Trying to do some tricky stuff but I can't seem to get the value of a type(enum in my case, but must work in general).

Basically, given a type T or an alias T(alias preferred), I'd like to be able to get the "default value" of that type.

e.g.,

if it is an enum and I have an alias to a member, instead of (Enum)3 I want 3.

if it is a member field, I want the default value.

if it is a function pointer, the address(probably 0, but whatever).

Basically a sort of generic "getValue" that attempts to get the CT value of any type, if it exists. (obviously the value of class is meaningless, so returns ""... class isn't even a type, but can still query).

In fact, I'd like to also naturally resolve the value back in to the expression that created it.

e.g.,

(Enum)3 -> Enum.ThirdEntry

On 7/14/17 1:51 PM, FoxyBrown wrote:
> Trying to do some tricky stuff but I can't seem to get the value of a type(enum in my case, but must work in general).
> 
> Basically, given a type T or an alias T(alias preferred), I'd like to be able to get the "default value" of that type.
> 
> e.g.,
> 
> if it is an enum and I have an alias to a member, instead of (Enum)3 I want 3.
> 
> if it is a member field, I want the default value.
> 
> if it is a function pointer, the address(probably 0, but whatever).
> 
> Basically a sort of generic "getValue" that attempts to get the CT value of any type, if it exists. (obviously the value of class is meaningless, so returns ""... class isn't even a type, but can still query).
> 
> In fact, I'd like to also naturally resolve the value back in to the expression that created it.
> 
> e.g.,
> 
> (Enum)3 -> Enum.ThirdEntry
> 
> 

.init is the default value.

I'm not sure you can get the default value of a non-default initializer, My attempts using init didn't work. e.g.:

void foo(alias T)()
{
   pragma(msg, T.init);
}

struct S
{
   int y = 5;
   void bar() { foo!y; } // prints 0
}

-Steve

On Friday, 14 July 2017 at 18:06:49 UTC, Steven Schveighoffer wrote:
>
> .init is the default value.
>
> I'm not sure you can get the default value of a non-default initializer, My attempts using init didn't work. e.g.:
>
> void foo(alias T)()
> {
>    pragma(msg, T.init);
> }
>
> struct S
> {
>    int y = 5;
>    void bar() { foo!y; } // prints 0
> }

See spec [1]:

"If applied to a variable or field, it is the default initializer for that variable or field's type."

If you want to get at the 5 in a static context, you'd have to use

---
S.init.y
---

i.e. get the default initializer for the struct and get its y member's value.

[1] https://dlang.org/spec/property.html#init