alias this private?

[Ali Çehreli]

On 12/09/2012 11:23 AM, js.mdnq wrote:

 > but b is not private, only the internal representation of it. e.g.,
 > `alias Value_ this` is public. I do realize that it sort of makes Value_
 > and b one and the same but they are not quite the same.
 >
 > To me, it breaks encapsulation. writeln(c.b) is accessing b, not Value_.
 > b is a "virtual type" in the sense that it wraps Value_. While it seems
 > to do something funky like change the protection of Value_ from private
 > to public it doesn't. There is no way to access Value_ when it is
 > private. i.e., we can't do c.b.Value_;

That's great. Value_ must remain private.

 > To drive the point home. We could/should be able to completely
 > encapsulate `Value_` by overriding opAssign and add getters and setters
 > so that `Value_` is actually never even used(although pointless then).

That is possible in D.

 > Another way to see this:
 >
 > struct sPassword
 > {
 > private:
 > string Password;
 > public:
 > alias this Password;
 > opAssign
 > opCast
 > opCmp
 > opGet { return "******"; }
 > Change(old pass, new pass) { if ... }
 > Validate(pass) { pass == Password; ... }
 > }
 >
 > sPassword myPass;
 > writeln(myPass) same as writeln(myPass.Get()) which prints ******

You may have a point but at least I need to see how the existing 
features of D are not acceptable. The following already does what you want:

struct sPassword
{
private:
     string Password_;

public:

     alias opGet this;

     string opGet() { return "******"; }
}

void main()
{
     sPassword myPass;
     assert(myPass == "******");
}

(Aside: Although its name may suggest so, opGet is not an operator 
function.)

 > In any case, it just seems to me that we should be able to use private
 > this way. If we want the additional functionality we just make it public.

Yes, private it good.

 > (it does sort of seem sPassword is like a readonly type but it's more
 > than that as it encapsulates completely)
 >
 > Now, if Password was public instead of private, anyone could do
 > myPass.Password to get the password.

Yeah, not a good idea.

Ali