C语言回调函数总结

/*
Main program ---calls--> Library function ---calls--> Callback funtion
*/
#include <stdio.h>
#include <stdlib.h>

typedef double (*op_fun)(double, double);/*函数指针的第一种定义方式，比较奇怪为什么不是 double () (double, double) * op_fun; 这种形式。*/

typedef struct _OP
{
    double (*add)(double, double);    /*函数指针第二种定义方式*/
    double (*sub)(double, double);
    double (*mul)(double, double);
    //double (*div)(double, double);
    op_fun div;
} OP;

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

double sub(double a, double b)
{
    return a - b;
}

double mul(double a, double b)
{
    return a * b;
}

double div(double a, double b)
{
    return a / b;
}

void OP_init(OP * op)
{
    op->add = add;
    op->sub = sub;    /*函数指针第一种赋值方式*/
    op->mul = &mul;    /*函数指针第二种赋值方式*/
    op->div = &div;
}

/*Library function*/
double lib_fun(double a, double b, double (*op)(double, double)) 
{
    return op(a, b);
    //return (*op)(a, b);
}

//double (*op_arr[5])(double, double);    /*声明函数指针数组的第一种方式*/
op_fun op_arr[5];    /*声明函数指针数组的第二种方式*/

void op_arr_init() {
    op_arr[1] = add;
    op_arr[2] = sub;
    op_arr[3] = &mul;
    op_arr[4] = &div;
}

double (* get_op(int num))(double, double)    /*函数指针作为返回值，get_op参数为(int num)，返回类型为double(*)(double, double)*/
{
    op_arr_init();
    if (1 <= num && num <= 4) return op_arr[num];
    return NULL;
}

int main(int argc, char const *argv[])
{
    OP * op = (OP *)malloc(sizeof(OP));
    OP_init(op);
    double a, b;
    scanf("%lf %lf", &a, &b);
    printf("ADD = %lf, SUB = %lf, MUL = %lf, DIV = %lf
", (op->add)(a, b), (op->sub)(a, b), /*函数指针第一种调用方式*/
        (*op->mul)(a, b), (*op->div)(a, b));    /*函数指针第二种调用方式*/
    printf("ADD = %lf, SUB = %lf, MUL = %lf, DIV = %lf
", lib_fun(a, b, add), lib_fun(a, b, sub), lib_fun(a, b, mul), lib_fun(a, b, div));
    free(op);

    int num;
    printf("Input operation(1:add, 2:sub, 3:mul, 4:div):
");
    scanf("%d", &num);
    if (num) printf("Answer is %lf
", lib_fun(a, b, get_op(num)));
    else printf("Wrong!
");
    return 0;
}