Interfacing with XPCOM

[Sergey Gromov]

Sun, 7 Dec 2008 07:04:22 +0000 (UTC), John Reimer wrote:

> Hello Jason,
> 
>> Sergey Gromov wrote:
>> 
>>> But what's the typical use case for extern(C++) objects ?  I think
>>> it's something like this:
>>> 
>>> extern(C++) class CppInterfaceClass { ... }
>>> 
>>> extern(C++) CppInterfaceClass create()
>>> {
>>> return new CppInterfaceClass(...);
>>> }
>>> extern(C++) void destroy(CppInterfaceClass obj)
>>> {
>>> delete obj;
>>> }
>> while extern(C++) will accept that code, but it won't work right.
>> 
> 
> (Sorry, I've been away for a bit)
> 
> I don't think the example is how extern(C++) was designed to work either.
> 
> When used for C++ classes, extern(C++), from what I gather, is intended to 
> be applied to an /interface/ in the D code, NOT a D class:
> 
> extern(C++) interface Foo { 
>     int foo(int a, b);
> }
> 
> This interface represents the link to the external C++ class.  An instance 
> of that class is created by the C++ code using a factory function which D 
> links to in its code (BUT does not implement):

This is one possible use.  But the opposite use is also perfectly
possible.  The ultimate example of the opposite use is a COM object
implemented in D.

> Of course, the documentation on extern(C++) already discusses this.  I just 
> thought it should be made clear here since the prior code sample implies 
> that the C++ object creation and destruction are done in D, which it cannot 
> be.

Of course it can.  My example wasn't quite correct though.  Assuming
your definition of interface Foo, here is the implementation:

> extern(C++) interface Foo { 
>     int foo(int a, int b);
> }

class FooImpl : Foo {
   override int foo(int a, int b) {  // implicit extern(C++), correct VTAB placement
        return a + b;
    }
}

extern(C++) Foo createFoo() {
    return new FooImpl;
}

extern(C++) void destroyFoo(Foo foo) {
    delete foo;
}

The only problem with this code is that FooImpl will be GCed as soon as
it leaves the createFoo().