C++ base class needs at least one virtual method

ezneh petitv.isat at gmail.com
Wed Feb 6 07:38:04 UTC 2019

On Wednesday, 6 February 2019 at 02:36:33 UTC, evilrat wrote:
> No surprises here, D can only inherit from virtual C++ classes 
> and that message says exactly that(class must have at least one 
> virtual method).
> This someclass looks for me as just a strange design decision, 
> if it's not just a POD data object (i.e. simple struct that is 
> declared as a class) then normally the class will have at least 
> virtual destructor to allow proper destruction.
> In case it is really just the POD class you can declare it as 
> struct, add it as a member and 'alias this'[1] it to provide 
> sort-of-inheritance chain (i.e. it will be implicitly accepted 
> using that alias part)
>     extern(C++)
>     struct someclass {}
>     extern(C++)
>     class otherclass {
>         someclass baseclass; // <- "inherited" data must go 
> before other members to preserve data layout
>         alias baseclass this; // <- pseudo inheritance alias
>         ...
>     }
> Though in some cases depending on semantic it is possible that 
> it might not work as expected. (Though I can't remeber any)
> If however otherclass is just a POD as well then you should 
> turn it into a struct as well, same for all descendants...

Thanks for the trick, I'll try  it and see how it goes.
Since the class have nothing in them, I just made some "alias 
otherclass = baseclass" statements and it seems it is working (at 
least it's compiling, have to really test that too). From what 
I saw about the code is that those types are just "placeholders" 
because they aren't used directly by the API.

> Just in case, there is few tools for generating bindings 
> available - dstep(AFAIK no C++)[2], dpp[3], gentool[4], and 
> more info on the wiki[5].

I know about those tools, but usually (and sadly enough) they 
either just ignore any extern(C++) or don't fully work on 
Windows. I didn't test gentool yet, so I might have more luck 
with it, but for the other ones they didn't provide full 
translation of the headers yet.

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