11. Void Pointers and Function Pointers
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| 11.1 Void Pointers |
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 C does not allow us to declare and use void variables.
void can be used only as return type or parameter of a function.
C allows void pointers
Question: What are some scenarios where you want to pass void pointers?
void pointers can be used to point to any data type |
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int x; void ∗p = &x; /∗ points to int ∗/
float f; void ∗p = &f; /∗ points to float ∗/ |
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| void pointers cannot be dereferenced. The pointers should always be cast before dereferencing. |
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void ∗p; printf ("%d", ∗p); /∗ invalid ∗/
void ∗p;
int ∗px = (int ∗)p; printf ("%d", ∗px); /∗ valid ∗/ |
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| 11.2 Function Pointers |
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 In C, function itself is not a variable. But it is possible to declare pointer to functions. |
| Function pointers can be assigned, pass to and from functions, placed in arrays etc. |
| Question: What are some scenarios where you want to pass pointers to functions? |
| Declaration examples: |
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int (∗fp )(int) /∗ notice the () ∗/
int (∗fp )(void∗, void∗) |
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| Example: Function Pointer |
/* 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; }
void switch(float a, float b, char opCode)
{
float result;
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;
}
printf(“%f\n”, result);
}
/* Solution with a function pointer */
void switch_pointer
(float a, float b, float(*pt2func)(float, float))
{
float result = pt2func(a, b); /* call using function pt */
printf(“result=%f\n”, result);
}
/* Execute example code */
void switch2()
{
switch(2, 5, ‘+’);
switch_pointer(2, 5, &minus);}
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| 11.3 Callback |
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| Definition: Callback is a piece of executable code passed to functions. In C, callbacks are implemented by passing function pointers. |
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#include <stdio.h>
#include <stdlib.h>
/* The calling function takes a single callback as a parameter. */
void PrintTwoNumbers(int (*numberSource)(void))
{
printf("%d and %d\n", numberSource(), numberSource());
}
/* One possible callback. */
int overNineThousand(void)
{
return (rand() % 1000) + 9000;
}
/* Another possible callback. */
int fortyTwo(void)
{
return 42;
}
/* Here we call PrintTwoNumbers() with three different callbacks. */
int main(void)
{
PrintTwoNumbers(rand);
PrintTwoNumbers(overNineThousand);
PrintTwoNumbers(fortyTwo);
} |
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