#1485 Change COM class implementation from C to C++

[Zufu Liu]

I can't find previous discussion (around 2018) for this with the search box.

COM classes (these with VFunction **vtbl; field) inside ScintillaWin.cxx can be changed to C++ class similar to class BlobInline : public IDWriteInlineObject in PlatWin.cxx. I think previous concern about GCC can't generate correct vtable (or class layout?) no longer applies.

[Zufu Liu]

reinterpret_cast<IXXX> can be removed after implement them with C++ inheritance.

[Zufu Liu]

Patch for the changes.

[Neil Hodgson]

Why are the default virtual destructors needed? I expect they can be omitted following rule-of-zero with final added to the classes to show that they can't be derived from:

class DropSource final : public IDropSource {
public:
    ScintillaWin *sci = nullptr;
    DropSource() noexcept = default;

    // IUnknown
    //...
};

[Zufu Liu]

virtual destructor copied from class BlobInline, for these in ScintillaWin.cxx both constructor and destructor can be omitted.

Patch added final, removed unneeded constructor and destructor.

[Zufu Liu]

There are other virtual destructors can also be removed, rg virtual\s*~.

[Neil Hodgson]

Many of these occurrences will be destroyed through deleting a derived class through a base class pointer. COM objects are deleted by calling Release (which calls delete from inside the class) and must never be deleted by other code.

It may be worthwhile finding other cases for rule-of-zero but that can rely on implementation subtleties. There'll also be linter warnings when defining base classes without virtual destructors as COM -style interfaces are not understood by linters.

[Zufu Liu]

It may be worthwhile finding other cases

What about class CaseConverter in CaseConvert.cxx?

[Neil Hodgson]

OK [58cf34].

Also changed CharacterIndexer / DocumentIndexer with [fc2f4c] as that is a simple case.

[Neil Hodgson]

Committed with [72cb31] and [074546].