c - C中的继承

Question

我正在尝试在 C 中实现继承。我编写了以下两个变体。方法一运行程序崩溃，方法二运行正常。我在方法 1 中做错了什么？

方法一:

#include<stdio.h>
#include<stdlib.h>

typedef struct base_class
{
    int a;
}Base;

typedef struct derived_class
{
    int b;
    Base *base_ptr;
}Derived;

int main(void){
    Derived *der_ptr;
    der_ptr = (Derived *)malloc(sizeof(Derived));
    der_ptr->b = 5;
    der_ptr->base_ptr->a=10;

    printf("%d %d",der_ptr->b,der_ptr->base_ptr->a);
}

方法二:

#include<stdio.h>
#include<stdlib.h>

typedef struct base_class
{
    int a;
}Base;

typedef struct derived_class
{
    int b;
    Base base_ptr;
}Derived;

int main(void){
    Derived *der_ptr;
    der_ptr = (Derived *)malloc(sizeof(Derived));
    der_ptr->b = 5;
    der_ptr->base_ptr.a=10;

    printf("%d %d",der_ptr->b,der_ptr->base_ptr.a);
}

Answer 1

方法 1 因这行代码而崩溃:

der_ptr->base_ptr->a=10;

问问自己:der_ptr->base_ptr 的值是多少？