Linh Ha writes:

...

Just on a first glance, looks like you're mistaking ::_1 (placeholder from Boost.Bind) for boost::mpl::_1 (placeholder from Boost.MPL). They're incompatible; try using the latter...?

- Jeff

Thanks, Jeff !

It works. But then my next question is why they are different and somehow incompatible even-though are both placeholders. Why we keep two different place holder if they do exactly a same functionality.

There are far more than just two placeholders in Boost. AFAIK Phoenix, Lambda and Bind come with their own. The mpl placeholder is different though: It is a placeholder for a type (it's a meta-programming library after all) and not for a value. Try to write a MPL lambda, that might clear up some of the confusion. -- Philipp Moeller, GeometryFactory