c++ - 重载后缀运算符不起作用

Question

#include <iostream>

using namespace std;

class NumDays
{
private:
    int hour;
    int day;
    void simplify();

public:
    NumDays()
    {
        day = 0;
        hour = 0;
    }

    void setData(int d, int h)
    {
        hour = h;
        day = d;
        simplify();
    }

    int getHour()
    {
        return hour;
    }

    int getDay()
    {
    return day;
    }

    NumDays operator++(int);
    NumDays operator--(int);

};

void NumDays::simplify()
{
    hour = 8*day + hour;
    day = hour / 8;
    hour = hour % 8;
}

NumDays NumDays::operator++(int)
{
    NumDays obj1;

    hour++;
    simplify();
    return obj1;
    }

NumDays NumDays::operator--(int)
{
    NumDays obj1;

    hour--;
    simplify();
    return obj1;
}

void setFirst(NumDays &);
void setSecond(NumDays &);

void addData(NumDays &, NumDays &, NumDays &);

int main()
{
    NumDays first, second, third;

    setFirst(first);
    setSecond(second);

    addData(first, second, third);
}

void setFirst(NumDays &obj1)
{
    int day, hour = 0;
    cout << "Please enter the amount of days followed by hours." << endl;
    cin >> day >> hour;
    obj1.setData(day, hour);
}

void setSecond(NumDays &obj2)
{
    int day, hour = 0;
    cout << "Please enter the amount of days followed by hours again." << endl;
    cin >> day >> hour;
    obj2.setData(day, hour);
}

void addData(NumDays &obj1, NumDays &obj2, NumDays &obj3)
{
    for (int k = 0; k < 8; k++)
    {
    obj3 = obj1++;
    cout << "  First Data: " << obj3.getDay() << " day(s), "
         << obj3.getHour() << " hour(s).";
    cout << "  First Data: " << obj1.getDay() << " day(s), "
         << obj1.getHour() << " hour(s).\n";
    }

    cout << endl;

    for (int l = 0; l < 8; l++)
    {
    obj3 = obj2++;
    cout << "   Second Data: " << obj3.getDay() << " day(s), "
         << obj3.getHour() << " hour(s).";
    cout << "   Second Data: " << obj2.getDay() << " day(s), "
         << obj2.getHour() << " hour(s).\n";
    }
}

当我运行该文件时，obj3 有 0 天和 0 小时，而 obj2 增加了。怎么会这样？当我尝试将它作为 obj3 = obj2 没有任何后缀符号时，它会复制它。所以我假设从 obj2 获取数据没有问题。但是当我包含后缀运算符时，数据变成了 0 和 0。

Answer 1

你几乎是对的。首先，我对你的类(class)做了一些改动。

class NumDays
{
private:
    int hour;
    int day;
    void simplify();

public:
    NumDays(int dy = 0, int hr = 0) : hour(hr), day(dy) { simplify(); }
    void setData(int d, int h)
    {
        hour = h;
        day = d;
        simplify();
    }
    int getHour() const { return hour; }
    int getDay() const { return day; }
    friend ostream& operator<<(ostream& out, const NumDays &nd) {
    return out << "  First Data: " << nd.getDay() << " day(s), "
         << nd.getHour() << " hour(s).";
    }
    NumDays operator++(int);
};

我做的第一件事是重写构造函数以使用默认参数并使用现代风格。

我做的第二件事是将 const 添加到不(也不应该)修改对象的各种成员函数中。

我做的第三件事是添加一个 ostream 提取器作为 friend 函数。这纯粹是为了方便排查问题。

这是新版本的后增量运算符的样子:

NumDays NumDays::operator++(int)
{
    NumDays obj1(*this);
    hour++;
    simplify();
    return obj1;
}

唯一的区别是 obj1 的创建现在使用编译器生成的默认复制构造函数。这很重要，因为我们需要为后增量运算符返回未增量值的拷贝。 (您的后缀递减运算符需要类似的修复。)

我没有对您的 simplify() 例程进行任何更改。

然后我用这段代码测试了它:

int main()
{
    NumDays nd(2,3);
    cout << nd++ << endl;   // prints 2, 3
    cout << nd << endl;     // prints 2, 4
    return 0;
}

现在一切正常。