Boost C++ Application Development Cookbook - Second Edition

How to do it...

The Boost.MPL metafunctions are actually structures that can be easily passed as a template parameter. The hard part is to use it correctly:

1. We need the following headers to write a higher-order metafunction:

#include <boost/mpl/apply.hpp>#include <boost/mpl/if.hpp>#include <boost/type_traits/is_same.hpp>

2. The next step is to evaluate our functions:

template <class Param1, class Param2, class Func1, class Func2>struct coalesce {    typedef typename boost::mpl::apply<Func1, Param1>::type type1;    typedef typename boost::mpl::apply<Func2, Param2>::type type2;

3. Now, we need to choose the correct result type:

    typedef typename boost::mpl::if_<        boost::is_same< boost::mpl::false_, type1>,        type2,        type1    >::type type;};

That's it! We ...