Re: alias this of non-public member

[Daniel Kozak 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();
}

module some;
import std.stdio;

Another way is use template mixin:

private mixin template M()
{
        int someVar = 7;
        public void callMe() {
                writeln("Call");
        }

        public void callMe2() {
                writeln("Call2");
        }
}

struct S
{
        mixin M;
}


////

module main;
import some;
void main(string[] args)
{
        S s;
        s.callMe();
        s.callMe2();
}