best/proper way to declare constants ?

[H. S. Teoh]

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.

Example:

	enum ctValue = [ "my", "data", "here", ... ];

	// Initialize this once with ctValue.
	static immutable string[] rtValue = ctValue;

	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.


T

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