http://d.puremagic.com/issues/show_bug.cgi?id=5380

           Summary: Subtyping with "alias this" doesn't mix with regular
                    inheritance
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: samukha@voliacable.com


--- Comment #0 from Max Samukha <samukha@voliacable.com> 2010-12-27 03:59:11 PST ---
class A
{
}

class B
{
    A a;
    alias a this;
}

class C : B
{
}

void main()
{
    A a = new C; // error
}

Error: cannot implicitly convert expression (new C) of type test.C to test.A

Since C is a subtype of B (via inheritance) and B is a subtype of A (via "alias
this"), C should be implicitly convertible to A.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla@digitalmars.com


--- Comment #1 from Walter Bright <bugzilla@digitalmars.com> 2010-12-27 13:33:28 PST ---
Yes, I see the problem. Never thought of that.

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--- Comment #2 from Max Samukha <samukha@voliacable.com> 2012-11-28 07:16:16 PST ---
This one is blocking a design for QtD (and potentially other C++ projects using
single inheritance with interfaces) that should allow at the cost of relatively
small constant overhead to avoid:
1) unnecessary and undesirable allocations or lookups of wrapper objects for
object pointers returned by the C++ code.
2) bloating the library with virtual call dispatchers for objects that are
never inherited in D code.

Very roughly:

// Instead of classes, fat pointers are generated for C++ classes:
struct QObject
{
    bool isDWrapper;
    void* pointerToCppObject;

    // a virtual
    void foo() {
        if (isDWrapper)
            // dispatch to the D override
        else
            // make static call to C++
    }

    enum dispatchers = q{
        void foo() {
            // dispatch to C++
        }
    };
}

// Library
class Inherit(T)
{
    T ref_;
    alias ref_ this;
    mixin(T.dispatchers);

    this() { isDWrapper = true; }
}

// User
class MyObject : Inherit!QObject
{
    override void foo()
    {
        ...
        super.foo();
        ...
    }
}

// Some function taking a QObject
void bar(QObject obj)
{
    obj.foo();
}

void main()
{
    // Here is the problem: MyObject is not a subtype of QObject
    bar(new MyObject);
}

The Inherit template subtypes the struct and provides the implementation of the overrides dispatching calls to the C++ side. If the user does not inherit from a Qt class (which will be true for most classes), the overrides are never compiled into the library. When receiving an object from the C++ side, we do not need to allocated a wrapper objects, just initialize the fat pointer with the relevant information.

I am not sure whether the craziness of the above is going to make it into the project but a prototype I've created does work.

Now that we can link to 64-bit C++ libraries directly on windows, we *maybe* could try to use (and improve) extern(C++) interfaces in order to avoid the complications. I am still not sure what to do with 32-bit Windows and would rather give a try to the design described.

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Max Samukha <samukha@voliacable.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |critical


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