base/tuple.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // A Tuple is a generic templatized container, similar in concept to std::pair
   6 // and std::tuple.  The convenient MakeTuple() function takes any number of
   7 // arguments and will construct and return the appropriate Tuple object.  The
   8 // functions DispatchToMethod and DispatchToFunction take a function pointer or
   9 // instance and method pointer, and unpack a tuple into arguments to the call.
  10 //
  11 // Tuple elements are copied by value, and stored in the tuple.  See the unit
  12 // tests for more details of how/when the values are copied.
  13 //
  14 // Example usage:
  15 //   // These two methods of creating a Tuple are identical.
  16 //   Tuple<int, const char*> tuple_a(1, "wee");
  17 //   Tuple<int, const char*> tuple_b = MakeTuple(1, "wee");
  18 //
  19 //   void SomeFunc(int a, const char* b) { }
  20 //   DispatchToFunction(&SomeFunc, tuple_a);  // SomeFunc(1, "wee")
  21 //   DispatchToFunction(
  22 //       &SomeFunc, MakeTuple(10, "foo"));    // SomeFunc(10, "foo")
  23 //
  24 //   struct { void SomeMeth(int a, int b, int c) { } } foo;
  25 //   DispatchToMethod(&foo, &Foo::SomeMeth, MakeTuple(1, 2, 3));
  26 //   // foo->SomeMeth(1, 2, 3);
  27
  28 #ifndef BASE_TUPLE_H__
  29 #define BASE_TUPLE_H__
  30
  31 #include "base/bind_helpers.h"
  32
  33 // Index sequences
  34 //
  35 // Minimal clone of the similarly-named C++14 functionality.
  36
  37 template <size_t...>
  38 struct IndexSequence {};
  39
  40 template <size_t... Ns>
  41 struct MakeIndexSequenceImpl;
  42
  43 template <size_t... Ns>
  44 struct MakeIndexSequenceImpl<0, Ns...> {
  45   using Type = IndexSequence<Ns...>;
  46 };
  47
  48 template <size_t N, size_t... Ns>
  49 struct MakeIndexSequenceImpl<N, Ns...>
  50     : MakeIndexSequenceImpl<N - 1, N - 1, Ns...> {};
  51
  52 template <size_t N>
  53 using MakeIndexSequence = typename MakeIndexSequenceImpl<N>::Type;
  54
  55 // Traits ----------------------------------------------------------------------
  56 //
  57 // A simple traits class for tuple arguments.
  58 //
  59 // ValueType: the bare, nonref version of a type (same as the type for nonrefs).
  60 // RefType: the ref version of a type (same as the type for refs).
  61 // ParamType: what type to pass to functions (refs should not be constified).
  62
  63 template <class P>
  64 struct TupleTraits {
  65   typedef P ValueType;
  66   typedef P& RefType;
  67   typedef const P& ParamType;
  68 };
  69
  70 template <class P>
  71 struct TupleTraits<P&> {
  72   typedef P ValueType;
  73   typedef P& RefType;
  74   typedef P& ParamType;
  75 };
  76
  77 // Tuple -----------------------------------------------------------------------
  78 //
  79 // This set of classes is useful for bundling 0 or more heterogeneous data types
  80 // into a single variable.  The advantage of this is that it greatly simplifies
  81 // function objects that need to take an arbitrary number of parameters; see
  82 // RunnableMethod and IPC::MessageWithTuple.
  83 //
  84 // Tuple<> is supplied to act as a 'void' type.  It can be used, for example,
  85 // when dispatching to a function that accepts no arguments (see the
  86 // Dispatchers below).
  87 // Tuple<A> is rarely useful.  One such use is when A is non-const ref that you
  88 // want filled by the dispatchee, and the tuple is merely a container for that
  89 // output (a "tier").  See MakeRefTuple and its usages.
