Hi,

I am 80% sure, the failing assertion is correct but please have a look.
Second assertion fails.

Kind regards
André

class A{}

class B{}

class C : B
{
	A a;
	alias a this;
	
	this()
	{
		a = new A();
	}
}

void main()
{
	B b = new C();

	// OK
	assert(cast(C)b);
	
	// fails
	assert(cast(A)b);	
}

V Thu, 11 Sep 2014 11:40:05 +0000
andre via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
napsáno:

> Hi,
> 
> I am 80% sure, the failing assertion is correct but please have a look.

No it is not

assert(cast(A)cast(C)b); // this is OK

b is B so it does not know about having alias to A;

> Second assertion fails.
> 
> Kind regards
> André
> 
> class A{}
> 
> class B{}
> 
> class C : B
> {
> 	A a;
> 	alias a this;
> 
> 	this()
> 	{
> 		a = new A();
> 	}
> }
> 
> void main()
> {
> 	B b = new C();
> 
> 	// OK
> 	assert(cast(C)b);
> 
> 	// fails
> 	assert(cast(A)b);
> }

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.

For me, using alias this, the object b has 3 represenations: A, B and C. It is a matter of steps.

Kind regards
André

On Thursday, 11 September 2014 at 11:53:30 UTC, Daniel Kozak via Digitalmars-d-learn wrote:
> V Thu, 11 Sep 2014 11:40:05 +0000
> andre via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
> napsáno:
>
>> Hi,
>> 
>> I am 80% sure, the failing assertion is correct but please have a look.
>
> No it is not
>
> assert(cast(A)cast(C)b); // this is OK
>
> b is B so it does not know about having alias to A;
>
>> Second assertion fails.
>> 
>> Kind regards
>> André
>> 
>> class A{}
>> 
>> class B{}
>> 
>> class C : B
>> {
>> 	A a;
>> 	alias a this;
>> 	
>> 	this()
>> 	{
>> 		a = new A();
>> 	}
>> }
>> 
>> void main()
>> {
>> 	B b = new C();
>> 
>> 	// OK
>> 	assert(cast(C)b);
>> 	
>> 	// fails
>> 	assert(cast(A)b);	
>> }

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