C++ base class needs at least one virtual method

evilrat evilrat666 at gmail.com
Wed Feb 6 02:36:33 UTC 2019

On Tuesday, 5 February 2019 at 14:23:00 UTC, ezneh wrote:
> Hello
> While trying to make a 1:1 binding to a C/C++ lib, I got the 
> following issue:
> I only have access to the .h header file, and in there I have 
> this:
> class someclass {};
> class otherclass : public someclass {};
> When trying to translate that header file to D, I ended with 
> the following code:
> extern(C++):
> class someclass {}
> class otherclass : someclass {}
> But when compiling, the following error comes up:
> Error: class `modulename.otherclass` C++ base class someclass 
> needs at least one virtual function
> Am I missing something or is there a bug somewhere ?
> I don't have access to the .cpp file and cannot see what are 
> the methods of someclass / otherclass are.
> Any suggestion ?

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)

     struct someclass {}

     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...

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].

[1] https://dlang.org/spec/class.html#alias-this
[2] https://github.com/jacob-carlborg/dstep
[3] https://github.com/atilaneves/dpp
[4] https://github.com/Superbelko/ohmygentool
[5] https://wiki.dlang.org/Bindings

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