如何处理错误：使用已删除的函数 c++？

Question

这是我的代码：

#include <iostream>
#include <cstring>
#include <string>
#include <cmath>
#include "dummy.h"
using namespace std;

#ifndef SORT_H
#define SORT_H

template <class T>
class LinkedList {
    struct Node {
        T data;
        Node * next;
    };

    Node * head;

public:
    LinkedList() {
        head = NULL;
    }
    LinkedList(T value) {
        head -> data = value;
        head -> next = NULL;
    }

    ~LinkedList() {
        while(head != NULL) {
            Node * n = head->next;
            delete head;
            head = n;
        }
    }

    void add(T value) {
        Node * n = new Node;  // the error starts here probably
        n->data = value;
        n->next = head;
        head = n;
    }

    void print() {
        Node *curr = head;
        while (curr) {
            cout << curr->data << endl;
            curr = curr->next;
        }
    }
};

#endif

当我尝试编译它时，我得到了错误：使用已删除的函数'LinkedList::Node::Node()'

我知道这可能是因为我事先无法知道在创建排序列表时是否会得到任何参数，我该怎么做？ 让它编译运行？

Dummy 类是这样的：

#include <iostream>
#include <cstring>
#include <string>
#include <cmath>

using namespace std;

#ifndef DUMB_H
#define DUMB_H

class Dummy {
    int num_of_teeth;
public:
    Dummy(int num) {
        num_of_teeth = num;
    }
    ~Dummy() {};
    void add(int num) {
        num_of_teeth += num;
    }
    void remove() {
        num_of_teeth --;
    }
    void print() {
        int num = num_of_teeth;
        while (num > 0) {
            cout << "D";
            if ((num-1) == (num_of_teeth/2)) {
                cout << "\n";
            }
            num --;
            if (num == 0) {
                cout << "\n";
            }
        }
    }
    friend ostream& operator << (ostream& output, Dummy& dumb)
    {
        output << dumb.num_of_teeth;
        return output;
    }
};

#endif

我的主要是：

#include <iostream>
#include "sortedList.h"
#include "dummy.h"

int main() {
    std::cout << "Hello, World!" << std::endl;

    Dummy teeth(24);
    teeth.add(7);
    Dummy slime(11);
    slime.add(1);
    Dummy josh(32);
    LinkedList<Dummy> teeth_list;
    teeth_list.add(teeth);  // The first time I try to put teeth into the list
    teeth_list.add(slime);
    teeth_list.add(josh);
    teeth_list.print();

    LinkedList <string> myList;
    myList.add("yossi");
    myList.add("Rikki");
    myList.add("Pavel <3");
    myList.print();

    return 0;
}

构建日志：

====================[ Build | exe_name | Debug ]================================
"C:\Program Files\JetBrains\CLion 2021.1.1\bin\cmake\win\bin\cmake.exe" --build C:\Users\User\CLionProjects\ex2.2\cmake-build-debug --target exe_name -- -j 3
Scanning dependencies of target exe_name
[ 33%] Building CXX object CMakeFiles/exe_name.dir/main.cpp.obj
[ 66%] Building CXX object CMakeFiles/exe_name.dir/sortedList.cpp.obj
In file included from C:\Users\User\CLionProjects\ex2.2\main.cpp:2:
C:\Users\User\CLionProjects\ex2.2\sortedList.h: In instantiation of 'void LinkedList<T>::add(T) [with T = Dummy]':
C:\Users\User\CLionProjects\ex2.2\main.cpp:14:25:   required from here
C:\Users\User\CLionProjects\ex2.2\sortedList.h:38:20: error: use of deleted function 'LinkedList<Dummy>::Node::Node()'
         Node * n = new Node;
                    ^~~~~~~~
C:\Users\User\CLionProjects\ex2.2\sortedList.h:13:12: note: 'LinkedList<Dummy>::Node::Node()' is implicitly deleted because the default definition would be ill-formed:
     struct Node {
            ^~~~
C:\Users\User\CLionProjects\ex2.2\sortedList.h:13:12: error: no matching function for call to 'Dummy::Dummy()'
In file included from C:\Users\User\CLionProjects\ex2.2\sortedList.h:5,
                 from C:\Users\User\CLionProjects\ex2.2\main.cpp:2:
C:\Users\User\CLionProjects\ex2.2\dummy.h:14:5: note: candidate: 'Dummy::Dummy(int)'
     Dummy(int num) {
     ^~~~~
C:\Users\User\CLionProjects\ex2.2\dummy.h:14:5: note:   candidate expects 1 argument, 0 provided
C:\Users\User\CLionProjects\ex2.2\dummy.h:11:7: note: candidate: 'constexpr Dummy::Dummy(const Dummy&)'
 class Dummy {
       ^~~~~
C:\Users\User\CLionProjects\ex2.2\dummy.h:11:7: note:   candidate expects 1 argument, 0 provided
mingw32-make.exe[3]: *** [CMakeFiles\exe_name.dir\build.make:81: CMakeFiles/exe_name.dir/main.cpp.obj] Error 1
mingw32-make.exe[3]: *** Waiting for unfinished jobs....
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:94: CMakeFiles/exe_name.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:101: CMakeFiles/exe_name.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:136: exe_name] Error 2

Answer 1

好吧，这很有趣。看看这里：

template <class T>
class LinkedList {
    struct Node {
        T data; // Here you create a T

当你在节点中创建一个 T 时，你是怎么做的？那么你必须调用它的构造函数。现在让我们看看你的Dummy。它没有默认构造函数，您必须使用Dummy(int) 调用它。因此，在 Node 中，您的 T 现在是 Dummy 必须使用此构造函数构造。但是如何！？

这是您向编译器提出的难题。编译器以一种奇怪但明智的方式回答了。我无法构造这个Node，因为默认构造函数已被删除。它被删除是因为它的T (Dummy) 不能被默认构造。

解决此问题的一个潜在方法是向Node 添加一个新构造函数，该构造函数获取并复制T，又名：

struct Node {
    Node(const T &in) : data(in) {}
    T data;

你应该尝试一下。

---

一点旁注。你有问题。考虑一下如何添加后续节点，以及何时构建第一个节点。您的构造函数 LinkedList(T value) { 具有未定义的行为，因为它取消了对空指针的引用。