[C++-sig] [Py++] Does pyplusplus respect explicit constructors?

[Roman Yakovenko]

On Thu, Oct 1, 2009 at 5:46 PM, Christopher Bruns <cmbruns at stanford.edu> wrote:
> I disagree.  Py++ does have a way to find out.  The only time
> "explicit" matters is when a constructor takes exactly one argument.
> gccxml sets "explicit=1" in all cases when the number of arguments is
> other than 1.  That is why you saw "explicit=1" in all of the
> constructors in the previous example.  However, in the case where
> there is exactly one argument, gccxml does the right thing.  Consider
> the following case:
>
> ### test.h ####
> struct Foo1 {};
> struct Foo2 {};
>
> struct Bar {
>    explicit Bar(Foo1&); // no implicit conversion
>    Bar(Foo2&); // allows implicit conversion
> };
> #############
>
>
> ### fragment of resulting xml file from gccxml ###
>  <Constructor id="_10" name="Bar" explicit="1" context="_3"
> access="public" mangled="_ZN3BarC1ER4Foo1 *INTERNAL* "
> demangled="Bar::Bar(Foo1&amp;)" location="f0:5" file="f0" line="5"
> extern="1">
>    <Argument type="_22" location="f0:5" file="f0" line="5"/>
>  </Constructor>
>  <Constructor id="_11" name="Bar" context="_3" access="public"
> mangled="_ZN3BarC1ER4Foo2 *INTERNAL* " demangled="Bar::Bar(Foo2&amp;)"
> location="f0:6" file="f0" line="6" extern="1">
>    <Argument type="_23" location="f0:6" file="f0" line="6"/>
>  </Constructor>
> #######################
>
> Notice that gccxml sets "explicit=1" for the first Bar constructor,
> but not for the second.

I didn't know that, so pygccxml didn't provided access to the information.

> However, Py++ sets "allow_implicit_conversion" to True for both
> constructors.  I think gccxml is doing the right thing here.  But I am
> not so sure about Py++ behavior.

Well, these days I would implement "allow_implicit_conversion" to be a
mix of the user desire and "explicit" attribute. But, this change will
introduce backward compatibility problem, which I would like to avoid.

> I would have assumed that the first
> Bar constructor would have "allow_implicit_conversion" set to False.
> I was wrong about being able to use "allow_implicit_conversion" for
> this task.  Is there another way to get at the "explicit" xml tag from
> the module_builder object?  Or do I need to parse the xml file or
> source code separately to discover this information?

No. I updated pygccxml package
http://pygccxml.svn.sourceforge.net/pygccxml/?rev=1761&view=rev

All you need is to write the following code:

mb = module_builder_t(...)
mb.constructors( lambda c: c.explicit == True
).allow_implicit_conversion = False

That's all

> Thank you Roman for your diligent feedback.  I sincerely appreciate
> the quick and thoughtful feedback you provide to everyone on this
> mailing list.

Wow :-). Thank you for such words!

-- 
Roman Yakovenko
C++ Python language binding
http://www.language-binding.net/