Hello,

class A{
  int i = 42;
}

Is there a way to get that 42? (Input is the class A and the name of the field 'i')

A.i.init returns 0, so it doesn't care about the class.

classinfo.m_init can be an option, but maybe there's a nicer way to do this?

Thx!

On Saturday, 8 February 2020 at 23:37:56 UTC, realhet wrote:
> Hello,
>
> class A{
>   int i = 42;
> }
>
> Is there a way to get that 42? (Input is the class A and the name of the field 'i')
>
> A.i.init returns 0, so it doesn't care about the class.
>
> classinfo.m_init can be an option, but maybe there's a nicer way to do this?
>
> Thx!

Try A.init.i

On Saturday, 8 February 2020 at 23:37:56 UTC, realhet wrote:
> classinfo.m_init can be an option, but maybe there's a nicer way to do this?

eh the initializer i think is the best. you can sometimes shortcut it

pragma(msg, (new A).i);

which will call the ctor at ctfe and get the value. but if there isn't a default constructor that's harder. (of course at runtime you can yank it out of the init member... but ctfe will fail that i believe because of the reinterpret cast involved).

i can't think of a better way right now.

On Sunday, 9 February 2020 at 00:15:47 UTC, Drug wrote:
> On Saturday, 8 February 2020 at 23:37:56 UTC, realhet wrote:
>> Hello,
>>
>> class A{
>>   int i = 42;
>> }
>>
>> Is there a way to get that 42? (Input is the class A and the name of the field 'i')
>>
>> A.i.init returns 0, so it doesn't care about the class.
>>
>> classinfo.m_init can be an option, but maybe there's a nicer way to do this?
>>
>> Thx!
>
> Try A.init.i

I tried and A.init.stringof == "void()"

That generates a new question, what is void() ?
It has no sizeof property.
I guess it's the voidest of the voids in D :D

On Sunday, 9 February 2020 at 00:27:21 UTC, Adam D. Ruppe wrote:
> On Saturday, 8 February 2020 at 23:37:56 UTC, realhet wrote:
>> classinfo.m_init can be an option, but maybe there's a nicer way to do this?
>
> eh the initializer i think is the best. you can sometimes shortcut it
>
> pragma(msg, (new A).i);
>
> which will call the ctor at ctfe and get the value. but if there isn't a default constructor that's harder. (of course at runtime you can yank it out of the init member... but ctfe will fail that i believe because of the reinterpret cast involved).
>
> i can't think of a better way right now.

Thank you! It works!
Somehow I thought that 'new' is too much in CTFE, now I can rethink that.

On Sunday, 9 February 2020 at 00:41:12 UTC, realhet wrote:
> On Sunday, 9 February 2020 at 00:27:21 UTC, Adam D. Ruppe wrote:

Using what I've learned:

class A{
  int i=42;

  void init(){ // reInitialize class fields
    alias T = typeof(this);
    mixin([FieldNameTuple!T].map!(n => n~"=(new T)."~n~";").join);
  }

}

(I also like to live dangerously with string mixins :D)

On Sunday, 9 February 2020 at 00:57:05 UTC, realhet wrote:
> On Sunday, 9 February 2020 at 00:41:12 UTC, realhet wrote:
>> On Sunday, 9 February 2020 at 00:27:21 UTC, Adam D. Ruppe wrote:


    mixin([FieldNameTuple!T].map!(n => n~"=(new T)."~n~";").join);

After checking it in the asm debugger, it turned out, that it evaluates (new T) in runtime. (Silly me, because the constructor can have side effects.)
So I first put the default values into an enum, and then do the assignment:

    foreach(n; FieldNameTuple!T){{
      mixin("enum def = (new T).@; @ = def;".replace("@", n));
    }}

And that became as fast as it can be:

    mov dword ptr ds:[rax+10], 2A

Thanks for the help again.

On Sunday, 9 February 2020 at 00:57:05 UTC, realhet wrote:
> On Sunday, 9 February 2020 at 00:41:12 UTC, realhet wrote:
>> On Sunday, 9 February 2020 at 00:27:21 UTC, Adam D. Ruppe wrote:
>
> Using what I've learned:
>
> class A{
>   int i=42;
>
>   void init(){ // reInitialize class fields
>     alias T = typeof(this);
>     mixin([FieldNameTuple!T].map!(n => n~"=(new T)."~n~";").join);
>   }
>
> }
>
> (I also like to live dangerously with string mixins :D)

Tip: don't name a member function `init`, since it will conflict with the built-in `.init` property.

Here's a version without string mixins, for comparison:

    void initialize() {
        static foreach (i, field; typeof(this).tupleof) {{
            // force compile-time evaluation
            enum initValue = (new typeof(this)).tupleof[i];
            field = initValue;
        }}
    }

On 2/8/20 7:39 PM, realhet wrote:
> On Sunday, 9 February 2020 at 00:15:47 UTC, Drug wrote:
>> On Saturday, 8 February 2020 at 23:37:56 UTC, realhet wrote:
>>> Hello,
>>>
>>> class A{
>>>   int i = 42;
>>> }
>>>
>>> Is there a way to get that 42? (Input is the class A and the name of the field 'i')
>>>
>>> A.i.init returns 0, so it doesn't care about the class.
>>>
>>> classinfo.m_init can be an option, but maybe there's a nicer way to do this?
>>>
>>> Thx!
>>
>> Try A.init.i
> 
> I tried and A.init.stringof == "void()"
> 
> That generates a new question, what is void() ?
> It has no sizeof property.
> I guess it's the voidest of the voids in D :D

Looks like you wrote a function void init() inside your class.

Don't do that. D has a default init property.

But as an aside, A.init has a value of null. So A.init.i => segfault.

The 42 lives inside the intializer, which is a void array stored in the TypeInfo. But you probably don't want to get it that way.

Adam's way is technically a good way to get it. But you don't necessarily need that (and it might not be what you are looking for -- the constructor could easily change i to a different value). Problem is that typeid cannot be read at compile time. https://issues.dlang.org/show_bug.cgi?id=7147

If you could do that, you could construct a "pre-constructed" class instance, and use it as the prototype for reading the int value.

-Steve

On Sunday, 9 February 2020 at 01:19:55 UTC, Paul Backus wrote:
> On Sunday, 9 February 2020 at 00:57:05 UTC, realhet wrote:
>> On Sunday, 9 February 2020 at 00:41:12 UTC, realhet wrote:
>>> On Sunday, 9 February 2020 at 00:27:21 UTC, Adam D. Ruppe
>             enum initValue = (new typeof(this)).tupleof[i];

Wow, thx! Accessing by index instead of name. Really powerful!