hphp/util/match.h

   1 /*
   2    +----------------------------------------------------------------------+
   3    | HipHop for PHP                                                       |
   4    +----------------------------------------------------------------------+
   5    | Copyright (c) 2010-present Facebook, Inc. (http://www.facebook.com)  |
   6    +----------------------------------------------------------------------+
   7    | This source file is subject to version 3.01 of the PHP license,      |
   8    | that is bundled with this package in the file LICENSE, and is        |
   9    | available through the world-wide-web at the following url:           |
  10    | http://www.php.net/license/3_01.txt                                  |
  11    | If you did not receive a copy of the PHP license and are unable to   |
  12    | obtain it through the world-wide-web, please send a note to          |
  13    | license@php.net so we can mail you a copy immediately.               |
  14    +----------------------------------------------------------------------+
  15 */
  16 #ifndef incl_HPHP_UTIL_MATCH_H_
  17 #define incl_HPHP_UTIL_MATCH_H_
  18
  19 #include <type_traits>
  20 #include <utility>
  21
  22 #include <boost/variant.hpp>
  23
  24 #include <folly/DiscriminatedPtr.h>
  25
  26 namespace HPHP {
  27
  28 //////////////////////////////////////////////////////////////////////
  29
  30 /*
  31  * This is a utility for short-hand visitors (using lambdas) with
  32  * boost::apply_visitor.
  33  *
  34  * Usage e.g.:
  35  *
  36  *   match<return_type>(
  37  *     thing,
  38  *     [&] (TypeA a) { ... },
  39  *     [&] (TypeB b) { ... }
  40  *   );
  41  */
  42
  43 //////////////////////////////////////////////////////////////////////
  44
  45 namespace match_detail {
  46
  47 template<class Ret, class... Lambdas> struct visitor;
  48
  49 template<class Ret, class L, class... Lambdas>
  50 struct visitor<Ret,L,Lambdas...> : L, visitor<Ret,Lambdas...> {
  51   using L::operator();
  52   using visitor<Ret,Lambdas...>::operator();
  53   visitor(L l, Lambdas&&... lambdas)
  54     : L(l)
  55     , visitor<Ret,Lambdas...>(std::forward<Lambdas>(lambdas)...)
  56   {}
  57 };
  58
  59 template<class Ret, class L>
  60 struct visitor<Ret,L> : L {
  61   typedef Ret result_type;
  62   using L::operator();
  63   /* implicit */ visitor(L l) : L(l) {}
  64 };
  65
  66 template<class Ret> struct visitor<Ret> {
  67   typedef Ret result_type;
  68   visitor() {}
  69 };
  70
  71 template<class Ret, class... Funcs>
  72 visitor<Ret,Funcs...> make_visitor(Funcs&&... funcs) {
  73   return { std::forward<Funcs>(funcs)... };
  74 }
  75
  76 template <typename T> struct is_variant {
  77   static constexpr bool value = false;
  78 };
  79 template <typename... T> struct is_variant<boost::variant<T...>> {
  80   static constexpr bool value = true;
  81 };
  82
  83 template <typename T> struct is_discriminated_ptr {
  84   static constexpr bool value = false;
  85 };
  86 template <typename... T>
  87 struct is_discriminated_ptr<folly::DiscriminatedPtr<T...>> {
  88   static constexpr bool value = true;
  89 };
  90
  91 }
  92
  93 template<class Ret, class Var, class... Funcs>
  94 typename std::enable_if<
  95   match_detail::is_variant<
  96     typename std::remove_cv<Var>::type
  97     >::value,
  98   Ret>::type
  99 match(Var& v, Funcs&&... funcs) {
 100   return boost::apply_visitor(
 101     match_detail::make_visitor<Ret>(std::forward<Funcs>(funcs)...),
 102     v
 103   );
 104 }
 105
 106 template<class Ret, class Var, class... Funcs>
 107 typename std::enable_if<
 108   match_detail::is_discriminated_ptr<
 109     typename std::remove_cv<Var>::type
 110     >::value,
 111   Ret>::type
 112 match(Var& v, Funcs&&... funcs) {
 113   return v.apply(
 114     match_detail::make_visitor<Ret>(std::forward<Funcs>(funcs)...)
 115   );
 116 }
 117
 118 //////////////////////////////////////////////////////////////////////
 119
 120 }
 121
 122 #endif