The language spec is ambiguous, and DMD can apparently do either. This program prints

[a __T3TagS23_D11smartstruct4Test1aiVi1Z b __T3TagS24_D11smartstruct4Test1bAaVi2Z]

Note: this is a lame-ass hack around the lack of real metadata attributes in D2. It'd be nice if the real thing was supported.


import std.stdio;

template Tag(alias m, int i) {
  mixin("const int tag_" ~ m.stringof ~ " = " ~ i.stringof ~ ";");
};

class Test {
  int    a;   mixin Tag!(a, 1);
  char[] b;   mixin Tag!(b, 2);
};

int main(char[][] args) {
  writefln(__traits(derivedMembers, Test));
  return 0;
}

On Sat, 06 Dec 2008 00:13:40 +0300, Mike Hearn <mike@plan99.net> wrote:

> The language spec is ambiguous, and DMD can apparently do either. This program prints
>
> [a __T3TagS23_D11smartstruct4Test1aiVi1Z b __T3TagS24_D11smartstruct4Test1bAaVi2Z]
>
> Note: this is a lame-ass hack around the lack of real metadata attributes in D2. It'd be nice if the real thing was supported.
>
>
> import std.stdio;
>
> template Tag(alias m, int i) {
>   mixin("const int tag_" ~ m.stringof ~ " = " ~ i.stringof ~ ";");
> };
>
> class Test {
>   int    a;   mixin Tag!(a, 1);
>   char[] b;   mixin Tag!(b, 2);
> };
>
> int main(char[][] args) {
>   writefln(__traits(derivedMembers, Test));
>   return 0;
> }
>

Yeah, I'm using the same hack (i.e. defining private static const members) to denote some members characteristics, e.g. whether a member is serializable, its value is written to log etc.

> Yeah, I'm using the same hack (i.e. defining private static const members) to denote some members characteristics, e.g. whether a member is serializable, its value is written to log etc.

Is the code open source? I was intending to put together some kind of better integrated protobufs equivalent, ie with the same concept of optional/required tagged fields and efficient serialization, but type safe and better integrated with the language.

Another oddity of __traits, this prints

dump!
a
__T3TagS23_D11smartstruct4Test1aiVi1Z
b
__T3TagS24_D11smartstruct4Test1bAaVi2Z
__T11SmartStructZ

So dump() isn't considered a member, but the name of the mixin does appear!



import std.stdio;

template Tag(alias m, int i) {
  mixin("const int tag_" ~ m.stringof ~ " = " ~ i.stringof ~ ";");
};

template SmartStruct() {
  void dump() {
    writefln("dump!");
  }
};

struct Test {
  int    a;   mixin Tag!(a, 1);
  char[] b;   mixin Tag!(b, 2);

  mixin SmartStruct;
};

int main(char[][] args) {
  Test t;
  t.dump();

  foreach (member; __traits(allMembers, Test)) {
    writefln(member);
  }

  return 0;
}