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)
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...
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
