Re: alias this of non-public member

[Namespace via Digitalmars-d-learn]

On Tuesday, 7 April 2015 at 17:21:09 UTC, Daniel Kozak wrote:

On Tue, 07 Apr 2015 16:40:29 +0000
via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> 
wrote:

Hi!

Excuse me if this is obvious, but I can't recall coming across 
anything similar and a quick search returns nothing relevant:

struct Foo {
}

struct FooWrapper {
   alias x_ this;
   private Foo* x_; // doesn't work, as x_ is private
}

Basically, I want x_ to never be visible, except through the 
"alias this" mechanism, at which point it should instead be 
seen as public.

Assuming something like this is not already possible in a 
clean way, I would like to suggest a tiny(I think) addition to 
the language:

struct FooWrapper {
   public alias x_ this; // overrides the visibility through 
the alias;
   private Foo* x_;
}


While I think this would be useful for the language, the 
reason I want such a wrapper, is because I want to give 
opIndex, toString, to a pointer, or, in fact just value 
semantics, while keeping the rest of the interface through the 
pointer.

I thought about using a class instead of a struct pointer, but 
I am not sure about the memory layout for classes, nor about 
the efficiency of overriding Object's methods, so I didn't 
want to risk making it any less efficient. If someone could 
shed some light about D's class memory layout and general 
performance differences to a simple struct (or a C++ class for 
that matter), that would also be great. In general, more 
information about these sort of things would be great for us 
also-C++ programmers. :)

Works for me:

struct M
{
        void callMe() {
                writeln("Ring...");
        }
}

struct S
{
        alias m this;
        private M m;
}

void main(string[] args)
{
        S s;
        s.callMe();
}

Modules are like friends in C++: Even private members can be 
accessed.

@ Márcio Martins:
You need a public getter function:
----
@property
@nogc
@safe
inout(Foo*) getFoo() inout pure nothrow {
    return x_;
}

alias getFoo this;
----