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[Benji Smith]

Sergey Gromov wrote:
> Thu, 09 Oct 2008 09:07:19 -0400,
> Benji Smith wrote:
>> 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?
> 
> Your "PI" and "author" cannot be optimized because they're public and 
> mutable, so every time you use PI in your code compiler must access a 
> variable just in case some other module changed its value to 180.  Value 
> of "North" on the other hand can never change so it can take part in 
> constant folding etc.  You "manifest" an identity between name "North" 
> and a number 0.
> 
> The closest to a manifest constant would be
> 
> invariant double PI = 3.14;
> invariant string author = "Walter";
> 
> I think it even works in D2.  I don't know why enum were introduced for 
> declaring constants.

Oops. Silly me. I meant to declare those variables as const (or 
invariant, like in your example).

I understand the optimization benefits of const values. I'v just never 
understood what people are talking about with "manifest" constants, and 
why they deserve to have some special keyword.

I take it, from your example code, that you agree with me that there's 
no need for a "define" or "manifest" keyword...

--benji