Re: alias this & cast
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
Re: alias this & cast
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 +
andre via Digitalmars-d-learn
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);
}
Re: alias this & cast
V Thu, 11 Sep 2014 11:40:05 +
andre via Digitalmars-d-learn
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);
> }
alias this & cast
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);
}
alias this : cast turns into endless loop
Trying with
U opCast(U : Object)() {
return new U();
}
work fine. It isn't a solution, but i don't understand why this
works and the following not:
U opCast(U : Object)() {
return cast(U) this._obj;
}
I hope really some of you can explain that to me and help me to
fix it.
And forgive me the old senseless thread title. ;)
