any_internal_impl.h revision 9ed0cab99f18acb3570a35e9408f24355f6b8324
1// Copyright 2014 The Chromium OS 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// Internal implementation of brillo::Any class. 6 7#ifndef LIBCHROMEOS_BRILLO_ANY_INTERNAL_IMPL_H_ 8#define LIBCHROMEOS_BRILLO_ANY_INTERNAL_IMPL_H_ 9 10#include <type_traits> 11#include <typeinfo> 12#include <utility> 13 14#include <base/logging.h> 15#include <brillo/dbus/data_serialization.h> 16#include <brillo/type_name_undecorate.h> 17 18namespace brillo { 19 20namespace internal_details { 21 22// An extension to std::is_convertible to allow conversion from an enum to 23// an integral type which std::is_convertible does not indicate as supported. 24template <typename From, typename To> 25struct IsConvertible 26 : public std::integral_constant< 27 bool, 28 std::is_convertible<From, To>::value || 29 (std::is_enum<From>::value && std::is_integral<To>::value)> {}; 30 31// TryConvert is a helper function that does a safe compile-time conditional 32// type cast between data types that may not be always convertible. 33// From and To are the source and destination types. 34// The function returns true if conversion was possible/successful. 35template <typename From, typename To> 36inline typename std::enable_if<IsConvertible<From, To>::value, bool>::type 37TryConvert(const From& in, To* out) { 38 *out = static_cast<To>(in); 39 return true; 40} 41template <typename From, typename To> 42inline typename std::enable_if<!IsConvertible<From, To>::value, bool>::type 43TryConvert(const From& in, To* out) { 44 return false; 45} 46 47////////////////////////////////////////////////////////////////////////////// 48// Provide a way to compare values of unspecified types without compiler errors 49// when no operator==() is provided for a given type. This is important to 50// allow Any class to have operator==(), yet still allowing arbitrary types 51// (not necessarily comparable) to be placed inside Any without resulting in 52// compile-time error. 53// 54// We achieve this in two ways. First, we provide a IsEqualityComparable<T> 55// class that can be used in compile-time conditions to determine if there is 56// operator==() defined that takes values of type T (or which can be implicitly 57// converted to type T). Secondly, this allows us to specialize a helper 58// compare function EqCompare<T>(v1, v2) to use operator==() for types that 59// are comparable, and just return false for those that are not. 60// 61// IsEqualityComparableHelper<T> is a helper class for implementing an 62// an STL-compatible IsEqualityComparable<T> containing a Boolean member |value| 63// which evaluates to true for comparable types and false otherwise. 64template<typename T> 65struct IsEqualityComparableHelper { 66 struct IntWrapper { 67 // A special structure that provides a constructor that takes an int. 68 // This way, an int argument passed to a function will be favored over 69 // IntWrapper when both overloads are provided. 70 // Also this constructor must NOT be explicit. 71 // NOLINTNEXTLINE(runtime/explicit) 72 IntWrapper(int dummy) {} // do nothing 73 }; 74 75 // Here is an obscure trick to determine if a type U has operator==(). 76 // We are providing two function prototypes for TriggerFunction. One that 77 // takes an argument of type IntWrapper (which is implicitly convertible from 78 // an int), and returns an std::false_type. This is a fall-back mechanism. 79 template<typename U> 80 static std::false_type TriggerFunction(IntWrapper dummy); 81 82 // The second overload of TriggerFunction takes an int (explicitly) and 83 // returns std::true_type. If both overloads are available, this one will be 84 // chosen when referencing it as TriggerFunction(0), since it is a better 85 // (more specific) match. 