Trait for "can be instantiated"?
ag0aep6g
anonymous at example.com
Tue May 10 05:45:25 UTC 2022
On 09.05.22 23:24, Ben Jones wrote:
> enum x;
> enum y = 5;
>
> struct Wrap(alias T) {
> static if(isType!T){
> T value; //When t == x this doesn't work because `x` is opaque
> and has no default initializer
> }
> }
[...]
>
> x is a "type" as far as isType is concerned (which makes sense to me),
> but I don't think you can ever declare a variable of that type since
> there's no way to initialize it... Is there a trait that can tell if you
> can initialize a variable of a certain type? The best I can think of is
> __traits(compiles, "T x;"), but it seems like it should be possible to
> do better?
`x` is a type, period.
You can use void initialization to declare values of types that don't
have an `init` value: `x value = void;`
As for an alternative to the brute force `__traits(compiles, ...)`, you
can check if `T.init` is a thing:
static if (is(typeof(T.init))) { T value; }
I'm not sure if that's really better, though.
By the way, what is your `Wrap` supposed to do with `x`? Treating it
like `y` will likely fail, too, because `x` is not a value.
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