Line *1 isn't doing the same thing as *2 -- see the documentation on nested binds [1].

In short, *1 is evaluating a() (which returns void), then attempting to pass that into the outer bind.  I'm not sure if cref is an acceptable call to use there -- I don't know if it would always work.  It would be more idiomatic to use protect:

bind(b, boost::protect(bind(a)))

All assuming that your actually case is more complicated than shown here, and can't be replaced with:

bind(b, a);
 

HTH,
Nate