86 // 87 // However this overload is available only for types that support operator==. 88 // This is achieved by employing SFINAE mechanism inside a template function 89 // overload that refers to operator==() for two values of types U&. This is 90 // used inside decltype(), so no actual code is executed. If the types 91 // are not comparable, reference to "==" would fail and the compiler will 92 // simply ignore this overload due to SFIANE. 93 // 94 // The final little trick used here is the reliance on operator comma inside 95 // the decltype() expression. The result of the expression is always 96 // std::true_type(). The expression on the left of comma is just evaluated and 97 // discarded. If it evaluates successfully (i.e. the type has operator==), the 98 // return value of the function is set to be std::true_value. If it fails, 99 // the whole function prototype is discarded and is not available in the 100 // IsEqualityComparableHelper<T> class. 101 // 102 // Here we use std::declval<U&>() to make sure we have operator==() that takes 103 // lvalue references to type U which is not necessarily default-constructible. 104 template<typename U> 105 static decltype((std::declval<U&>() == std::declval<U&>()), std::true_type()) 106 TriggerFunction(int dummy); 107 108 // Finally, use the return type of the overload of TriggerFunction that 109 // matches the argument (int) to be aliased to type |type|. If T is 110 // comparable, there will be two overloads and the more specific (int) will 111 // be chosen which returns std::true_value. If the type is non-comparable, 112 // there will be only one version of TriggerFunction available which 113 // returns std::false_value. 114 using type = decltype(TriggerFunction<T>(0)); 115}; 116 117// IsEqualityComparable<T> is simply a class that derives from either 118// std::true_value, if type T is comparable, or from std::false_value, if the 119// type is non-comparable. We just use |type| alias from 120// IsEqualityComparableHelper<T> as the base class. 121template<typename T> 122struct IsEqualityComparable : IsEqualityComparableHelper<T>::type {}; 123 124// EqCompare() overload for non-comparable types. Always returns false. 125template<typename T> 126inline typename std::enable_if<!IsEqualityComparable<T>::value, bool>::type 127EqCompare(const T& v1, const T& v2) { 128 return false; 129} 130 131// EqCompare overload for comparable types. Calls operator==(v1, v2) to compare. 132template<typename T> 133inline typename std::enable_if<IsEqualityComparable<T>::value, bool>::type 134EqCompare(const T& v1, const T& v2) { 135 return (v1 == v2); 136} 137 138////////////////////////////////////////////////////////////////////////////// 139 140class Buffer; // Forward declaration of data buffer container. 141 142// Abstract base class for contained variant data. 143struct Data { 144 virtual ~Data() {} 145 // Returns the type information for the contained data. 146 virtual const std::type_info& GetType() const = 0; 147 // Copies the contained data to the output |buffer|. 148 virtual void CopyTo(Buffer* buffer) const = 0; 149 // Moves the contained data to the output |buffer|. 150 virtual void MoveTo(Buffer* buffer) = 0; 151 // Checks if the contained data is an integer type (not necessarily an 'int'). 152 virtual bool IsConvertibleToInteger() const = 0; 153 // Gets the contained integral value as an integer. 154 virtual intmax_t GetAsInteger() const = 0; 155 // Writes the contained value to the D-Bus message buffer. 156 virtual void AppendToDBusMessage(dbus::MessageWriter* writer) const = 0; 157 // Compares if the two data containers have objects of the same value. 