Re: [boost] Re: [any] ostream inserter

From: Sérgio Vale e Pace (svpace.forum_at_[hidden])
Date: 2005-01-26 07:13:14

 Jonathan Turkanis wrote:
> Sérgio Pace wrote:
> > Hi,
> >   I was playing with the any.hpp library trying to add a generic
> > inserter to an ostrem. To acomplish that I created a pure virtual
> > method print in the placeholder:
> 
>     <snip code>
> 
> > .. I had a nice
> > surprise to find out that as long I don´t actually invoke the inserter
> > on non inserter implementing contained objects everything work fine,
> > at least in bcc 5.5 and gcc(mingw) 3.4.2. when I do use the inserert
> > operator for a nom implementing contained object I get a stack
> > overflow exception ...
> 
> Your code looks fine, and works for me on VC7.1, gcc 3.4.1 (Cygwin) and Intel
> 8.0 for windows.
> 
> Two comments, though:
> 
> 1. Your code breaks boost::any for use with non-OutputStreamable types.

that´s the odd part it didn´t. as long I don´t use the insert operator
i works fine and I don´t understant why it don´t break. see:

class A {};

void foo() {
  A a; int b=0; any x;
  x = b;
  cout << x;
  b = any_cast<int>(x);

  x = a;
  // cout << x; // as long as this line is commented everything works fine
  a = any_cast<A>(x);
}

on the other hand if I do invoke the insert operator (<<) on a
non-OutputStreamable type I get a stack overflow also I don´t
understand why.

> ...You can
> fix this by dispatching on whether a type is output OutputStreamable, using the
> metafunction is_default_insertable, here:

  great I was looking for some way of doing that. template
metaprogramming rules! If I understood correctly you create a last
choice overloaded operator<< that return tag and verify if check gets
an ostrem or a tag is that correct?

> 2. You should probably support narrow and wide streams.

good idea

> >   Beeing able to strem out a unknow object is a huge plus in my
> > current project and I thing it will be a great addition to the
> > library, but I´m wondering how portable (standard compliant?) is my
> > solution and why this recursive call happens with non inserter
> > objects.
> 
> I'm about to post an announcement for an interface library which can be thought
> of as a generalization of Boost.Any. Very roughly, while an instance of
> boost::any can be bound to any object (actually any instance of a value type),
> an interface reference can be bound to any object of a type which
> (non-intrusively) implements the interface. Although I haven't implemented
> operator overloading yet, use will be as follows:

I am following the "BIL" discussion for a while now, and I´m looking
forward to put my hands on it

> > Sérgio
> 
> Jonathan