Concepts in modern C++

Interesting article from DrDobs

enable_if is quite simple in and of itself -- if the first template parameter (which must be a boolean constant expression) evaluates to true then the nested "type" member is a typedef to the second template parameter. If the the first parameter evaluates to False then there is no nested "type" member.

The SFINAE rules mean that if enable_if::type is used in the signature of a function template (as here), then that function overload is discarded if some-expression is False -- that overload is only enabled if some-expression is True.