C語言講義——指針函數和函數指針

指針函數

返回值是指針的函數，如void* malloc(...)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int *newAdd(int a, int b); // 聲明指針函數

main() {
	int *p1 = NULL;
	p1 = newAdd(1, 2);
	printf("p1 = 0x%x \n", p1);
	printf("*p1 = %d \n", *p1);
	getchar();
}

int *newAdd(int a, int b) {
	int *p = (int *)malloc(sizeof(int));
	memset(p, 0, sizeof(int));
	printf("   函數內:p = 0x%x \n", p);

	*p = a + b;
	printf("   函數內:*p = %d \n", *p);

	return p;
}

函數中創建“數組”

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int *createArray(int n) {
    int *p = (int *)malloc(n * sizeof(int));
    memset(p, 0, n * sizeof(int));
    int *q = p;
    for(int  i = 0 ; i<n; i++) {
        *(q++) = i*i;
    }
    return p;
}
main() {
    int *p = NULL;
    p = createArray(10);
    int *q = p;
    while(p-q<10) {
        printf("%d ", *p);
        p++;
    }
    //TODO:釋放內存
}

函數指針

指向函數的指針

#include<stdio.h>
int addition(int x,int y) {
	return (x+y);
}
int subtraction(int x,int y) {
	return (x-y);
}
int multiplication(int x,int y) {
	return (x*y);
}
int division(int x,int y) {
	return (x/y);
}
int main() {
	int a, b, symbol, ret;
	scanf("%d",&a);
	getchar();// Enter會被當做輸入字符,丟棄
	symbol = getchar();
	scanf("%d",&b);
	switch(symbol) {
		case '+':
			ret = addition(a, b);
			break;
		case '-':
			ret =subtraction(a,b);
			break;
		default:
			ret = -999;
			break;
	}
	printf("%d%c%d=%d", a,symbol,b,ret);
	return 0;
}

每個函數有一個地址，即該函數第一條指令的地址，
可以用一個指針來保存這個地址，而這個指針就是函數指針。
可以用該指針來調用這個函數。

#include<stdio.h>
int addition(int x,int y) {
	return (x+y);
}
int subtraction(int x,int y) {
	return (x-y);
}
int multiplication(int x,int y) {
	return (x*y);
}
int division(int x,int y) {
	return (x/y);
}
int main() {
	int (*p)(int, int);
	int a, b, symbol, ret;
	scanf("%d",&a);
	getchar();// Enter會被當做輸入字符,丟棄
	symbol = getchar();
	scanf("%d",&b);
	switch(symbol) {
		case '+':
			p = addition;
			break;
		case '-':
			p =subtraction;
			break;
		default:
			ret = -999;
			break;
	}
	ret = (*p)(a,b);
	printf("%d%c%d=%d", a,symbol,b,ret);
	return 0;
}

函數指針數組

一組指向函數的指針
例：直接計算100*5

#include<stdio.h>
int addition(int x,int y) {
	return (x+y);
}
int subtraction(int x,int y) {
	return (x-y);
}
int multiplication(int x,int y) {
	return (x*y);
}
int division(int x,int y) {
	return (x/y);
}
int main() {

	int a, b, symbol, ret;
	a = 100;
	b = 5;

	//int (*p)(int, int);
	int (*p[])(int, int) = {addition,subtraction,multiplication,division};

	ret = (*p[2])(a,b);
	printf("%d * %d=%d \n", a,b,ret);

	ret = p[2](a,b);
	printf("%d * %d=%d \n", a,b,ret);
	return 0;
}

運行結果：

100 * 5=500
100 * 5=500