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 at 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();
}