Writing the kind of code that you see in the Boost libraries is something of a step change from the day-to-day production code that I write, and I suspect most of us write (Joel, Eric, Dave, Peter and others excepted of course! :-) ), and writing it, I find, is a rather different matter from reading it and broadly understanding it! So,....enable_if.... Is enable_if intended only to include or exclude complementary instantiations of different templates from the possible set, or also to control whether a particular instantiation is available at all? Say I have a class which I want to be able to instantiate only for vectors. Is it sensible to create an is_vector<> template and then use enable_if, or should I simply allow instantiation to fail when the code uses some aspect of vectors which is specific to vectors? Does the situation change if it is vectors or lists that are permissible? Or if it's anything with begin() & end() defined? Or am I making a complete hash of this, and ought to be thinking in a different paradigm? Thanks in advance. - Rob.