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 // For legacy compatibility, we name the first 8 tuple elements "a", "b", ...
 128 // TODO(mdempsky): Update users to use get<N>() (crbug.com/440675).
 129
 130 #define DEFINE_TUPLE_LEAF(N, x)                                        \
 131   template <typename T>                                                \
 132   struct TupleLeaf<N, T> {                                             \
 133     TupleLeaf() {}                                                     \
 134     explicit TupleLeaf(typename TupleTraits<T>::ParamType x) : x(x) {} \
 135                                                                        \
 136     T& get() { return x; }                                             \
 137     const T& get() const { return x; }                                 \
 138                                                                        \
 139     T x;                                                               \
 140   }
 141
 142 DEFINE_TUPLE_LEAF(0, a);
 143 DEFINE_TUPLE_LEAF(1, b);
 144 DEFINE_TUPLE_LEAF(2, c);
 145 DEFINE_TUPLE_LEAF(3, d);
 146 DEFINE_TUPLE_LEAF(4, e);
 147 DEFINE_TUPLE_LEAF(5, f);
 148 DEFINE_TUPLE_LEAF(6, g);
 149 DEFINE_TUPLE_LEAF(7, h);
 150
 151 #undef DEFINE_TUPLE_LEAF
 152
 153 // Deprecated compat aliases
 154 // TODO(mdempsky): Update users to just use Tuple instead (crbug.com/440675).
 155
 156 using Tuple0 = Tuple<>;
 157 template <typename A>
 158 using Tuple1 = Tuple<A>;
 159 template <typename A, typename B>
 160 using Tuple2 = Tuple<A, B>;
 161 template <typename A, typename B, typename C>
 162 using Tuple3 = Tuple<A, B, C>;
 163 template <typename A, typename B, typename C, typename D>
 164 using Tuple4 = Tuple<A, B, C, D>;
 165 template <typename A, typename B, typename C, typename D, typename E>
 166 using Tuple5 = Tuple<A, B, C, D, E>;
 167 template <typename A,
 168           typename B,
 169           typename C,
 170           typename D,
 171           typename E,
 172           typename F>
 173 using Tuple6 = Tuple<A, B, C, D, E, F>;
 174 template <typename A,
 175           typename B,
 176           typename C,
 177           typename D,
 178           typename E,
 179           typename F,
 180           typename G>
 181 using Tuple7 = Tuple<A, B, C, D, E, F, G>;
 182 template <typename A,
 183           typename B,
 184           typename C,
 185           typename D,
 186           typename E,
 187           typename F,
 188           typename G,
 189           typename H>
 190 using Tuple8 = Tuple<A, B, C, D, E, F, G, H>;
 191
 192 // Tuple getters --------------------------------------------------------------
 193 //
 194 // Allows accessing an arbitrary tuple element by index.
 195 //
 196 // Example usage:
 197 //   Tuple<int, double> t2;
 198 //   get<0>(t2) = 42;
 199 //   get<1>(t2) = 3.14;
 200
 201 template <size_t I, typename T>
 202 T& get(TupleLeaf<I, T>& leaf) {
 203   return leaf.get();
 204 }
 205
 206 template <size_t I, typename T>
 207 const T& get(const TupleLeaf<I, T>& leaf) {
 208   return leaf.get();
 209 }
 210
 211 // Tuple types ----------------------------------------------------------------
 212 //
 213 // Allows for selection of ValueTuple/RefTuple/ParamTuple without needing the
 214 // definitions of class types the tuple takes as parameters.
 215
 216 template <typename T>
 217 struct TupleTypes;
 218
 219 template <typename... Ts>
 220 struct TupleTypes<Tuple<Ts...>> {
 221   using ValueTuple = Tuple<typename TupleTraits<Ts>::ValueType...>;
 222   using RefTuple = Tuple<typename TupleTraits<Ts>::RefType...>;
 223   using ParamTuple = Tuple<typename TupleTraits<Ts>::ParamType...>;
 224 };
 225
 226 // Tuple creators -------------------------------------------------------------
 227 //
 228 // Helper functions for constructing tuples while inferring the template
 229 // argument types.
 230
 231 template <typename... Ts>
 232 inline Tuple<Ts...> MakeTuple(const Ts&... arg) {
 233   return Tuple<Ts...>(arg...);
 234 }
 235
 236 // The following set of helpers make what Boost refers to as "Tiers" - a tuple
 237 // of references.
