Hi All, I am going through Chapter 3 of the MPL book ("A Deeper Look at Metafunctions") and have some questions on the placeholders _1 and _2. They are first introduced in the section 3.1.5 as follows: typename mpl::transform<D1, D2, mpl::minus<_1, _2> >::type I understand from the context that _1 represents D1 and _2 represents D2. From this I understood that _1 will represent the first template argument and _2 will represent the 2nd template argument. This made sense in this context. But later I came across the following in section 3.3: template <class X> struct two_pointers : twice<boost::add_pointer<_1>, X> {}; In this context, it looks like _1 represents X, which is the 2nd template argument. So what exactly _1 and _2 represents? The book doesn't seem to define them. While I haven't completely read the book yet, if it is defined at some other place, it would have been helpful to provide a cross-reference when they are first introduced in chapter 3. Can anybody provide a reference that explains what is a placeholder and what _1 and _2 mean? And what is the maximum number of placeholders? Thank you. Rgds, anna -- Two Party Fraud: Staged Presidential Debates http://missingrainbow.blogspot.com/2008/10/two-party-fraud-staged-presidenti...