On Thu, Oct 4, 2012 at 1:28 PM, Matt Calabrese <rivorus@gmail.com> wrote:
When the expression isn't valid, then substitution when trying to match the specialization fails, meaning it falls back to the default defintiion. When the expression is valid, the specialization is a better match because the last parameter (void) is an exact match with the template parameters that were passed (void was implicitly passed as the default parameter because of the original template declaration). It's the same way enable_if works only we don't need a boolean condition since we want the specialization to be picked for any time the expression is valid, regardless of the result type.
I made some enable if macros a while ago to make expresion checks like this simpler, I'll boostify them and put them up on the sandbox. -- -Matt Calabrese