  90
  91 template <typename IxSeq, typename... Ts>
  92 struct TupleBaseImpl;
  93 template <typename... Ts>
  94 using TupleBase = TupleBaseImpl<MakeIndexSequence<sizeof...(Ts)>, Ts...>;
  95 template <size_t N, typename T>
  96 struct TupleLeaf;
  97
  98 template <typename... Ts>
  99 struct Tuple : TupleBase<Ts...> {
 100   Tuple() : TupleBase<Ts...>() {}
 101   explicit Tuple(typename TupleTraits<Ts>::ParamType... args)
 102       : TupleBase<Ts...>(args...) {}
 103 };
 104
 105 // Avoids ambiguity between Tuple's two constructors.
 106 template <>
 107 struct Tuple<> {};
 108
 109 template <size_t... Ns, typename... Ts>
 110 struct TupleBaseImpl<IndexSequence<Ns...>, Ts...> : TupleLeaf<Ns, Ts>... {
 111   TupleBaseImpl() : TupleLeaf<Ns, Ts>()... {}
 112   explicit TupleBaseImpl(typename TupleTraits<Ts>::ParamType... args)
 113       : TupleLeaf<Ns, Ts>(args)... {}
 114 };
 115
 116 template <size_t N, typename T>
 117 struct TupleLeaf {
 118   TupleLeaf() {}
 119   explicit TupleLeaf(typename TupleTraits<T>::ParamType x) : x(x) {}
 120
 121   T& get() { return x; }
 122   const T& get() const { return x; }
 123
 124   T x;
 125 };
 126
 127 // Tuple getters --------------------------------------------------------------
 128 //
 129 // Allows accessing an arbitrary tuple element by index.
 130 //
 131 // Example usage:
 132 //   Tuple<int, double> t2;
 133 //   get<0>(t2) = 42;
 134 //   get<1>(t2) = 3.14;
 135
 136 template <size_t I, typename T>
 137 T& get(TupleLeaf<I, T>& leaf) {
 138   return leaf.get();
 139 }
 140
 141 template <size_t I, typename T>
 142 const T& get(const TupleLeaf<I, T>& leaf) {
 143   return leaf.get();
 144 }
 145
 146 // Tuple types ----------------------------------------------------------------
 147 //
 148 // Allows for selection of ValueTuple/RefTuple/ParamTuple without needing the
 149 // definitions of class types the tuple takes as parameters.
 150
 151 template <typename T>
 152 struct TupleTypes;
 153
 154 template <typename... Ts>
 155 struct TupleTypes<Tuple<Ts...>> {
 156   using ValueTuple = Tuple<typename TupleTraits<Ts>::ValueType...>;
 157   using RefTuple = Tuple<typename TupleTraits<Ts>::RefType...>;
 158   using ParamTuple = Tuple<typename TupleTraits<Ts>::ParamType...>;
 159 };
 160
 161 // Tuple creators -------------------------------------------------------------
 162 //
 163 // Helper functions for constructing tuples while inferring the template
 164 // argument types.
 165
 166 template <typename... Ts>
 167 inline Tuple<Ts...> MakeTuple(const Ts&... arg) {
 168   return Tuple<Ts...>(arg...);
 169 }
 170
 171 // The following set of helpers make what Boost refers to as "Tiers" - a tuple
 172 // of references.
 173
 174 template <typename... Ts>
 175 inline Tuple<Ts&...> MakeRefTuple(Ts&... arg) {
 176   return Tuple<Ts&...>(arg...);
 177 }
 178
 179 // Dispatchers ----------------------------------------------------------------
 180 //
 181 // Helper functions that call the given method on an object, with the unpacked
 182 // tuple arguments.  Notice that they all have the same number of arguments,
 183 // so you need only write:
 184 //   DispatchToMethod(object, &Object::method, args);
 185 // This is very useful for templated dispatchers, since they don't need to know
 186 // what type |args| is.
 187
 188 // Non-Static Dispatchers with no out params.
