alias this & cast

[Ali Çehreli via Digitalmars-d-learn]

On 09/11/2014 09:18 AM, andre wrote:

 > I am not sure. b is C but everything not in super class B is hidden.
 > Using cast I can cast b to a full C.
 >
 > The cast "cast(C)b" has the same information about b like the cast
 > "cast(A)b": The memory area of b knows compatitibility to C and also the
 > alias.

That's only because 'b' really is a C.

 > For me, using alias this, the object b has 3 represenations: A, B and C.

Correct but it cannot be known whether any B is an A:

void foo(B b)
{
     // ...
}

Can that 'b' used as an A? Who knows...

It may be desirable that the compiler did static code analysis and saw 
that the 'b' in your code is always a C, therefore can be casted to an 
A. Compilers do not and most of the time cannot do that.

Consider one line added to you program:

 >>> class A{}
 >>>
 >>> class B{}
 >>>
 >>> class C : B
 >>> {
 >>>     A a;
 >>>     alias a this;
 >>>
 >>>     this()
 >>>     {
 >>>         a = new A();
 >>>     }
 >>> }
 >>>
 >>> void main()
 >>> {
 >>>     B b = new C();

Add this:

     takesBbyReference(b);

Now nobody knows whether the object has changed to something other than 
C. For example:

class Z : B
{}

void takesBbyReference(ref B b)
{
     b = new Z;
}

Now the first assert fails as well:

 >>>     assert(cast(C)b);

Ali