best/proper way to declare constants ?

[Steven Schveighoffer]

On 8/4/21 11:20 PM, H. S. Teoh wrote:
> On Thu, Aug 05, 2021 at 01:39:42AM +0000, someone via Digitalmars-d-learn wrote:
> [...]
>> What happens in the following case ?
>>
>> public immutable enum gudtLocations = [
>>     r"BUE"d : structureLocation(r"arg"d, r"Buenos Aires"d, r"ART"d),
>>     r"GRU"d : structureLocation(r"bra"d, r"São Paulo"d, r"BRT"d),
>>     r"HHN"d : structureLocation(r"deu"d, r"Frankfurt am Main"d, r"CET"d),
>>     r"LHR"d : structureLocation(r"gbr"d, r"London"d, r"UTC"d),
>>     r"NYC"d : structureLocation(r"usa"d, r"New York"d, r"EST"d)
>>     ];
>>
>> This is something that I also need at compilation time.
> [...]
> 
> If you need a constant array value both at compile-time and runtime, one
> way to do it is to declare an enum that is used only by compile-time
> code, and the same enum is used once to declare the runtime static
> immutable.

A static immutable *array* is usable at compile time. No need for the enum.

> 
> Example:
> 
> 	enum ctValue = [ "my", "data", "here", ... ];
> 
> 	// Initialize this once with ctValue.
> 	static immutable string[] rtValue = ctValue;
> 
> 	if (ctfe) {

`if(__ctfe) {`

> 		// Compile-time: use ctValue
> 		foreach (value; ctValue) {
> 			...
> 		}
> 	} else {
> 		// Runtime: use rtValue instead
> 		foreach (value; rtValue) {
> 			...
> 		}
> 	}
> 
> Just be sure you don't use ctValue during runtime, otherwise it will
> incur an allocation per use.

H.S. Teoh, I know you know better than this ;) None of this is 
necessary, you just need `rtValue` for both runtime and CTFE (and 
compile time parameters)!

Now, the original question is about *associative arrays*, which are a 
different animal. Those, you actually have to initialize using a static 
constructor, and does indeed need both an enum and a static immutable, 
as CTFE currently does not understand runtime AAs. This is a huge issue 
since you do need silly things like the `if(__ctfe)` statement you 
wrote, and keep an enum handy for those cases which is identical to the 
static immutable. We really need to fix this.

-Steve