Toon Knapen wrote:

Inside a templated class, I need to provide two different implementations for a function based on the compile-time decision ::boost::is_convertible< value_type,double >::value. How can I do this using MPL ?

1) A simple way: template< typename T > class my { public: typedef typename T::value_type value_type; void foo() { typedef boost::is_convertible< value_type,double > c; do_foo(mpl::bool_c()); } private: void do_foo(mpl::true_c) { /*...*/ } void do_foo(mpl::false_c) { /*...*/ } }; Andrei has written an article that covers this technique - http://www.cuj.com/experts/1810/alexandr.htm. 2) A sophisticated one (becomes more relevant when you have several "foo"'s to handle): template< typename T, typename Derived > struct foo_impl { void foo() { Derived& self = static_cast(*this); // ... } }; template< typename Derived > struct foo_double_impl { void foo() { Derived& self = static_cast(*this); // ... } }; template< typename T > class my : public mpl::select_if< boost::is_convertible< typename T::value_type,double > , foo_impl , foo_double_impl<T> >::type { // ... private: friend class foo_impl; friend class foo_double_impl<T>; // ... }; HTH, Aleksey