C++ 类中的运算符

Question

我有以下代码，它一直给我错误，我不确定该怎么做，因为我声明了运算符。

#ifndef BOOK_H
#define BOOK_H
#include<vector>
#include<string>
using namespace std;
class Book {
public:
    Book();
    Book(vector<string>*, string, int);
    Book(const Book&);
    ~Book();
    Book& operator=(const Book&);
    void update(vector<string>*);
    void update(string); 
    void update(int); 
    int getYear() const{
        return year;
    };
    string getTitle() const{
        return title;
    };
    bool operator==(const Book&);
    bool operator!=(const Book&);
    friend ostream& operator<<(ostream&, const Book&);
    void getAuthors();
private:
    vector<string>* authors;
    string title;
    int year;
};

#endif


#include "Book.h"
#include <vector>
#include <string>
#include <cstdlib>
using namespace std;
Book::Book():year(0), title(NULL), authors(NULL){}
Book::Book(vector<string>* bookauthors,string booktitle, int bookyear ){
    authors = bookauthors;
    title = booktitle;
    year = bookyear;
}
Book::Book(const Book& aBook){
    authors = aBook.authors;
    title = aBook.title;
    year = aBook.year;
}
bool Book::operator==(const Book &aBook){
    if(getYear() == aBook.getYear() && getTitle() == aBook.getTitle())
        return true;
    else return false;
}
bool Book::operator != (const Book &aBook){
    if(getYear() != aBook.getYear() && getTitle() == aBook.getTitle())
        return true;
    else return false;
}
Book& Book::operator =(const Book& rhs){
    if(this != rhs){
        authors = rhs.authors;
        title = rhs.title;
        year = rhs.year;
    }
    return *this;
}
void Book::update(int newyear){
    year = newyear;
}
void Book::update(string newtitle){
    title = newtitle;    
}
void Book::update(vector<string>* newauthors){
    authors = newauthors;
}
ostream& operator<<(ostream& os, const Book& b){
    os<<b.getTitle()<<"/"<<b.getYear();
    return os;
}

我不断收到这个错误，我不知道如何解决它。

Book.cc: In member function 'Book& Book::operator=(const Book&)':
Book.cc:28: error: no match for 'operator!=' in 'this != rhs'
Book.cc: In function 'std::ostream& operator<<(std::ostream&, const Book&)':
Book.cc:45: error: no match for 'operator<<' in 'std::operator<< [with _CharT = char,   _Traits = std::char_traits<char>, _Alloc = std::allocator<char>](((std::basic_ostream<char,   std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)os)), ((const  std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >*)(& Book::getTitle()  const())))) << "/"'
Book.cc:44: note: candidates are: std::ostream& operator<<(std::ostream&, const Book&)

感谢您的帮助。

Answer 1

替换

bool operator==(const Book&);
bool operator!=(const Book&);
friend ostream& operator<<(ostream&, const Book&);

由

bool operator==(const Book&) const;
bool operator!=(const Book&) const;
friend std::ostream& operator<<(std::ostream&, const Book&);

当检查是否相等时，试试

if (this != &rhs)

或

if (*this != rhs)

取决于您是要进行深度比较还是浅度比较。

Answer 2

对于第一个错误，您将this（一个Book 指针）与Book 引用进行比较。您需要将this 与参数的地址进行比较，即比较指针：

if(this != &rhs){ .... }

关于第二个错误，我在您发布的代码中没有发现任何问题。但是，您不需要将 ostream& << 运算符声明为友元，因为它只访问公共方法。

您还应该将比较运算符设为== 和!= const，尽管这与您报告的错误无关。