Re: [Boost-users] boost lambda weirdness (possible bug?)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-02-04 13:02:57

David Sankel:

...

> class T
> {
> public:
>    char m_c;
>    T( char c ) : m_c( c )
>    {
>    }
>    virtual T * clone() const=0;
>    virtual ~T(){}
> };
> typedef boost::shared_ptr<T> TPtr;
> typedef std::vector<TPtr> TVec;
> using namespace boost::lambda;
>
> This works:
>
>    const boost::function< void( T const * const) > f =
>        bind( &TVec::push_back, &tVec,
>            bind( constructor<TPtr>(),
>                bind( &T::clone, _1)));
>
> But this doesn't:
>
>    const boost::function< void( const T & ) > g =
>        bind( &TVec::push_back, &tVec,
>            bind( constructor<TPtr>(),
>                bind( &T::clone, boost::cref(_1) )));

Note that cref(_1) isn't valid; the parameters are already taken by 
reference, _1 should suffice.

> The compilation error I'm getting is that it cannot create a new
> object of type T and that is expected since T is abstract. Why would
> lambda be making copies instead of passing on the reference?

This is a known Lambda bug; internally, it tries to create the type tuple<T, 
_1_type> and fails. If you add

template<class T> struct constructor
{
    typedef T result_type;

    template<class A1> T operator()( A1 const & a1 ) const
    {
        return T( a1 );
    }
};

you can compile the above with boost::bind, although a better solution would 
probably be to rewrite it with a loop. :-)