c++ - 运算符重载时出现 Unresolved external 错误，即使在定义了所有方法之后

Question

编辑:需要删除帖子——问题微不足道(打字错误)，对其他人没有任何帮助

尝试在我的一个 cpp 文件中使用运算符重载时，出现错误 LNK2019，即 Unresolved external 错误。我仔细查看了一下，许多人已经能够通过确保在他们的类原型(prototype)中定义每个方法来解决问题。

老实说，我认为这与我的项目设计有很大关系，但我真的无法准确指出为什么会发生此错误

代码如下:

//a.cpp
//
// ... skipped all code to bottom to where i modified it
//OVERLOADED FUNCTIONS
int operator+(const int n, const a& entry){
   return n + entry.getTime();
}
ostream& operator<<(ostream & out, const a& entry){
   out << entry.getTitle() << " by " << entry.getArtist()
      << " (" << entry.getTime() << ") ";
   return out;
}

//*********************************************************
// a.h
//... only posting what I changed
//
// Inside the class..

class a
{
public:
   friend ostream& operator<<(const ostream& out, const a& entry);
   friend int operator+(const int n, const a& entry);
//..
//.. SNIPPED
//..
}

当我尝试在 show() 方法中输出一个 b 对象时遇到错误。

//b.cpp
#include "b.h"

b b::etnsl(const int &indexOfItemToAdd) const{ 
   if (originalObjects != NULL && indexOfItemToAdd >= (*originalObjects).size()){
      throw "Index out of bounds";
   }
   b x(originalObjects);
   vector<int> *iCopy = x.getIndices();
   (*iCopy) = indices;
   iCopy->push_back(indexOfItemToAdd);
   x.setSum(sum + (*originalObjects)[indexOfItemToAdd].getTime());
   return x;
}

void b::show() const{
   cout << "    Item at loc " << "0x" << this << ":" << endl;
   //int j = indices.size();
   //if (j == 0)
   if (size == 0)
      cout << "    Empty item." << endl;
   else{
      for (int i = 0; i < size; i++) //ERROR IN LOOP
         cout << "    Index " << indices[i] << " : " << (*originalObjects)[indices[i]] << endl;
   }
}
int b::getSum() const{
   return sum;
}
void b::setSum(const int& num){
   sum = num;
}
vector<int>* b::getIndices(){
   return &indices;
}

//*********************************************************
//b header class

#ifndef B_H
#define B_H

#include <iostream>
#include <vector>
#include "a.h"
using namespace std;

class b{
private:
   int sum, size;
   vector <a> *originalObjects;
   vector <int> indices;
public:
   b(vector<a> *orig = NULL) //counts as 2 constructors: default and a custom one.
      : sum(0), originalObjects(orig), size(indices.size()) {
   }
   b etnsl(const int &indexOfItemToAdd) const; 
   void show() const;
   int getSum() const;
   void setSum(const int& num);
   vector<int> *getIndices();
};

#endif

Answer 1

ostream& operator<<(ostream & out, const iTunesEntry& tune)

不一样

ostream& operator<<(const ostream& out, const iTunesEntry& entry);
//                  ~~~~~

在 iTunesEntry 类中(友元方法)

并且不应使用 const，因为您将不得不修改 ostream 的 out 对象