generating @property from structs

[user1234]

On Saturday, 30 September 2017 at 08:20:44 UTC, Nicholas Wilson 
wrote:
> struct MyType
> {
>     void* ptr;
>     static struct Info
>     {
>         @(42) int foo;
>     }
>
>     // Should be generated by the mixin below
>     @property int foo()
>     {
>          int ret;
>          getMyTypeInfo(ptr,42,int.sizeof,&ret);
>          return ret;
>     }
>     mixin generateInfo!getMyTypeInfo;
> }
>
> extern(C) void getMyTypeInfo(void*,int, size_t,void*);
>
> How do I write generateInfo info?
>
> mixin template generateInfo(alias func)
> {
>     foreach(field; typeof(this).Info.tupleof)
>     {
>         //???
>     }
> }
>
> I know I'll probably have to resort to string mixins, but how 
> do I get the attribute so that I call
> func(ptr,attibute, T.sizeof, &ret)? There will be a couple of 
> values that dont have an attribute but they are all of known 
> types and I want to handle them separately.
>
> Thanks
> Nic

You can use a struct that contains the value because apparently 
without you can only retrieve a specific value:

---
struct ValueHolder{int value;}

struct Foo
{
     @ValueHolder(42) int foo;

     int getTheValueDuringTheCall(string member)()
     {
         import std.traits;
         alias uda = getUDAs!(__traits(getMember, Foo, member), 
ValueHolder);
         static if (uda.length == 1)
             return uda[0].value;
         else
             return 0;
     }

     void test()
     {
         import std.stdio;
         writeln(getTheValueDuringTheCall!"foo");
     }
}

void main()
{
     Foo foo; foo.test();
}
---

I mean that `getUDAs!(__traits(getMember, Foo, member), int);` 
wouldn't work.