是否有输入枚举类型变量而不是使用 static_cast 的替代方法

Question

I was trying to input an enumerated type variable but i could not do it without a using a static_cast operation

#include<iostream>
using namespace std;

enum month
{
    JAN,
    FEB,
    MAY,
    JUNE,
    OCTOBER,
    DECEMBER,
};
int main()
{
    month This;
    cin >> This; <<_______________________ Causes a compiler error
    system("pause");
    return 0;
}

One workaround is to read in an integer, and use a static_cast to force the compiler to put an integer value into an enumerated type

{
int input_month;
cin >> input_month;

month This = static_cast<month>(input_month);<<_____________Works
}

So is there an alternative to inputting an enumerated type value

Answer 1

Here is an example how I would approach this (extending the answer of jacobi).

#include <iostream>
#include <stdexcept>

enum month
{
    JAN,
    FEB,
    MAY,
    JUNE,
    OCTOBER,
    DECEMBER,
};

month int2month(int m)
{
    switch(m)
    {
        case 1: return month::JAN;
        case 2: return month::FEB;
        case 3: return month::MAY;
        case 6: return month::JUNE;
        case 10: return month::OCTOBER;
        case 12: return month::DECEMBER;
        default: throw std::invalid_argument("unknown month");
    }
}

std::istream& operator>>(std::istream& is, month& m)
{
    int tmp;
    if (is >> tmp)
        m = int2month(tmp);
    return is;
}

int main()
{
    month This;
    try{
       std::cin >> This;
    }
    catch(std::invalid_argument& e){
    // TODO: handle
    }
    return 0;
}

online example

Notice that there are many more way how you could map 'int's to month. For example I think your code will give the same results as:

month int2month(int m)
{
    switch(m)
    {
        case 0: return month::JAN;
        case 1: return month::FEB;
        case 2: return month::MAY;
        case 3: return month::JUNE;
        case 4: return month::OCTOBER;
        case 5: return month::DECEMBER;
        default: throw std::invalid_argument("unknown month");
    }
}

Out of scope for this question:

Also note that you could write an 'string2month' function. Then you could make 'tmp' an string. Depending on 'tmp' containg only digits, you could convert 'tmp' to an 'int' to convert this to an month, or try to convert 'tmp' to an 'month'. This would allow for inputs like JAN or January, depending on the implementation of 'string2month'.

Answer 2

You would have to implement your own operator >> for the enum 'month' to do what you want.

Example:

std::istream& operator>>(std::istream& is, month& m)
{
    int tmp;
    if (is >> tmp)
        m = static_cast<month>(tmp);
    return is;
}