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

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 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. :-)