Re: Proposal: user defined attributes

[Tove]

On Wednesday, 21 March 2012 at 15:11:47 UTC, Andrei Alexandrescu 
wrote:
class Foo
{
int a;
int b;

mixin NonSerialized!(b);
}

I think the liability here is that b needs to appear in two 
places, once in the declaration proper and then in the 
NonSerialized part. (A possible advantage is that sometimes it 
may be advantageous to keep all symbols with a specific 
attribute in one place.) A possibility would be to make the 
mixin expand to the field and the metadata at once.


In case my proof of concept which was posted in another thread 
was overlooked... it was my goal to address this very issue... 
also it's possible to change the datatype of the members in the 
"parallel annotation class"(Foo_Serializable in my limited 
example), and store any extra user data there if so desired while 
keeping the real class clean... and then simply traverse with 
__traits.

import std.stdio;
import std.array;
import std.string;
import std.algorithm;

string attr(string complex_decl)
{
  string org_struct;
  string ser_struct;

  auto lines = splitLines(complex_decl);

  {
    auto decl = split(stripLeft(lines[0]));

    if(decl[0]=="struct")
    {
      org_struct = decl[0] ~ " " ~ decl[1];
      ser_struct = decl[0] ~ " " ~ decl[1] ~ "_Serializable";
    }
    else
      return complex_decl;
  }

  foreach(line; lines[1..$])
  {
    auto attr = findSplitAfter(stripLeft(line), "@NonSerialized 
");

    if(attr[0]=="@NonSerialized ")
      org_struct ~= attr[1];
    else
    {
      org_struct ~= attr[1];
      ser_struct ~= attr[1];
    }
  }

  return ser_struct ~ "\n" ~ org_struct;
}

mixin(attr(q{struct Foo
{
  @NonSerialized int x;
  @NonSerialized int y;
  int z;
}}));

void main()
{
  auto m = [ __traits(allMembers, Foo) ];
  writeln("Normal members of Foo:", m);

  auto n = [ __traits(allMembers, Foo_Serializable) ];
  writeln("Serializable members of Foo:", n);
}