“對'std::operator<<(std::ostream&, std::LinkedList const&)的未定義引用”C++

Question

大家好，這是一個 uni 項目，目前在編譯期間遇到了這個問題 /usr/lib/gcc/x86_64-pc-cygwin/9.3.0/../../../../x86_64 -pc-cygwin/bin/ld: LinkedListDemo.o:LinkedListDemo.cpp:(.text+0x178): 對std::operator<<(std::ostream&, std::LinkedList const&)' LinkedListDemo.o:LinkedListDemo.cpp:(.text+0x178): relocation truncated to fit: R_X86_64_PC32 against undefined symbol未定義引用std::operator<<(std::ostream&, std::LinkedList const&)' LinkedListDemo.o:LinkedListDemo.cpp:(.text+0x178): relocation truncated to fit: R_X86_64_PC32 against undefined symbol std::operator<<(std::ostream&, std::LinkedList const&)' collect2: 錯誤：ld 返回 1 個退出狀態使： *** [makefile:11: a1] 錯誤 1

#include <iostream>
#include <cstdlib>
#include <string>
#include "LinkedList.h"
using namespace std;

void initialize(LinkedList &l1, LinkedList &l2)
{
    l1.add("the black cat was sitting on the black mat that was on the black floor");
    l2.add("the dog scared the cat and the cat ran away");
}

int main()
{
    LinkedList firstList;
    LinkedList secondList;
    
    initialize(firstList, secondList);

    cout << "Start lists:" << endl;
    cout << "List 1: " << firstList <<  endl;
    //cout << "List 2: " << secondList <<  endl << endl;

    cout << "Concatenating the two lists onto list '1':" << endl;
    firstList += secondList;
   // cout << "List 1: " << firstList  << endl;
    //cout << "List 2: " << secondList << endl << endl;

    cout << "Removing the word 'was' from list '1':" << endl;
    firstList.remove("was");
   // cout << "List 1: " << firstList  << endl;
    //cout << "List 2: " << secondList << endl << endl;

    cout << "Removing the word 'away' from list '2':" << endl;
    secondList.remove("away");
   // cout << "List 1: " << firstList  << endl;
    //cout << "List 2: " << secondList << endl << endl;

    cout << "Removing the word 'cat' from both lists:" << endl;
    firstList.remove("cat");
    secondList.remove("cat");
    //cout << "List 1: " << firstList  << endl;
    //cout << "List 2: " << secondList << endl << endl;

    cout << "Number of occurrences of 'black' in list 1: ";
    cout << firstList.count("black") << endl << endl;
    
//  Uncomment this section if you are implementing the extended version of the method remove()  
//  cout << "Removing 'on the black' from both lists:" << endl;
//  firstList.remove("on the black");
//  secondList.remove("on the black");
//  cout << "List 1: " << firstList  << endl;
//  cout << "List 2: " << secondList << endl << endl;

    cout << "Sorting list 1:" << endl;
    firstList.sort();
    //cout << firstList << endl << endl;

    cout << "The program has finished." << endl;
    return 0;
}

這是主文件，旨在工作而不是更改

#ifndef will_PC
#define will_PC 
#include <cstdlib>
#include <string>
#include "node.h"

namespace std{
    class LinkedList{
    public:

        node* get_Head() const;
        void add(string input);
        void remove(string Input);

        void sort();
        int count(string Input);
        string getText();

        void operator += (const LinkedList& list);
    private:
        node* head;
        node* tail;
        node* n;

    };
    //this is what outputs the object
    std::ostream& operator << ( std::ostream &out,  LinkedList const& lst);

}

#endif

#include <iostream>
#include <cstdlib>
#include <string>
#include "LinkedList.h"
using namespace std;

void LinkedList::add(string Input){

    tail = NULL;
    int count =0;
    string word = "";
    int len=Input.length();
    for (int i=0; i< len;i++)
    {
        if (Input[i]==' ')
        {

            n = new node;
            n->set_Data(word);
            n->set_Previous(tail);
            n->set_Next(NULL);
            if(tail !=NULL){
                tail->set_Next(n);
            }
            tail = n;
            if (count ==0)
            {
                head = n;
            }
            word = "";
            count +=1;
        } else
        {
            word +=Input[i];
        }
    }
    n = new node;
    n->set_Data(word);
    n->set_Previous(tail);
    n->set_Next(NULL);
    tail = n;
    

