Library Typedefs are fundamentally broken

[Adam D. Ruppe via Digitalmars-d]

On Tuesday, 23 September 2014 at 17:36:33 UTC, Andrei 
Alexandrescu wrote:
> (b) there are no significant ways to improve Typedef in ways 
> that would be difficult to the struct-based approach (e.g. 
> disallow implicit conversion to base type, adding constructors 
> etc).

I think disallowing stuff is exactly why typedef (and Typedef) 
are suboptimal - they are a package approach. And once you start 
adding configuration options, you get a parameter list so long 
you might as well just write the struct.

I do think there's value in some generic mixins for proxy member 
forwarding though. Typedef is built on a Proxy mixin but it 
brings everything. I'd say ideally, we'd break it up into various 
pieces:

mixin Addition;
mixin Multiplication; /* these two might come together with mixin 
Arithmetic; */
mixin Dereferencing;
mixin Indexing;

you know something like that. So now you can add operations 
selectively... or add them all with alias this and selectively 
disable them!

mixin template Addition(string proxy = "_") {
         typeof(this) opBinary(string op : "+")(typeof(this) rhs) {
                 return typeof(this)(mixin(proxy ~ op ~ "rhs." ~ 
proxy));
         }
}

struct Foo {
         int _;
         mixin Addition; /* Foo(10) + Foo(20) now yields Foo(30) */
}

struct NoAdd {
    int _;
    alias _ this; /* Foo(10) + Foo(20) yields int(30) */
    mixin Addition; /* now we have all alias this but addition 
gives Foo */
    @disable mixin Multiplication; /* multiply is now statically 
disallowed */
}



These mixins would take a wee bit more care than I showed here 
and we'd want a battery of building blocks AND common 
combinations, but I think this would blow even the ideal Typedef 
utterly out of the water.

I think I posted this before, but you don't actually want 
deferencing of an opaque HANDLE. typedef lets you do that since 
it thinks it is a void*. alias this lets you do it, since it will 
implicitly convert. But with the individual feature mixins, alias 
this, and @disable, you can do whatever you want and have pretty 
short, readable code.