C++ 链表值追溯改变

Question

编辑:解决方案代码包含在最后。

我正在尝试实现一个链表类，该链表类使用作业中定义的节点类。下面的代码块按预期打印输出:

#include <iostream>
using namespace std;

// Node class as provided
class node {
    void *info;
    node *next;
public:
    node (void *v) {info = v; next = 0; }
    void put_next (node *n) {next = n;}
    node *get_next ( ) {return next;}
    void *get_info ( ) {return info;}
};

// Linked list class
class list {
    //Start of the linked list
    node *start;
public:
    list (int v) {
        start = new node (&v);
    }

    void insert (int value, int place=-1) {
        node *temp = new node (&value);

        if (place == 0) {
            temp->put_next(start);
            start = temp;
        } else {
            node *before = start;
            for (int i = 1; before->get_next() != 0; i++) {
                if (i == place) { 
                    break;
                }
                before = before->get_next();
            }

            temp->put_next(before->get_next());
            before->put_next(temp);
        }
    }

    void remove(int place) {
        if (place == 0) {
            start = start->get_next();
        } else {
            node *curr = start;
            for (int i = 1; curr != 0; i ++) {
                if (i == place) {
                    curr->put_next(curr->get_next()->get_next());
                    break;
                }
                curr = curr->get_next();
            }
        }
    }

    void display() {
        for (node *current = start; current != 0; current = current->get_next()) {
            cout << *(static_cast<int*>(current->get_info())) << endl;
        }
    }
};

int main() {

    list *tst = new list(10);
    tst->display();
    cout << "Prepending 9" << endl;
    tst->insert(9,0);
    tst->display();
    cout << "Inserting 8" << endl;
    tst->insert(8,1);
    tst->display();
    cout << "Prepending 7" << endl;
    tst->insert(7,0);
    tst->display();

    tst->remove(0);

    cout << "Removed the first element:" << endl;
    tst->display();
    cout << endl;

//  cout << "Prepending 6" << endl;
//  tst->insert(6,0);
//  tst->display();

}

创建此输出:

10
Prepending 9
9
10
Inserting 8
9
8
10
Prepending 7
7
9
8
10
Removed the first element:
9
8
10

但是，当我将这最后一条语句添加到 main 中程序流的末尾时:

tst->insert(6,0);

我的输出更改为:

10
Prepending 9
9
10
Inserting 8
8
8
10
Prepending 7
7
7
7
10
Removed the first element:
134515798
134515798
10

我错过了什么？稍后在执行中添加一个值如何改变在我到达程序流中的那个点之前发生的输出？

我使用 ideone.com 作为我的 IDE/来运行程序，我以前从未遇到过问题，但这是问题所在吗？

解决方案

#include <iostream>
using namespace std;

// Provided node class
class node {
    void *info;
    node *next;
public:
    node (void *v) {info = v; next = 0; }
    void put_next (node *n) {next = n;}
    node *get_next ( ) {return next;}
    void *get_info ( ) {return info;}
};

// List class template
template <class T>
class list {
    node *start;
public:
    list (T v) {
        start = new node (&v);
    }

    // Insert method
    void insert (T *value, int place=-1) {
        node *temp = new node (value);

        // If we're putting it at the beginning, then change the reference to start
        if (place == 0) {
            temp->put_next(start);
            start = temp;
        }

        // We're inserting it somewhere other than the beginning, handle appropriately
        else {
            node *before = start;

            // Loop to find preceeding node
            for (int i = 1; before->get_next() != 0; i++) {
                if (i == place) { 
                    break;
                }
                before = before->get_next();
            }

            // Insert after preceeding node, and point at subsequent node
            temp->put_next(before->get_next());
            before->put_next(temp);
        }
    }

    // Remove function
    void remove(int place) {

        // If we're removing hte beginning, then change start pointer
        if (place == 0) {
            start = start->get_next();
        }

        // Find node to remove
        else {
            node *curr = start;
            for (int i = 1; curr != 0; i ++) {
                if (i == place) {
                    // Cut target node out of list
                    curr->put_next(curr->get_next()->get_next());
                    break;
                }
                curr = curr->get_next();
            }
        }
    }

    // Print nodes
    void display() {
        for (node *current = start; current != 0; current = current->get_next()) {
            cout << *(static_cast<T*>(current->get_info())) << endl;
        }
        cout << endl;
    }
};

int main() {
    int nine = 9;
    int eight = 8;
    int seven = 7;
    int six = 6;
    int five = 5;

    cout << "Create list holding '10'" << endl;
    list<int> *tst = new list<int>(10);

    cout << "Prepending 9" << endl;
    tst->insert(&nine,0);

    cout << "Inserting 8 at 2nd place" << endl;
    tst->insert(&eight,1);

    cout << "Appending 7" << endl;
    tst->insert(&seven);

    cout << "Prepending 6" << endl;
    tst->insert(&six,0);

    cout << "Inserting 5 at 3rd place" << endl;
    tst->insert(&five,2);

    cout << "Show completed list:" << endl;
    tst->display();

    cout << "Removing the first element:" << endl;
    tst->remove(0);
    tst->display();

    cout << "Removing the last element:" << endl;   
    tst->remove(4); 
    tst->display();

    cout << "Removing the second element:" << endl;
    tst->remove(1);
    tst->display();
}

Answer 1

您的代码中有未定义的行为，因为您保存了指向局部变量的指针，这些变量一旦函数返回就超出范围。

我说的变量是insert函数里面的参数value，一旦insert函数返回，那个指针就不再是有效。

快速解决方案？不要存储指针，存储整数列表。或者可以使列表(和节点)成为模板化类并按值存储。

如果您真的想要一个可以包含任何内容的列表，请考虑使用例如Boost any .