[C++-sig] Re: value_holder_back_reference::holds()
dave at boost-consulting.com
Sun Nov 2 20:37:56 CET 2003
"Niall Douglas" <s_sourceforge at nedprod.com> writes:
> This goes as follows:
> template <class Value, class Held>
> void* value_holder_back_reference<Value,Held>::holds(
> type_info dst_t)
> type_info src_t = python::type_id<Value>();
> Value* x = &m_held;
> if (dst_t == src_t)
> return x;
> else if (dst_t == python::type_id<Held>())
> return &m_held;
> return find_static_type(x, src_t, dst_t);
> Q: Why is find_static_type() used here? Shouldn't it use
> find_dynamic_type() when the value is polymorphic?
No, we already know the most-derived type it can be: Held. Unless
you've introduced multiple levels of inheritance between Value and
Held (you're not supposed to do that) there's no way you can
correctly downcast to anything else from Value.
> I only ask because I am passing "self" in python to a class
> constructor which expects a FXComposite pointer.
There's too much information missing and the little you've provied is
too vague. How are you passing it? What is a "class constructor?"
Metaclasses construct classes when called. Do you actually mean an
Instead of trying to describe it in English, please just show a small
> self is python class
Really, a class? Not an instance of the class?
> MainWindow which inherits off C++ class FXMainWindow
More accurately, I presume: you derived self.__class__ in Python from
the Python class wrapper for the C++ class FXMainWindow.
> which someway down the line inherits off FXComposite, so the above
> should work.
I don't understand what you mean by "the above should work". When
constructing a C++ class instance in a Python object, there is no
check to see that the Python object already holds an instance of the
C++ class, since, after all, the C++ object has not been
> Unfortunately find_static_type() is not finding the FXComposite and
> is returning zero. If however I alter the "polymorphic" parameter in
> convert_type() (called by find_static_type()) to true, it *does* find
> the FXComposite correctly. Of course FXComposite has virtual
> functions, therefore a virtual function table for RTTI to work with.
That surprises me, but I guess anything is possible.
> If this is not known behaviour (it seems odd that find_static_type()
> is not finding base classes), I'll get to work on a repeatable
> example. Just say the word.
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