alias this - am I using it wrong?

[Adam D Ruppe]

On Wednesday, 25 August 2021 at 12:11:01 UTC, Johann Lermer wrote:
> I have a little problem understanding alias this. I always 
> thought, that alias this only makes implicit conversions from 
> the aliased object to this.

What it does is if "a SOMETHING b" doesn't compile, it instead 
tries "a.alias_this_member SOMETHING b" instead, or "a SOMETHING 
b.alias_this_member" instead if that's on the other side. The 
object with alias this must already exist though, so constructors 
are an exception (though another object's constructor can trigger 
some existing object's alias this when used as a param to that).

The "SOMETHING" there can be operators like + or = or a .member.

Only if both fail to compile do you actually get an error.

> 17     Test_Struct ts = ac;  // compiles

So what really happens here is the compiler sees ts = ac; fails 
to compile, so it is rewritten into "ts = ac.ts;"

> 18     ac = ts;              // compiles as well - why?

So ac = ts fails, meaning it rewrites into `ac.ts = ts;`

> 20     auto tc = new Test_Class;
> 21     ts = tc.ac;           // compiles

So here it is rewritten into `ts = tc.ac.ts`.

> 22     tc.ac = ts;           // again this compiles, but seg 
> faults

And now

tc.ac.ts = ts;

is the rewrite since the plain one didn't compile, thus accessing 
the null member.



Note too that alias this can be to a function, in which case the 
rewrite will call the function.

Implicit conversion isn't really what alias this is about. It 
kinda works (though note with a alias this struct it can be 
passed by value and thus copy, again the compiler just does the 
rewrite). It is just giving transparent access to a member.