C|数组指针、函数指针做函数参数和返回值

数组和函数不能用做参数和返回值，但数组指针和函数指针可以。

1 数组指针做函数参数

数组指针做函数参数，数组名用做实参时，其形参为指向数组首元素的指针（指针目标类型为数组元素）。

#include <stdio.h>
#include <stdlib.h>
#define ROWS 3
#define COLS 2
void fun1(int (*)[COLS], int);
int main()
{
    int array_2D[ROWS][COLS] = { {1, 2}, {3, 4}, {5, 6} };
    int rows = ROWS;
    /* works here because array_2d is still in scope and still an array */
    printf("MAIN: %zu\n",sizeof(array_2D)/sizeof(array_2D[0]));
    fun1(array_2D, rows);
    getchar();
    return EXIT_SUCCESS;
}
void fun1(int (*a)[COLS], int rows)
{
    int i, j;
    int n, m;
    n = rows;
    /* Works, because that information is passed (as "COLS").
    It is also redundant because that value is known at compile time (in "COLS"). */
    m = (int) (sizeof(a[0])/sizeof(a[0][0]));
    /* Does not work here because the "decay" in "pointer decay" is meant
    literally--information is lost. */
    printf("FUN1: %zu\n",sizeof(a)/sizeof(a[0]));
    for (i = 0; i < n; i++) {
        for (j = 0; j < m; j++) {
            printf("array[%d][%d]=%d\n", i, j, a[i][j]);
        }
    }
}

2 数组指针做函数返回值

数组指针做函数返回值时，数组指针是指指针目标类型为数组元素的指针。

#include <stdio.h>
#include <malloc.h>

char(*weekday())[5]
{
    char(*wee)[5] = (char(*)[5])malloc(sizeof(char)*5*7);
    char* str[] = {"Mon.","Tue","Wed.","Thu.","Fri.","Sat.","Sun."};
    for(int i=0;i<7;i++)
        for(int j=0;j<5;j++)
            wee[i][j] = str[i][j];
    return wee;
}

int main()
{
    char(*week)[5] = weekday();
    for(int i=0;i<7;i++)
        printf("%s\n",week[i]);
    free(week);
    setbuf(stdin,NULL);
    getchar();
}

3 函数指针做函数参数

函数指针做函数参数，可以封装函数体中因因应不同需求而需“变化”的代码，避免“硬”编程，让代码更加通用和泛化。

#include <stdio.h>
void sort(int arr[],int size,bool(*comp)(int,int))
{
    for(int i=0;i<size-1;i++)
        for(int j=0;j<size-i-1;j++)
            if(comp(arr[j],arr[j+1]))
            {
                int t = arr[j+1];
                arr[j+1] = arr[j];
                arr[j]=t;
            }
}
bool Comp(int a,int b)
{
    return a<b;
}

int main(void)
{
    int arr[]={2,1,4,5,3,9,6,8,7};
    int n = sizeof arr / sizeof *arr;
    sort(arr,n,Comp);
    for(int i=0;i<n;i++)
        printf("%d ",arr[i]);
    getchar();
    return 0;
}

4 函数指针做函数返回值

用函数返回函数指针，可以让同类函数具有相同的接口。

#include <stdio.h>

enum Op
{
    ADD = '+',
    SUB = '-',
};

int add(int a, int b)
{
    return a + b;
}

int sub(int a, int b)
{
    return a - b;
}
/* getmath: return the appropriate math function */
int (*getmath(enum Op op))(int,int)
{
    switch (op)
    {
    case ADD:
        return &add;
    case SUB:
        return ?
    default:
        return NULL;
    }
}

int main(void)
{
    int a, b, c;
    int (*fp)(int,int);
    fp = getmath(ADD);
    a = 1, b = 2;
    c = (*fp)(a, b);
    printf("%d + %d = %d\n", a, b, c);
    getchar();
    return 0;
}

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