[boost] Re: Iterator adaptors <-> boost::bind interaction

From: David Abrahams (dave_at_[hidden])
Date: 2005-05-24 14:30:23

Aleksey Gurtovoy <agurtovoy_at_[hidden]> writes:

> It became way too easy to write
> an innocently looking transformation that in fact invokes undefined
> behavior -- and the latter is not even diagnosed by the most compilers
> (neither at compile nor at run time). Consider, for instance, this:
>
>     #include <boost/iterator/transform_iterator.hpp>
>     #include <boost/bind.hpp>
>     #include <algorithm>
>
>     struct X
>     {
>         X( int i = 7 ) : i_( i ) {}
>         ~X() { this->i_ = -1; }
>         int const& get_i() const { return this->i_; }
>  
>      private:
>         int i_;
>     };
>
>     struct Y { X x; };
>     X make_x( int i ) { return X( i ); }
>
>     int main()
>     {
>         int i[] = { 8 };
>         Y y[1];
>
>         // undefined behavior #1 (returning reference to a temporary)
>         int r = *boost::make_transform_iterator(
>               boost::make_transform_iterator( i, std::ptr_fun( &make_x ) )
>             , std::mem_fun_ref( &X::get_i )
>             );
>
>         assert( r == 8 );
>
>         // undefined behavior #2 (returning reference to a temporary)
>         r = *boost::make_transform_iterator(
>               boost::make_transform_iterator( y, boost::bind( &Y::x, _1 ) )
>             , std::mem_fun_ref( &X::get_i )
>             );
>
>         assert( r == 7 );
>     }
>
>
> IMO this is way too subtle and error-prone to leave it as
> is. Opinions?

Well I'm afraid there's too much going on in that code for me to see
where the problems are without some narrative.  You could take that as
an argument that it's too subtle, but personally I would never write
something that involved and "just expect it to work," so I don't see
it that way.  Maybe you could add some commentary?

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