extern (C++) including bodies of member functions?

[Jacob Carlborg via Digitalmars-d-learn]

On 2016-07-15 19:52, cy wrote:
> I would never (ever) do this myself, but trying to understand dmd, the
> code is absolutely packed with things like this:
>
> extern(C++) class Package : ScopeDSymbol
> {
> ...
>   override const(char)* kind() const
>   {
>      return "package";
>   }
> ...
>   override final inout(Package) isPackage() inout
>   {
>      return this;
>   }
> ...etc...
> }
>
> How exactly does that... work? Is the body of such a member function
> written in the D Programming Language?

Yes. Just as it's possible to call C function from D, it's possible to 
implement functions in D that can be called from C. This compatibility 
applies C++ and Objective-C as well.

When the D compiler sees a "extern(C++)" declaration it will adopt the 
code and output the data structures to be compatible with C++. For 
example, the location of the vtable, name mangling and so on.

> The documents say nothing about this, only defining their examples as
> stubs, like
> extern(C++) class Foo {
>   void bar();
> }
>
> The documentation also avoids any memory allocation in D, instead
> assuming C++ routines like createInstance to use malloc to allocate the
> classes. But the code in dmd makes extensive use of things like this:
> extern(C++) ... {
>   final extern(D) this(...) { ... }
> }

IIRC, it's not possible to call a C++ constructor from D. That's why 
"extern(D) this" is used to be able to do the memory allocation from the 
D side.

> How would you create new objects of types that are declared extern(C++)?
> Just new, and mark the constructor as extern(D)?
>
> I realize DMD is a horrible program that was Frankensteined from C++
> code ages ago and is practically still C++ itself

The front end was only fairly recently converted to D.

-- 
/Jacob Carlborg