c++ - 为什么我没有使用我类(class)的 toString 方法获得输出？

Question

日期.h

#include <string>

#ifndef DATE_H_
#define DATE_H_

class Date
{
public:
    static const unsigned int monthsPerYear{12};
    explicit Date(unsigned int d = 1, unsigned int m = 1, unsigned int y = 1900);
    std::string toString() const;
private:
    unsigned int day;
    unsigned int month;
    unsigned int year;
    unsigned int checkDay(unsigned int ) const;
};


#endif /* DATE_H_ */

日期.cpp

#include <iostream>
#include <sstream>
#include <stdexcept>
#include <array>
#include "Date.h"

using namespace std;

Date::Date(unsigned int d, unsigned int m, unsigned int y) : day{checkDay(d)}, month{m}, year{y} {
    if ( month < 1 || month > monthsPerYear ) {
        throw invalid_argument("wrong entry for month");
    }
}

string Date :: toString() const {
    ostringstream output;
    output << day << ":" << month << "/" << year;
    return output.str();
}

unsigned int Date :: checkDay(unsigned int testDay) const {
    static const array<unsigned, monthsPerYear + 1> daysPerMonth{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (testDay <= daysPerMonth[month] && testDay > 0) {
        return testDay;
    }
    return testDay;
}

主要.cpp

#include <iostream>
#include "Date.h"

using namespace std;

int main()
{
    Date d2;
    cout << d2.toString() << endl;
    return 0;
}

我在输出控制台中什么也得不到。

Answer 1

除了 一些程序员 识别的 UB 和 I do get output 之外，我看不出有什么问题(在将您的代码片段转换为单文件测试用例之后)。

您可以通过使 checkDay 取天 和 月来修复 UB:

unsigned int checkDay(unsigned int, unsigned int) const;

和:

Date::Date(unsigned int d, unsigned int m, unsigned int y) : day{checkDay(d, m)}, month{m}, year{y} {

和:

unsigned int Date :: checkDay(unsigned int testDay, unsigned int testMonth) const {
    static const array<unsigned, monthsPerYear + 1> daysPerMonth{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (testDay <= daysPerMonth[testMonth] && testDay > 0) {
        return testDay;
    }
    return testDay;
}

或者初始化 month before 调用函数(你也必须在 day 之前声明 month，然后)但我认为考虑到函数的作用，上面的解决方案具有更好的对称性。实际上，我现在也考虑将 checkDay 设为 static 成员函数。

然后 it should work reliably .

当然，对于非学习程序，您将使用 boost::gregorian::date。 :)