Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Passes through anything that is already deferred.
template<
typename T>
constraint< is_deferred< typename decay< T >::type >::value, typename decay< T >::type >::type operator()(
T && t) const;