enum

[Dennis Schulz]

Hi this is my problem:

enum num{Jan=1, Feb=2, Mar=3, Apr=4, May=5, Jun=6, Jul=7, Aug=8,
Sep=9, Okt=10, Nov=11, Dec=12};

std::string month = "Dec";

num n1 = Dec; //this is working

num n2 = static_cast<num >(month); //this is not working

All i want is the number of the month as return. what to do?

Thank you Dennis

[Rolf Magnus]

Re: enum

Dennis Schulz wrote:
[color=blue]
> Hi this is my problem:
>
> enum num{Jan=1, Feb=2, Mar=3, Apr=4, May=5, Jun=6, Jul=7, Aug=8,
> Sep=9, Okt=10, Nov=11, Dec=12};
>
> std::string month = "Dec";
>
> num n1 = Dec; //this is working
>
> num n2 = static_cast<num >(month); //this is not working[/color]

month is a string. Enums are compile time constants. The name of an enum
value is only existing as identifier in the soruce code and doesn't have a
meaning at runtime.
A static_cast won't parse the string and tell you which enum value the
contained text would represent if it were an identifier. If you need
something like that, you have to do it yourself.
[color=blue]
> All i want is the number of the month as return. what to do?[/color]

Write a function:

num stringToMonth(c onst std::string& name)
{
if (name == "Jan")
return Jan;
else if (name == "Feb")
return Feb;
// ...
}

[Thomas Matthews]

Re: enum

Dennis Schulz wrote:[color=blue]
> Hi this is my problem:
>
> enum num{Jan=1, Feb=2, Mar=3, Apr=4, May=5, Jun=6, Jul=7, Aug=8,
> Sep=9, Okt=10, Nov=11, Dec=12};
>
> std::string month = "Dec";
>
> num n1 = Dec; //this is working
>
> num n2 = static_cast<num >(month); //this is not working
>
> All i want is the number of the month as return. what to do?
>
> Thank you Dennis[/color]

As Rolf said, there is no facilities in the C++ language
to convert from string to enum, so you will have to invent
one.

Here is an example of a Month class. This encapsulates
the enum for safety purposes:
class Month
{
public:
enum {Unknown = 0,
Jan, Feb, Mar, Apr, May, Jun,
Jul, Aug, Sep, Okt, Nov, Dec,
Maximum_Months} ;

Month(unsigned int new_value)
: value(new_value )
{ }
Month(char * text);
Month(const Month& m)
: value(m.value)
{ }
Month& operator=(const Month& m)
{
if (this != &m)
{
value = m.value;
}
return *this;
}
char * const to_text(void) const;
private:
unsigned int value;
const char * name_table;
};

const char * Month::names[] =
{ "Unknown",\
"Jan", "Feb", "Mar", "Apr", "May", "Jun",
"Jul", "Aug", "Sep", "Okt", "Nov", "Dec"
};

Month ::
Month(char * text)
: value(0)
{
for (unsigned int i = 0, i < Maximum_Months; ++i)
{
if (strcmp(text, names[i]) == 0)
{
value = i;
break;
}
}
}

char * const
Month ::
to_text(void) const
{
return names[value];
}


Other methods can be added such as:
operator<< and operator>> for streams
++, --, for incrementing and decrementing a month
+, -, for arithmetic.
to_str() for converting to a std::string

--
Thomas Matthews

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