User-defined conversions?

[Daniel Keep]

Jeff McGlynn wrote:
> I'm in the process of translating my MySQL library from PHP to D.  When
> doing research I found that MySQL++ uses class operator overloading to
> automatically convert classes to basic types like so:
> 
> C++ ------------------
> class convert {
>     char * myVal;
>     
>     MyClass(char * inString) {
>         myVal = inString;
>     }
>     
>     operator int() {
>         // Convert myVal to an int here ...
>         return myValAsInt;
>     }
>     
>     operator float() {
>         // Convert myVal to float here ...
>         return myValAsFloat;
>     }
> };
> 
> int someInt = convert("346");
> float someFloat = convert("346.3");
> -------------------------
> 
> Is there any way to do something similar in D?  The opCast function just
> attempts to cast whatever is returned to the proper type, which doesn't
> work for strings to ints.

import std.conv : toInt, toFloat;

T fromString(T)(char[] str)
{
    static if( is( T == int ) )
        return toInt(str);
    else static if( is( T == float ) )
        return toFloat(str);
    else
        static assert( false,
            "Whoopsie!  Don't support fromString!("
            ~T.stringof~"); sorry.");
}

Or, if you want something more extensible...

import std.boxer;

alias Box function(char[]) ConvFn;
ConvFn[TypeInfo] convfns;

T fromString(T)(char[] str)
{
    static if( is( T == int ) )
        return toInt(str);
    else static if( is( T == float ) )
        return toFloat(str);
    else
    {
        if( typeinfo(T) in convfns )
            return unbox!(T)(convfns[typeinfo(T)](str));
        else
            throw new Exception("Type "
                ~T.stringof~" not supported.");
    }
}

Note: none of the above is tested, but should work.  If the compiler
complains about that type alias, switch "Box" and "function": I can
never remember which way around they go :P

	-- Daniel

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