链表的返回值错误,无法得到正确的值

Question

为什么这行代码返回的是0，不应该是1吗

```c++
typedef struct sqlist{
    string str;
    sqlist* next;
}sqlist;

string car_str_sqlist(sqlist s){
    return s.str;
}
sqlist cdr_sqlist(sqlist s){
    return *s.next;
}
sqlist cons_sqlist(string str,sqlist s){
    sqlist h;
    h.str=str;
    h.next=&s;
    return h;
}
sqlist empty_sqlist(){
    sqlist s;
    s.next=NULL;
    return s;
}
bool empty_sqlist_W(sqlist s){
    return s.next=NULL;
}
int main{
sqlist s=cons_sqlist("ss",cons_sqlist("sl",empty_sqlist()));
cout<<empty_sqlist_W(cdr_sqlist(cdr_sqlist(s)));
}

Answer 1

• 这是 赋值语句，你应该改成 ==

return s.next=NULL;
// 改成
return s.next==NULL;

• C++中 NULL 可以理解为 0