Function Pointer

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Concepts

Function pointer is a kind of pointer whicn points to function. There two commom uses of function pointers, jump tables and passing a function pointer as an argument in a function call.

Uses&Examples

The pointer to function must be initialized to point to something before indirection can be performed on it, which means, to use function pointers, we should firstly have a function already declared, and then we can assign a pointer that fits this function to it.

int addValue(int a, int b) {
    return a+b;
}
int (*pf)(int a, int b) = &addValue;

As we can see, the second declaration create pf, a pointer to function, with two arguments, making its type the same as function addValue. Thus we this initialization is avaliable. To use the function pointer, we have three ways.

int a = 20;
int b = 10;
int ans;
//1th method
ans = addValue(a, b);
//2nd method
ans = (*pf)(a, b);
//3nd method
ans = pf(a, b);

The outcome was shown in pf1.c

Callback Functions

The more important way of function pointer is callback function. we can implement an operation function, and we pass a function pointer to the operation, then we can decide indeed which operation is used when we call operation.

void operation(int a, int b, int(*op)(int, int));

we pass an function pointer to the operation function, and we can use the function pointer paseed in the block.

void operation(int a, int b, int(*op)(int, int)){
    int ans = op(a,b);
    printf("%d \n", ans);
}

Function Pointer Array

This time, we declare an array of function pointers, and initialize each of them

int (*pfArray[4])(int, int);
pfArray[0] = addValue;
pfArray[1] = subValue;
pfArray[2] = plusValue;
pfArray[3] = divValue;
int addValue(int a, int b) {
    printf("a + b = ");
    return a+b;
}
int subValue(int a, int b){
    printf("a - b = ");
    return a-b;
}
int plusValue(int a, int b){
    printf("a * b = ");
    return a*b;
}
int divValue(int a, int b){
    printf("a / b = ");
    return a/b;
}

And we can call them each time by just access each item of the array.

for (int i = 0; i < 4; i++){
     operation(a, b, pfArray[i]);
}

Conclusion

From all above, we have had a basic knowledge of function pointers, and the executable code used in this report are as follows.

#include<stdio.h>
int addValue(int a, int b);
int subValue(int a, int b);
int plusValue(int a, int b);
int divValue(int a, int b);
void operation(int a, int b, int(*op)(int, int));
int main(){
    int (*pf)(int a, int b) = &addValue;
    int a = 20;
    int b = 10;
    int ans;
    //Show the use of function pointer
    //1th method
    printf("call addValue(10, 20):\n");
    ans = addValue(a, b);
    printf("%d \n", ans);
    //2nd method
    printf("call (*pf)(10, 20):\n");
    ans = (*pf)(a, b);
    printf("%d \n", ans);
    //3nd method    
    printf("call (*pf)(10, 20): \n");
    ans = pf(a, b);
    printf("%d \n", ans);
    //Show callback function
    printf("Call back function and function pointer array used:\n");
    int (*pfArray[4])(int, int);
    pfArray[0] = addValue;
    pfArray[1] = subValue;
    pfArray[2] = plusValue;
    pfArray[3] = divValue;
    for (int i = 0; i < 4; i++){
        operation(a, b, pfArray[i]);
    }
    return 0;
}
int addValue(int a, int b) {
    printf("a + b = ");
    return a+b;
}
int subValue(int a, int b){
    printf("a - b = ");
    return a-b;
}
int plusValue(int a, int b){
    printf("a * b = ");
    return a*b;
}
int divValue(int a, int b){
    printf("a / b = ");
    return a/b;
}
void operation(int a, int b, int(*op)(int, int)){
    int ans = op(a,b);
    printf("%d \n", ans);
}

Reference

• Pointers on C
• How do function pointers in C work?
• C语言-函数指针（Function Pointer）及进阶