Re: [boost] Boost::python: implicit conversions

From: David Abrahams (david.abrahams_at_[hidden])
Date: 2001-07-17 12:18:58

----- Original Message -----
From: <Peter.Bienstman_at_[hidden]>
> Consider
>
>   class_builder<Term> Term_(module, "Term");
>   Term_.def(constructor<>());
>
>   class_builder<Expression> Expression_(module, "Expression");
>   Expression_.def(constructor<>());
>   Expression_.def(constructor<const Term&>());
>
>   class_builder<User> User_(module, "User");
>   User_.def(constructor<const Expression&>();
>
> Using the original C++ code, I could write
>
>   Term t=Term();
>   User u=User(t);
>
> , because C++ is smart enough to figure out how to construct an
> Expression from a Term.
>
> Now, trying the same in Python, we get
>
> >>>t = Term()
> >>>u = User(t)
>
> TypeError: extension class 'Term' is not convertible
> into 'Expression'.
>
> Short of modifying the original C++ code to add a User::User(const
> Term&) constructor, is there anything I can do to get the Boost
> typesystem to apply Expression::Expression(const Term&) for me?

I am assuming that Term can be implicitly converted to (but is not derived
from) Expression here. I think that in this case, you can just add the
following:

  User_.def(constructor<const Term&>);

OTOH, that will only work for constructors. I think that getting it to work
for other functions in general is a problem closely related to the default
arguments issue also explored here recently.