, char b, char c)  __attribute__((cdecl));     // GNU GCC

 

2.3  Assign an address to a Function Pointer

It's quite easy to assign the address of a function to a function pointer. You simply take the name of a suitable and known function or member function. Although it's optional for most compilers you should use the address operator & infront of the function's name in order to write portable code. You may have got to use the complete name of the member function including class-name and scope-operator (::). Also you have got to ensure, that you are allowed to access the function right in scope where your assignment stands.

// 2.3 assign an address to the function pointer
//     Note: Although you may ommit the address operator on most compilers
//     you should always use the correct way in order to write portable code.

// C
int DoIt  (float a, char b, char c){ printf("DoIt\n");   return a+b+c; }
int DoMore(float a, char b, char c)const{ printf("DoMore\n"); return a-b+c; }

pt2Function = DoIt;      // short form
pt2Function = &DoMore;   // correct assignment using address operator


// C++
class TMyClass
{
public:
   int DoIt(float a, char b, char c){ cout << "TMyClass::DoIt"<< endl; return a+b+c;};
   int DoMore(float a, char b, char c) const
         { cout << "TMyClass::DoMore" << endl; return a-b+c; };

   /* more of TMyClass */
};

pt2ConstMember = &TMyClass::DoMore; // correct assignment using address operator
pt2Member = &TMyClass::DoIt; // note:  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" << endl;

 

2.5  Calling a Function using a Function Pointer

In C you call a function using a function pointer by explicitly dereferencing it using the * operator. Alternatively you may also just use the function pointer's instead of the funtion's name. In C++ the two operators .* resp. ->* 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 << result << endl;
}

// execute example code - 'DoIt' is a suitable function like defined above in 2.1-4
void Pass_A_Function_Pointer()
{
   cout << endl << "Executing 'Pass_A_Function_Pointer'" << endl;
   PassPtr(&DoIt);
}

 

2.7  How to Return a Function Pointer ?

It's a little bit tricky but a function pointer can be a function's return value. In the following example there are two solutions of how to return a pointer to a function which is taking two float arguments and returns a float. If you want to return a pointer to a member function you have just got to change the definitions/declarations of all function pointers.

//------------------------------------------------------------------------------------
// 2.7 How to Return a Function Pointer
//     'Plus' and 'Minus' are defined above. They return a float and take two float


// Direct solution: Function takes a char and returns a pointer to a
// function which is taking two floats and returns a float. 
// 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
}


// 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 << endl << "Executing 'Return_A_Function_Pointer'" << endl;

   // define a function pointer and initialize it to NULL
   float (*pt2Function)(float, float) = NULL;

   pt2Function=GetPtr1('+');   // get function pointer from function 'GetPtr1'
   cout << (*pt2Function)(2, 4) << endl;   // call function using the pointer


   pt2Function=GetPtr2('-');   // get function pointer from function 'GetPtr2'
   cout << (*pt2Function)(2, 4) << endl;   // call function using the pointer
}

 

2.8  How to Use Arrays of Function Pointers ?

Operating with arrays of function pointers is very interesting. This offers the possibility to select a function using an index. The syntax appears difficult, which frequently leads to confusion. Below you find two ways of how to define and use an array of function pointers in C and C++. The first way uses a typedef, the second way directly defines the array. It's up to you which way you prefer.

//------------------------------------------------------------------------------------
// 2.8 How to Use Arrays of Function Pointers

// C ---------------------------------------------------------------------------------

// type-definition: 'pt2Function' now can be used as type
typedef int (*pt2Function)(float, char, char);

// illustrate how to work with an array of function pointers
void Array_Of_Function_Pointers()
{
   printf("\nExecuting 'Array_Of_Function_Pointers'\n");

   // define arrays and ini each element to NULL,  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};


   // 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 << endl << "Executing 'Array_Of_Member_Function_Pointers'" << endl;

   // define arrays and ini each element to NULL,  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] = funcArr2nd use an array of function pointers in C and C++. The first way uses a typedef, the second way directly defines the array. It's up to you which way you prefer.
[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') << endl;
   cout << (instance.*funcArr1[0])(12, 'a', 'b') << endl;
   cout << (instance.*funcArr2[1])(34, 'a', 'b') << endl;
   cout << (instance.*funcArr2[0])(89, 'a', 'b') << endl;
}

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
   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;
   }

   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!