158 virtual bool CompareEqual(const Data* other_data) const = 0; 159}; 160 161// Concrete implementation of variant data of type T. 162template<typename T> 163struct TypedData : public Data { 164 explicit TypedData(const T& value) : value_(value) {} 165 // NOLINTNEXTLINE(build/c++11) 166 explicit TypedData(T&& value) : value_(std::move(value)) {} 167 168 const std::type_info& GetType() const override { return typeid(T); } 169 void CopyTo(Buffer* buffer) const override; 170 void MoveTo(Buffer* buffer) override; 171 bool IsConvertibleToInteger() const override { 172 return std::is_integral<T>::value || std::is_enum<T>::value; 173 } 174 intmax_t GetAsInteger() const override { 175 intmax_t int_val = 0; 176 bool converted = TryConvert(value_, &int_val); 177 CHECK(converted) << "Unable to convert value of type '" 178 << GetUndecoratedTypeName<T>() << "' to integer"; 179 return int_val; 180 } 181 182 template<typename U> 183 static typename std::enable_if<dbus_utils::IsTypeSupported<U>::value>::type 184 AppendValueHelper(dbus::MessageWriter* writer, const U& value) { 185 brillo::dbus_utils::AppendValueToWriterAsVariant(writer, value); 186 } 187 template<typename U> 188 static typename std::enable_if<!dbus_utils::IsTypeSupported<U>::value>::type 189 AppendValueHelper(dbus::MessageWriter* writer, const U& value) { 190 LOG(FATAL) << "Type '" << GetUndecoratedTypeName<U>() 191 << "' is not supported by D-Bus"; 192 } 193 194 void AppendToDBusMessage(dbus::MessageWriter* writer) const override { 195 return AppendValueHelper(writer, value_); 196 } 197 198 bool CompareEqual(const Data* other_data) const override { 199 return EqCompare<T>(value_, 200 static_cast<const TypedData<T>*>(other_data)->value_); 201 } 202 203 // Special methods to copy/move data of the same type 204 // without reallocating the buffer. 205 void FastAssign(const T& source) { value_ = source; } 206 // NOLINTNEXTLINE(build/c++11) 207 void FastAssign(T&& source) { value_ = std::move(source); } 208 209 T value_; 210}; 211 212// Buffer class that stores the contained variant data. 213// To improve performance and reduce memory fragmentation, small variants 214// are stored in pre-allocated memory buffers that are part of the Any class. 215// If the memory requirements are larger than the set limit or the type is 216// non-trivially copyable, then the contained class is allocated in a separate 217// memory block and the pointer to that memory is contained within this memory 218// buffer class. 219class Buffer final { 220 public: 221 enum StorageType { kExternal, kContained }; 222 Buffer() : external_ptr_(nullptr), storage_(kExternal) {} 223 ~Buffer() { Clear(); } 224 225 Buffer(const Buffer& rhs) : Buffer() { rhs.CopyTo(this); } 226 // NOLINTNEXTLINE(build/c++11) 227 Buffer(Buffer&& rhs) : Buffer() { rhs.MoveTo(this); } 228 Buffer& operator=(const Buffer& rhs) { 229 rhs.CopyTo(this); 230 return *this; 231 } 232 // NOLINTNEXTLINE(build/c++11) 233 Buffer& operator=(Buffer&& rhs) { 234 rhs.MoveTo(this); 235 return *this; 236 } 237 238 // Returns the underlying pointer to contained data. Uses either the pointer 239 // or the raw data depending on |storage_| type. 240 inline Data* GetDataPtr() { 241 return (storage_ == kExternal) ? external_ptr_ 242 : reinterpret_cast<Data*>(contained_buffer_); 243 } 244 inline const Data* GetDataPtr() const { 245 return (storage_ == kExternal) 246 ? external_ptr_ 247 : reinterpret_cast<const Data*>(contained_buffer_); 248 } 249 250 // Destroys the contained object (and frees memory if needed). 251 void Clear() { 252 Data* data = GetDataPtr(); 253 if (storage_ == kExternal) { 254 delete data; 255 } else { 256 // Call the destructor manually, since the object was constructed inline 257 // in the pre-allocated buffer. We still need to call the destructor 258 // to free any associated resources, but we can't call delete |data| here. 259 data->~Data(); 260 } 261 external_ptr_ = nullptr; 262 storage_ = kExternal; 263 } 264 265 // Stores a value of type T. 266 template<typename T> 267 void Assign(T&& value) { // NOLINT(build/c++11) 268 using Type = typename std::decay<T>::type; 269 using DataType = TypedData<Type>; 270 Data* ptr = GetDataPtr(); 271 if (ptr && ptr->GetType() == typeid(Type)) { 272 // We assign the data to the variant container, which already 273 // has the data of the same type. Do fast copy/move with no memory 274 // reallocation. 