[C++-sig] virtual functions with default implementation

[Stefan Seefeld]

On 17/08/15 02:12 PM, Nikolay Mladenov wrote:
> If you export an abstract class, create an object from python and call
> its virtual from C++ it should not work without the default
> implementation.

The tutorial explains that well:

for the case of an abstract base, use

  def("func", bpl::pure_virtual(&Base::func))

which will provide an implementation that does nothing but raise a
Python exception to tell the caller that the method isn't implemented.

For the case with default implementation, the tutorial gives this wrapper:

  struct BaseWrap : Base, bpl::wrapper<Base>
  {
    virtual std::string func() const
    {
      if (bpl::override f = this->get_override("func"))
        return f();
      else
        return Base::func();
    }
    std::string default_func() const { return this->Base::func();}
  };

and it seems to me much more natural to implement the "default" case
directly in the "else" branch above, rather than add it to the "def()" call.

        Stefan


>
> On Mon, Aug 17, 2015 at 8:03 AM, Stefan Seefeld <stefan at seefeld.name
> <mailto:stefan at seefeld.name>> wrote:
>
>     Hi,
>
>     I'm reviewing the Boost.Python tutorial, and I'm stumbling over the
>     section on "Virtual Functions with Default implementations"
>     (http://boostorg.github.io/python/doc/html/tutorial/tutorial/exposing.html#tutorial.exposing.virtual_functions_with_default_i).
>     It mentions the need to provide a separate default implementation
>     function as third argument to "def()", without explaining why that is
>     needed.
>     In fact, I'm trying various alternatives (abstract, non-abstract),
>     and I
>     can't find a need for it. All my tests work fine without it.
>
>     Does anyone know why this is needed, and could perhaps even provide a
>     little test case ?
>
>     Thanks,
>             Stefan
>
>     --
>
>           ...ich hab' noch einen Koffer in Berlin...
>
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      ...ich hab' noch einen Koffer in Berlin...