[dmd-beta] dmd 2.063 beta

[Kenji Hara]

2013/4/29 Sönke Ludwig <sludwig at outerproduct.org>

> I noticed that the following doesn't work anymore:
>
> ---
> class Test {
>     this()
>     {
>         // ...
>     }
> }
>
> void main()
> {
>     auto test = new shared Test; // Error: non-shared method
> shared_obj.Test.this is not callable using a shared object
>     //auto test cast(shared)new Text; // still works, but ugly and with
> dangerous semantics
> }
> ---
>

First of all, this is a new feature from 2.063 - qualified constructor
support.
In the definition of the class Test, mutable constructor this() could
construct only non-shared objects. Then you cannot create shared object by
using it.


> This one hurts a lot because it (again) destroys the only way to use
> shared objects in a generally acceptable way (i.e. without coding
> everything twice, once with "shared" and once without, and without casts
> everywhere):
>
> 1. Write the class without explicit "shared" support
> 2. Creates shared objects where needed using "new shared ClassName"
> 3. Use a special "lock()" function that locks the object and safely
> casts away shared during the lock


One proper way is adding shared constructor overload. Now it would be
correctly selected for the construction.
class Test {
    this() {}   // for non-shared mutable construction
    this() shared {}    // for shared mutable object construction
}
new Test();   // this() is used
new shared Test();  // this() shared is used

One another way is make the constructor 'pure'. If a pure constructor
satisfies some conditions, it would become to 'unique constructor'.

class Test {
    this() pure {}   //
}
new Test();   // this() pure is used
new shared Test();  // this() pure is used!

In default, this() is used for non-shared mutable object.
But pure attribute enforces that the constructed object does not hold any
"non-shared mutable global data" implicitly, then compiler can guarantee
that the constructed object is unique.
Unique object can be convertible to any qualifier, then implicit conversion
to shared is accepted.

Thanks.

Kenji Hara
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