双链接列表问题-项目包括在内，但不出现

Question

我试着做一个双链接列表，作为一种练习，我遇到了一些奇怪的行为--如果我尝试在列表中间使用insert_before，就像在trooper3和trooper4中看到的那样，当我尝试从头到尾检查所有元素时，trooper4并没有出现。反向迭代显示所有元素确实都在那里。我应该指出，我只是在学习c++，很可能错过了一些东西。

我在gdb中运行了几次代码，并观察到以下内容:下一个和以前的指针只在第二个for循环期间更新。当我尝试从头到尾迭代时，trooper2的下一个指针直接指向trooper3。

#include<cstdio>

struct Element{
    Element* next{};
    Element* previous{};
    void insert_after(Element* new_element) {
        new_element->previous = this;
        new_element->next = this->next;
        this->next = new_element;   
    }
    void insert_before(Element* new_element) {
        new_element->previous = this->previous;
        new_element->next = this;
        this->previous = new_element;
    }
    char prefix[2];
    short operating_number;
};

int main() {
    Element trooper1, trooper2, trooper3, trooper4, trooper5;
    trooper1.prefix[0] = 'T';
    trooper1.prefix[1] = 'K';
    trooper1.operating_number = 421;
    trooper1.insert_before(&trooper5);
    trooper5.prefix[0] = 'X';
    trooper5.prefix[1] = 'X';
    trooper5.operating_number = 6767;
    trooper1.insert_after(&trooper2);
    trooper2.prefix[0] = 'F';
    trooper2.prefix[1] = 'N';
    trooper2.operating_number = 2187;
    trooper2.insert_after(&trooper3);
    trooper3.prefix[0] = 'L';
    trooper3.prefix[1] = 'S';
    trooper3.operating_number = 005;
    trooper3.insert_before(&trooper4);
    trooper4.prefix[0] = 'Y';
    trooper4.prefix[1] = 'Q';
    trooper4.operating_number = 768;

    printf("going forward\n");
    for (Element *cursor = &trooper5; cursor; cursor = cursor->next) {
        printf("stormtrooper %c%c-%d\n",
                cursor->prefix[0],
                cursor->prefix[1],
                cursor->operating_number);
    }
    printf("going back\n");
    for (Element *cursor = &trooper3; cursor; cursor = cursor->previous) {
        printf("stormtrooper %c%c-%d\n",
                cursor->prefix[0],
                cursor->prefix[1],
                cursor->operating_number);
    }
}

所以，实际输出是

going forward
stormtrooper XX-6767
stormtrooper TK-421
stormtrooper FN-2187
stormtrooper LS-5
going back
stormtrooper LS-5
stormtrooper YQ-768
stormtrooper FN-2187
stormtrooper TK-421
stormtrooper XX-6767

而在前进之后的那一节应该是

stormtrooper XX-6767
stormtrooper TK-421
stormtrooper FN-2187
stormtrooper YQ-768
stormtrooper LS-5

UPDATE I根据建议修改了我的代码，现在这些方法如下所示：

    void insert_after(Element* new_element) {
        new_element->previous = this;
        new_element->next = this->next;
        this->next->previous = new_element;
        this->next = new_element;   
    }
    void insert_before(Element* new_element) {
        new_element->previous = this->previous;
        new_element->next = this;
        this->previous->next = new_element;
        this->previous = new_element;
    }

完全解决了问题。

Answer 1

这是因为你忘记了一个链接，当你必须做4个链接时，你只做3个链接。

看下面你错过了什么：

void insert_after(Element* new_element) {
    new_element->previous = this;
    new_element->next = this->next;
    this->next->previous = new_element; // You missed it
    this->next = new_element;   
}
void insert_before(Element* new_element) {
    new_element->previous = this->previous;
    new_element->next = this;
    this->previous->next = new_element; // You missed it
    this->previous = new_element;
}

编辑：

我忘记提到，这解决了元素“消失”的问题。

但是..。函数不处理列表为空或只包含一个元素的特殊情况。访问未分配元素(或超出范围的元素)的是未定义的行为，您可能会在某个时候出现分段错误。