}
 



void LinkedList::remove(string Input){
    node* temp;
    node* hold;
    temp =head;
    while (temp!=NULL){
        if(Input.compare(temp->get_Data())==0){
            hold = temp->get_Next();
            temp=temp->get_Previous();
            temp->set_Next(hold);
            hold->set_Previous(temp);
            temp=temp->get_Next();
            temp=temp->get_Next();
        } else{
            temp = temp->get_Next();
        }
    }
}
int LinkedList::count(string Input){
    node* temp;
    temp =head;
    int count= 0 ;
    while (temp != NULL){
        if(Input.compare(temp->get_Data())==0){
            count +=1;
        }
        temp = temp->get_Next();
    }
    return count;
}

void LinkedList::operator += (const LinkedList& list){
    node* temp;
    cout << list.get_Head()->get_Next()->get_Data()<<endl;
    temp = list.get_Head();
    while(temp!=NULL){
        n=new node;
        n->set_Data(temp->get_Data());
        n->set_Previous(tail);
        n->set_Next(NULL);
        tail->set_Next(n);
        tail= n;
        temp=temp->get_Next();
    }
    
}


void LinkedList::sort(){
    node* temp;
    temp =head;
    while(temp!=NULL){
        cout<<temp->get_Data()<<" ";
        temp = temp->get_Next();
    }
    cout<<endl;
    
}

node* LinkedList::get_Head() const {
    return head;
}


std::ostream& operator << ( std::ostream &out, LinkedList const& lst)
{
    node* temp = lst.get_Head();
    while(temp != NULL){
        out<< " "<< temp->get_Data() <<" ";
        temp = temp->get_Next();
    }
    return out;
}

#include <cstdlib>
#include <string>
#include "node.h"
using namespace std;

void node::set_Next(node* nextLink){
    next = nextLink;
}
void node::set_Previous(node* previousLink){
    previous = previousLink;
}

node* node::get_Next(){
    return next;
}
node* node::get_Previous(){
    return previous;
}
void node::set_Data(string input){
    text = input;
}
string node::get_Data(){
    return text;
}

#ifndef Will_Node
#define Will_Node

#include <cstdlib>
#include <string>
namespace std{

    class node{
    public:
        void set_Next(node* nextLink);
        void set_Previous(node* previousLink);

        void set_Data(string input);
        node* get_Next();
        node* get_Previous();

        string get_Data();
        

    private:
        string text;
        node* next;
        node* previous;
    };
}
#endif

非常感謝任何幫助我花了太多時間詛咒試圖解決這個問題

Answer 1

你在命名空間std聲明你的operator << （這是非法的，如注釋中所述），但你在全局命名空間中定義它。

using namespace與類方法的定義一起工作，因為編譯器知道在 namespace std中有一個類LinkedList ，所以它可以連接它：

如果add()是LinkedList的成員，並且LinkedList是namespace std的成員，則完全限定名稱必須是::std::LinkedList::add()

但是操作符是一個自由函數，所以編譯器沒有將它與之前的聲明聯系起來，它被放置在全局命名空間中。

解決方案：

0. 將您的命名空間更改為與std不同的名稱
1. 不要在 cpp 文件中添加using namespace ，而是將整個內容包裝在namespace{} ：

---

#include <iostream>
#include <cstdlib>
#include <string>
#include "LinkedList.h"

namespace X {

void LinkedList::add(string Input){
// all of the member definitions...

std::ostream& operator << ( std::ostream &out, LinkedList const& lst)
{
    node* temp = lst.get_Head();
    while(temp != NULL){
        out<< " "<< temp->get_Data() <<" ";
        temp = temp->get_Next();
    }
    return out;
}

} // namespace X
//file ends here

您也可以僅將您的運算符定義包裝在namespace {} ，但如果默認情況下將整個內容放在命名空間中，則更容易避免此類問題。