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[Yigal Chripun]

Benji Smith wrote:
> Steven Schveighoffer wrote:
>> "Benji Smith" wrote
>>> Gide Nwawudu wrote:
>>>> 2) Finalise const/invariant stuff and change manifest const from
>>>> 'enum' to 'define' (or whatever).
>>>>
>>>> define {
>>>> double PI = 3.14;
>>>> string author = "Walter";
>>>> }
>>>> define enum Direction { North, South, East, West };
>>> I've never quite understood what people are talking about when they
>>> refer to a "manifest" constant. What does that mean?
>>>
>>> And why do we need any special keyword? What does the "define"
>>> keyword give you that an ordinary variable declaration doesn't? Why
>>> not just write the code from above like this:
>>>
>>>   double PI = 3.14;
>>>   string author = "Walter";
>>>   enum Direction { North, South, East, West };
>>>
>>> What am I missing here?
>>
>> You cannot take the address of a manifest constant, and it doesn't
>> live in a static data space or in memory anywhere.  Instead it is
>> built directly into the code.
>>
>> So when you say
>>
>> define double PI = 3.14;
>>
>> And then use PI:
>>
>> auto x = PI;
>>
>> This generates code that declares a variable x, then assigns it to
>> 3.14. the PI symbol isn't stored in the final code.
>>
>> This has a huge benefit when you are declaring lots of constants, but
>> only few will be used.  You don't have to pay the penalty of storing
>> all the constants in your code, only the ones you use, and only where
>> you use them.
>>
>> -Steve 
> 
> Okay. That makes sense.
> 
> I was just working with the tango.sys.win32.Types module yesterday, and
> it uses "enum" to declare something like five or six thousand different
> named constants, so I can see where that kind of thing would be helpful.
> 
> --benji

instead of having a special syntax for manifest constants, they could be
part of AST macros, or templates, or other compile time feature of D.
also, can't we use: "invariant PI = 3.14;" and have the linker optimize
this when necessary?