275 DataType* typed_ptr = static_cast<DataType*>(ptr); 276 // NOLINTNEXTLINE(build/c++11) 277 typed_ptr->FastAssign(std::forward<T>(value)); 278 } else { 279 Clear(); 280 // TODO(avakulenko): [see crbug.com/379833] 281 // Unfortunately, GCC doesn't support std::is_trivially_copyable<T> yet, 282 // so using std::is_trivial instead, which is a bit more restrictive. 283 // Once GCC has support for is_trivially_copyable, update the following. 284 if (!std::is_trivial<Type>::value || 285 sizeof(DataType) > sizeof(contained_buffer_)) { 286 // If it is too big or not trivially copyable, allocate it separately. 287 // NOLINTNEXTLINE(build/c++11) 288 external_ptr_ = new DataType(std::forward<T>(value)); 289 storage_ = kExternal; 290 } else { 291 // Otherwise just use the pre-allocated buffer. 292 DataType* address = reinterpret_cast<DataType*>(contained_buffer_); 293 // Make sure we still call the copy/move constructor. 294 // Call the constructor manually by using placement 'new'. 295 // NOLINTNEXTLINE(build/c++11) 296 new (address) DataType(std::forward<T>(value)); 297 storage_ = kContained; 298 } 299 } 300 } 301 302 // Helper methods to retrieve a reference to contained data. 303 // These assume that type checking has already been performed by Any 304 // so the type cast is valid and will succeed. 305 template<typename T> 306 const T& GetData() const { 307 using DataType = internal_details::TypedData<typename std::decay<T>::type>; 308 return static_cast<const DataType*>(GetDataPtr())->value_; 309 } 310 template<typename T> 311 T& GetData() { 312 using DataType = internal_details::TypedData<typename std::decay<T>::type>; 313 return static_cast<DataType*>(GetDataPtr())->value_; 314 } 315 316 // Returns true if the buffer has no contained data. 317 bool IsEmpty() const { 318 return (storage_ == kExternal && external_ptr_ == nullptr); 319 } 320 321 // Copies the data from the current buffer into the |destination|. 322 void CopyTo(Buffer* destination) const { 323 if (IsEmpty()) { 324 destination->Clear(); 325 } else { 326 GetDataPtr()->CopyTo(destination); 327 } 328 } 329 330 // Moves the data from the current buffer into the |destination|. 331 void MoveTo(Buffer* destination) { 332 if (IsEmpty()) { 333 destination->Clear(); 334 } else { 335 if (storage_ == kExternal) { 336 destination->Clear(); 337 destination->storage_ = kExternal; 338 destination->external_ptr_ = external_ptr_; 339 external_ptr_ = nullptr; 340 } else { 341 GetDataPtr()->MoveTo(destination); 342 } 343 } 344 } 345 346 union { 347 // |external_ptr_| is a pointer to a larger object allocated in 348 // a separate memory block. 349 Data* external_ptr_; 350 // |contained_buffer_| is a pre-allocated buffer for smaller/simple objects. 351 // Pre-allocate enough memory to store objects as big as "double". 352 unsigned char contained_buffer_[sizeof(TypedData<double>)]; 353 }; 354 // Depending on a value of |storage_|, either |external_ptr_| or 355 // |contained_buffer_| above is used to get a pointer to memory containing 356 // the variant data. 357 StorageType storage_; // Declare after the union to eliminate member padding. 358}; 359 360template <typename T> 361void TypedData<T>::CopyTo(Buffer* buffer) const { 362 buffer->Assign(value_); 363} 364template <typename T> 365void TypedData<T>::MoveTo(Buffer* buffer) { 366 buffer->Assign(std::move(value_)); 367} 368 369} // namespace internal_details 370 371} // namespace brillo 372 373#endif // LIBCHROMEOS_BRILLO_ANY_INTERNAL_IMPL_H_ 374