On Thursday, 5 August 2021 at 01:14:26 UTC, H. S. Teoh wrote:
1) If the constant is a POD (int, float, etc.), use:
enum myValue = ...;
crystal-clear.
2) If the constant is a string or some other array:
static immutable string myString = "...";
crystal-clear.
2) If the constant is a string or some other array:
static immutable Data[] myData = [ ... ];
Unless you have a specific reason to, avoid using `enum` with
string and array literals, because they will trigger a memory
allocation *at every single reference to them*, which is
probably not what you want.
enum myArray = [ 1, 2, 3 ];
...
int[] data = myArray; // allocates a new array
int[] data2 = myArray; // allocates another array
// they are separate arrays with the same contents
assert(data !is data2);
assert(data == data2);
// allocates a temporary array, does the comparison, then
// discards the temporary
if (data == myArray) ...
foreach (i; 0 .. 10) {
int[] input = getUserInput(...);
// allocates a new array at every single loop iteration
if (input == myArray) { ... }
}
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.
Throwing away the enum and recoding as following:
static immutable structureLocation[stringUTF32] 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)
];
gives me:
Error: non-constant expression `["BUE"d:structureLocation(true,
"arg"d, null, "Buenos Aires"d, "ART"d),
"GRU"d:structureLocation(true, "bra"d, null, "S\xe3o Paulo"d,
"BRT"d), "HHN"d:structureLocation(true, "deu"d, null, "Frankfurt
am Main"d, "CET"d), "LHR"d:structureLocation(true, "gbr"d, null,
"London"d, "UTC"d), "NYC"d:structureLocation(true, "usa"d, null,
"New York"d, "EST"d)]`
Don't do this. Use static immutable for arrays and strings, use
enum only for PODs.
crystal-clear.
