diff options
Diffstat (limited to 'boost/hana/fwd/type.hpp')
| -rw-r--r-- | boost/hana/fwd/type.hpp | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/boost/hana/fwd/type.hpp b/boost/hana/fwd/type.hpp index d3390cf5bd..c3c220db78 100644 --- a/boost/hana/fwd/type.hpp +++ b/boost/hana/fwd/type.hpp @@ -2,7 +2,7 @@ @file Forward declares `boost::hana::type` and related utilities. -@copyright Louis Dionne 2013-2016 +@copyright Louis Dionne 2013-2017 Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) */ @@ -37,18 +37,25 @@ BOOST_HANA_NAMESPACE_BEGIN //! //! //! @note - //! For subtle reasons having to do with ADL, the actual representation of - //! `hana::type` is implementation-defined. In particular, `hana::type` - //! may be a dependent type, so one should not attempt to do pattern - //! matching on it. However, one can assume that `hana::type` _inherits_ - //! from `hana::basic_type`, which can be useful when declaring overloaded - //! functions: + //! For subtle reasons, the actual representation of `hana::type` is + //! implementation-defined. In particular, `hana::type` may be a dependent + //! type, so one should not attempt to do pattern matching on it. However, + //! one can assume that `hana::type` _inherits_ from `hana::basic_type`, + //! which can be useful when declaring overloaded functions: //! @code //! template <typename T> //! void f(hana::basic_type<T>) { //! // do something with T //! } //! @endcode + //! The full story is that [ADL][] causes template arguments to be + //! instantiated. Hence, if `hana::type` were defined naively, expressions + //! like `hana::type<T>{} == hana::type<U>{}` would cause both `T` and `U` + //! to be instantiated. This is usually not a problem, except when `T` or + //! `U` should not be instantiated. To avoid these instantiations, + //! `hana::type` is implemented using some cleverness, and that is + //! why the representation is implementation-defined. When that + //! behavior is not required, `hana::basic_type` can be used instead. //! //! //! @anchor type_lvalues_and_rvalues @@ -86,6 +93,8 @@ BOOST_HANA_NAMESPACE_BEGIN //! The hash of a type is just that type itself. In other words, `hash` //! is the identity function on `hana::type`s. //! @include example/type/hashable.cpp + //! + //! [ADL]: http://en.cppreference.com/w/cpp/language/adl #ifdef BOOST_HANA_DOXYGEN_INVOKED template <typename T> struct type { |