SFINAE
SFINAE stands for Substitution Failure Is Not An Error, and is a principle essential to the good function of compilers in the identification of function templates. Basically, SFINAE states that if the specialization of a function template for a given template argument would result in an error, it is not deemed to be an error so long as a "good" alternative is available. Thankfully, having a deep and profound understanding of SFINAE is not necessary to making good use of it, as ably demonstrated by the tendency of this author to forget the fine details within a few minutes of comprehension, and yet still craft adaptive code that depends on it. Excellent explanations are to be found in [4] and [5].