Fw: [C++-sig] Pyste and STL types

[David Abrahams]

Nicodemus <nicodemus at globalite.com.br> writes:

> Now I see what you mean. Indeed, that would be a nice interface.
> One problem that I see is that there's no way to know when the
> "active" member changes in the C++ side, unless the user gives a
> mechanism that allows us to know, like you suggested. 


Right.

> But then the user would have to change the wrapping code to update
> this mechanism. Consider this:
>
> struct Test
> {
>     bool is_float;
>     union
>     {
>         float d;
>         char* s;
>     } val;
> };
>
> void use(Test& t)
> {
>     t.val.d = 3.0;
> }
>
> There's no way to know that now the active member is "d". The user
> would have to change it to:
>
> void use(Test& t)
> {
>     t.val.d = 3.0;
>     t.is_float = true;
> }

My point is that if there's no way to know that the active member is
"d", the union is useless anyway, so the user "always" has some member
like is_float which she is keeping in synch to indicate the correct
member to access.

> (unless this is a common practice: like I said before, I know little
> about unions). 

I'm saying it's a common practice.  How would you use the union
otherwise?

> So I don't think this solution is optimal. Sure, the user could
> provide wrappers for functions like "use", but it's a lot of work
> compared to the gains, in my opinion. 

That's debatable.  Part of the philosophy of Boost.Python is that you
should be able to wrap C++ code in such a way that it can't be abused
from Python to cause a crash.  Some unions are like that naturally,
e.g.:

        union { int x; char bytes[sizeof(int)]; };

but others like the union in Test above are not.

> Given this, I think exporting it to python as it is seen in C++ is
> enough. Don't get me wrong: I would gladly implement in Pyste a
> system to export unions like you suggested (I'm not lazy), but I
> don't know if it is possible to do it in a reasonable way for the
> user.

It requires the user to write some extra code.  Worth it?  I don't
know; I hardly ever use unions myself ;-)

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com