 238
 239 template <typename... Ts>
 240 inline Tuple<Ts&...> MakeRefTuple(Ts&... arg) {
 241   return Tuple<Ts&...>(arg...);
 242 }
 243
 244 // Dispatchers ----------------------------------------------------------------
 245 //
 246 // Helper functions that call the given method on an object, with the unpacked
 247 // tuple arguments.  Notice that they all have the same number of arguments,
 248 // so you need only write:
 249 //   DispatchToMethod(object, &Object::method, args);
 250 // This is very useful for templated dispatchers, since they don't need to know
 251 // what type |args| is.
 252
 253 // Non-Static Dispatchers with no out params.
 254
 255 template <typename ObjT, typename Method, typename A>
 256 inline void DispatchToMethod(ObjT* obj, Method method, const A& arg) {
 257   (obj->*method)(base::internal::UnwrapTraits<A>::Unwrap(arg));
 258 }
 259
 260 template <typename ObjT, typename Method, typename... Ts, size_t... Ns>
 261 inline void DispatchToMethodImpl(ObjT* obj,
 262                                  Method method,
 263                                  const Tuple<Ts...>& arg,
 264                                  IndexSequence<Ns...>) {
 265   (obj->*method)(base::internal::UnwrapTraits<Ts>::Unwrap(get<Ns>(arg))...);
 266 }
 267
 268 template <typename ObjT, typename Method, typename... Ts>
 269 inline void DispatchToMethod(ObjT* obj,
 270                              Method method,
 271                              const Tuple<Ts...>& arg) {
 272   DispatchToMethodImpl(obj, method, arg, MakeIndexSequence<sizeof...(Ts)>());
 273 }
 274
 275 // Static Dispatchers with no out params.
 276
 277 template <typename Function, typename A>
 278 inline void DispatchToMethod(Function function, const A& arg) {
 279   (*function)(base::internal::UnwrapTraits<A>::Unwrap(arg));
 280 }
 281
 282 template <typename Function, typename... Ts, size_t... Ns>
 283 inline void DispatchToFunctionImpl(Function function,
 284                                    const Tuple<Ts...>& arg,
 285                                    IndexSequence<Ns...>) {
 286   (*function)(base::internal::UnwrapTraits<Ts>::Unwrap(get<Ns>(arg))...);
 287 }
 288
 289 template <typename Function, typename... Ts>
 290 inline void DispatchToFunction(Function function, const Tuple<Ts...>& arg) {
 291   DispatchToFunctionImpl(function, arg, MakeIndexSequence<sizeof...(Ts)>());
 292 }
 293
 294 // Dispatchers with out parameters.
 295
 296 template <typename ObjT,
 297           typename Method,
 298           typename In,
 299           typename... OutTs,
 300           size_t... OutNs>
 301 inline void DispatchToMethodImpl(ObjT* obj,
 302                                  Method method,
 303                                  const In& in,
 304                                  Tuple<OutTs...>* out,
 305                                  IndexSequence<OutNs...>) {
 306   (obj->*method)(base::internal::UnwrapTraits<In>::Unwrap(in),
 307                  &get<OutNs>(*out)...);
 308 }
 309
 310 template <typename ObjT, typename Method, typename In, typename... OutTs>
 311 inline void DispatchToMethod(ObjT* obj,
 312                              Method method,
 313                              const In& in,
 314                              Tuple<OutTs...>* out) {
 315   DispatchToMethodImpl(obj, method, in, out,
 316                        MakeIndexSequence<sizeof...(OutTs)>());
 317 }
 318
 319 template <typename ObjT,
 320           typename Method,
 321           typename... InTs,
 322           typename... OutTs,
 323           size_t... InNs,
 324           size_t... OutNs>
 325 inline void DispatchToMethodImpl(ObjT* obj,
 326                                  Method method,
 327                                  const Tuple<InTs...>& in,
 328                                  Tuple<OutTs...>* out,
 329                                  IndexSequence<InNs...>,
 330                                  IndexSequence<OutNs...>) {
 331   (obj->*method)(base::internal::UnwrapTraits<InTs>::Unwrap(get<InNs>(in))...,
 332                  &get<OutNs>(*out)...);
 333 }
 334
 335 template <typename ObjT, typename Method, typename... InTs, typename... OutTs>
 336 inline void DispatchToMethod(ObjT* obj,
 337                              Method method,
 338                              const Tuple<InTs...>& in,
 339                              Tuple<OutTs...>* out) {
 340   DispatchToMethodImpl(obj, method, in, out,
 341                        MakeIndexSequence<sizeof...(InTs)>(),
 342                        MakeIndexSequence<sizeof...(OutTs)>());
 343 }
 344
 345 #endif  // BASE_TUPLE_H__