LinkedList 实现 C++ 的编译器错误 LNK2019

Posted 2023-02-22

【中文标题】LinkedList 实现 C++ 的编译器错误 LNK2019

【英文标题】：Compiler Error LNK2019 with LinkedList implementation C++

【发布时间】：2015-04-14 04:46:55

【问题描述】：

好吧，我的 LinkedList 类出现了这个错误，但我终其一生都无法弄清楚错误来自哪里，尽管很明显我已经做了一些声明事情。 错误：

Error   1   error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class LinkedList &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@AAVLinkedList@@@Z) referenced in function _main c:\Users\Cameron\documents\visual studio 2012\Projects\cs260_2\Project6\app.obj Project6

头文件：

    #ifndef _LINKED_LIST_
#define _LINKED_LIST_

#include <ostream>
using namespace std;

struct Node

    char item;
    Node * next;
;
//Node* head;

class LinkedList

public:
    Node* head;
    LinkedList();
    LinkedList(ostream& out, LinkedList& alist);
    ~LinkedList();

    void add(char ch);  //check for ch, if it does not exist, add to list
    bool find(char ch); //find and return ch from list
    bool del(char ch);  //find and delete ch from list

    friend ostream& operator<<(ostream& out, LinkedList& alist);
private:
;

#endif // _LINKED_LIST_

LinkedList.cpp

    #include "linkedlist.h"

LinkedList::LinkedList() :
        head(nullptr)
     
LinkedList::~LinkedList()




LinkedList::LinkedList(ostream& out, LinkedList& alist)




void LinkedList::add(char data)

    /*Node* prev = NULL;
    Node* curr = head;

    Node* newNode = new Node;
    newNode->item = data;
    newNode->next = NULL;

    prev = head;
    newNode->next = curr;
    if(prev == NULL)
    
        head = newNode;
    
    else
    
        prev->next = newNode;
    */
    Node* temp = new Node;
    temp->item = data;
    temp->next = head;
    head = temp;


bool LinkedList::del(char data)

    bool returnVal = false;

    if(head == nullptr)
    
        //return default
    
    else
    
        Node* prev = head;
        Node* curr = head->next;
        while(prev->next)
        
            if(data == curr->item)
            
                prev->next = curr->next;
                delete curr;
                returnVal = true;
            
            else
            
                prev = prev->next;
            
            curr = curr->next;
         
    

     return returnVal;


bool LinkedList::find(char data)

    bool returnVal = false;
    if(head == nullptr)
    
        //return default
    
    else
    
        Node* prev = head;
        while(prev->next)
        
            if(data == prev->item)
            
                returnVal = true;
            
            prev = prev->next;
        
     
    return returnVal;

app.cpp

#include <iostream>
#include "linkedlist.h"

void find(LinkedList& list, char ch)

    if (list.find(ch))
        std::cout << "found ";
    else
        std::cout << "did not find ";
    std::cout << ch << std::endl;


int main()

    LinkedList  list;

    list.add('x');
    list.add('y');
    list.add('z');
    std::cout << list;
    find(list, 'y');

    list.del('y');
    std::cout << list;
    find(list, 'y');

    list.del('x');
    std::cout << list;
    find(list, 'y');

    list.del('z');
    std::cout << list;
    find(list, 'y');

    return 0;

【参考方案1】：

friend ostream& operator<<(ostream& out, LinkedList& alist);

在您的标头中定义，但从未实际实现。实施它，然后链接器错误应该消失。

您正在尝试cout 一个结构，并且由于您尚未实际实现该运算符，因此链接器找不到它。

类似：http://www.cplusplus.com/forum/windows/37248/

【讨论】：

是的，成功了。类似的链接非常有用，谢谢。