结构体的而指针属性不能直接做左值

#include<bits/stdc++.h>
using namespace std;
struct ListNode {
    int val;
    ListNode *next =NULL;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};


int main() {
    int a[] = {1,2,3};
    ListNode* l = new ListNode(a[0]);
    cout<<"l->val:"<<l->val<<endl;
    ListNode  *pnew;

    for (int i = 1; i <3; i++) {
        pnew = new ListNode(a[i], NULL);
        l->next = pnew;／／
        l = pnew; //将tmp指
        cout<<"l->val:"<<l->val<<endl;
    }

}

#include<bits/stdc++.h>
using namespace std;
struct ListNode {
    int val;
    ListNode *next =NULL;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};


int main() {
    int a[] = {1,2,3};
    ListNode* l = new ListNode(a[0],NULL);
    ListNode  *ptemp = l,*pnew;
    cout<<"l->val:"<<ptemp->val<<endl;


    for (int i = 1; i <3; i++) {
        pnew= new ListNode(a[i], NULL);
        ptemp->next = pnew;//pnew是必须的l->next不能直接做左值,可能是因为l->next不存在未分配空间
        ptemp =ptemp->next; //将tmp指
        cout<<"l->val:"<<ptemp->val<<endl;
    }
    /* 这种写法随着新元素插入,l不断指向下一个,即只能遍历这一便,
     * 结束后链表就丢失了,此时l指向最后一个元素的下一个元素即null
     * for (int i = 1; i <3; i++) {
        l->next = new ListNode(a[i], NULL);
         l =l->next;
        cout<<"l->val:"<<l->val<<endl;
    }*/
    ListNode  *pn;
    pn = l;
    for (int i = 0; i <3; i++) {
        cout<<"pn->val:"<<pn->val<<endl;
        pn= pn->next;
    }

}