 189
 190 template <typename ObjT, typename Method, typename A>
 191 inline void DispatchToMethod(ObjT* obj, Method method, const A& arg) {
 192   (obj->*method)(base::internal::UnwrapTraits<A>::Unwrap(arg));
 193 }
 194
 195 template <typename ObjT, typename Method, typename... Ts, size_t... Ns>
 196 inline void DispatchToMethodImpl(ObjT* obj,
 197                                  Method method,
 198                                  const Tuple<Ts...>& arg,
 199                                  IndexSequence<Ns...>) {
 200   (obj->*method)(base::internal::UnwrapTraits<Ts>::Unwrap(get<Ns>(arg))...);
 201 }
 202
 203 template <typename ObjT, typename Method, typename... Ts>
 204 inline void DispatchToMethod(ObjT* obj,
 205                              Method method,
 206                              const Tuple<Ts...>& arg) {
 207   DispatchToMethodImpl(obj, method, arg, MakeIndexSequence<sizeof...(Ts)>());
 208 }
 209
 210 // Static Dispatchers with no out params.
 211
 212 template <typename Function, typename A>
 213 inline void DispatchToMethod(Function function, const A& arg) {
 214   (*function)(base::internal::UnwrapTraits<A>::Unwrap(arg));
 215 }
 216
 217 template <typename Function, typename... Ts, size_t... Ns>
 218 inline void DispatchToFunctionImpl(Function function,
 219                                    const Tuple<Ts...>& arg,
 220                                    IndexSequence<Ns...>) {
 221   (*function)(base::internal::UnwrapTraits<Ts>::Unwrap(get<Ns>(arg))...);
 222 }
 223
 224 template <typename Function, typename... Ts>
 225 inline void DispatchToFunction(Function function, const Tuple<Ts...>& arg) {
 226   DispatchToFunctionImpl(function, arg, MakeIndexSequence<sizeof...(Ts)>());
 227 }
 228
 229 // Dispatchers with out parameters.
 230
 231 template <typename ObjT,
 232           typename Method,
 233           typename In,
 234           typename... OutTs,
 235           size_t... OutNs>
 236 inline void DispatchToMethodImpl(ObjT* obj,
 237                                  Method method,
 238                                  const In& in,
 239                                  Tuple<OutTs...>* out,
 240                                  IndexSequence<OutNs...>) {
 241   (obj->*method)(base::internal::UnwrapTraits<In>::Unwrap(in),
 242                  &get<OutNs>(*out)...);
 243 }
 244
 245 template <typename ObjT, typename Method, typename In, typename... OutTs>
 246 inline void DispatchToMethod(ObjT* obj,
 247                              Method method,
 248                              const In& in,
 249                              Tuple<OutTs...>* out) {
 250   DispatchToMethodImpl(obj, method, in, out,
 251                        MakeIndexSequence<sizeof...(OutTs)>());
 252 }
 253
 254 template <typename ObjT,
 255           typename Method,
 256           typename... InTs,
 257           typename... OutTs,
 258           size_t... InNs,
 259           size_t... OutNs>
 260 inline void DispatchToMethodImpl(ObjT* obj,
 261                                  Method method,
 262                                  const Tuple<InTs...>& in,
 263                                  Tuple<OutTs...>* out,
 264                                  IndexSequence<InNs...>,
 265                                  IndexSequence<OutNs...>) {
 266   (obj->*method)(base::internal::UnwrapTraits<InTs>::Unwrap(get<InNs>(in))...,
 267                  &get<OutNs>(*out)...);
 268 }
 269
 270 template <typename ObjT, typename Method, typename... InTs, typename... OutTs>
 271 inline void DispatchToMethod(ObjT* obj,
 272                              Method method,
 273                              const Tuple<InTs...>& in,
 274                              Tuple<OutTs...>* out) {
 275   DispatchToMethodImpl(obj, method, in, out,
 276                        MakeIndexSequence<sizeof...(InTs)>(),
 277                        MakeIndexSequence<sizeof...(OutTs)>());
 278 }
 279
 280 #endif  // BASE_TUPLE_H__