c++ - C++运算符重载错误:在只读对象中分配成员“”

Question

错误信息:

class.cpp: In function 'RationalNumber operator++(const RationalNumber&, int)':
class.cpp:27:28: error: assignment of member 'RationalNumber::numerator' in read-only object
  r.numerator = r.numerator+1;

为什么它是“在只读对象中”？

class.h文件

#ifndef CLASS_H
#define CLASS_H
#include <iostream>
using namespace std;
// #include "class2.h"

class RationalNumber
{
 private:
  int numerator,denominator;
 public:
    RationalNumber(int x,int y);
    RationalNumber();
    friend ostream& operator << (ostream& os,const RationalNumber& x);
    friend RationalNumber operator++ (const RationalNumber& r, int dummy);
    RationalNumber operator- (const RationalNumber& r) {
        RationalNumber r3;
        //cout << numerator << " " << r.numerator<<endl;
        r3.numerator = (numerator * r.denominator)-(r.numerator * denominator);
        r3.denominator = r.denominator * denominator;
        return r3;
    }

};
#endif

main.cpp

#include <iostream>
using namespace std;
#include "class.h"
// #include "class2.h"

int main()
{
    RationalNumber r1(21,7),r2(67,31),r3;
    r3 = r1 - r2;
    cout << r3;
    r1++;
    cout << r1;
}

cpp部分出现错误

RationalNumber operator++ (const RationalNumber& r, int dummy) 
{
    RationalNumber temp;
    temp = r;
    r.numerator = r.numerator+1;
    return temp;
}

Answer 1

大概您的意图是在temp中更改operator++()的值；如果是这样，您在尝试修改r对象const时遇到错字，导致错误。您可能打算写:

RationalNumber operator++ (const RationalNumber& r, int dummy) 
{
    RationalNumber temp;
    temp = r;
    temp.numerator = temp.numerator+1; // NOT "r.numerator = ..."
    return temp;
}

(可以稍微缩短或以各种方式进行调整；为了清楚起见，我保留了原始的编码样式。您可能还会收到编译器警告，因为未使用 int参数 dummy)。

鉴于您实际上并没有使用 int dummy，这表明您可能正在尝试编写 前缀 ++而不是您编写的后缀。从 this answer可以看到，语法略有不同。你会写像

RationalNumber& operator++() // no args: prefix
{
    ++(this->numerator); 
    return *this;
}

尽管这将解决编译器错误，但您应该备份一些步骤，并确定 operator++()是否对您的类确实有意义；可以为 ++建立多种合理的含义。您可能希望 operator+()添加两个 RationalNumber，而不是“递增”。