| // Copyright 2014 The Chromium OS Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| // Internal implementation of brillo::Any class. |
| |
| #ifndef LIBBRILLO_BRILLO_ANY_INTERNAL_IMPL_H_ |
| #define LIBBRILLO_BRILLO_ANY_INTERNAL_IMPL_H_ |
| |
| #include <type_traits> |
| #include <typeinfo> |
| #include <utility> |
| |
| #include <base/check.h> |
| #include <base/logging.h> |
| #include <brillo/dbus/data_serialization.h> |
| #include <brillo/type_name_undecorate.h> |
| |
| namespace brillo { |
| |
| namespace internal_details { |
| |
| // An extension to std::is_convertible to allow conversion from an enum to |
| // an integral type which std::is_convertible does not indicate as supported. |
| template <typename From, typename To> |
| struct IsConvertible |
| : public std::integral_constant<bool, |
| std::is_convertible<From, To>::value || |
| (std::is_enum<From>::value && |
| std::is_integral<To>::value)> {}; |
| |
| // TryConvert is a helper function that does a safe compile-time conditional |
| // type cast between data types that may not be always convertible. |
| // From and To are the source and destination types. |
| // The function returns true if conversion was possible/successful. |
| template <typename From, typename To> |
| inline typename std::enable_if<IsConvertible<From, To>::value, bool>::type |
| TryConvert(const From& in, To* out) { |
| *out = static_cast<To>(in); |
| return true; |
| } |
| template <typename From, typename To> |
| inline typename std::enable_if<!IsConvertible<From, To>::value, bool>::type |
| TryConvert(const From& /* in */, To* /* out */) { |
| return false; |
| } |
| |
| ////////////////////////////////////////////////////////////////////////////// |
| // Provide a way to compare values of unspecified types without compiler errors |
| // when no operator==() is provided for a given type. This is important to |
| // allow Any class to have operator==(), yet still allowing arbitrary types |
| // (not necessarily comparable) to be placed inside Any without resulting in |
| // compile-time error. |
| // |
| // We achieve this in two ways. First, we provide a IsEqualityComparable<T> |
| // class that can be used in compile-time conditions to determine if there is |
| // operator==() defined that takes values of type T (or which can be implicitly |
| // converted to type T). Secondly, this allows us to specialize a helper |
| // compare function EqCompare<T>(v1, v2) to use operator==() for types that |
| // are comparable, and just return false for those that are not. |
| // |
| // IsEqualityComparableHelper<T> is a helper class for implementing an |
| // an STL-compatible IsEqualityComparable<T> containing a Boolean member |value| |
| // which evaluates to true for comparable types and false otherwise. |
| template <typename T> |
| struct IsEqualityComparableHelper { |
| struct IntWrapper { |
| // A special structure that provides a constructor that takes an int. |
| // This way, an int argument passed to a function will be favored over |
| // IntWrapper when both overloads are provided. |
| // Also this constructor must NOT be explicit. |
| // NOLINTNEXTLINE(runtime/explicit) |
| IntWrapper(int /* dummy */) {} // do nothing |
| }; |
| |
| // Here is an obscure trick to determine if a type U has operator==(). |
| // We are providing two function prototypes for TriggerFunction. One that |
| // takes an argument of type IntWrapper (which is implicitly convertible from |
| // an int), and returns an std::false_type. This is a fall-back mechanism. |
| template <typename U> |
| static std::false_type TriggerFunction(IntWrapper dummy); |
| |
| // The second overload of TriggerFunction takes an int (explicitly) and |
| // returns std::true_type. If both overloads are available, this one will be |
| // chosen when referencing it as TriggerFunction(0), since it is a better |
| // (more specific) match. |
| // |
| // However this overload is available only for types that support operator==. |
| // This is achieved by employing SFINAE mechanism inside a template function |
| // overload that refers to operator==() for two values of types U&. This is |
| // used inside decltype(), so no actual code is executed. If the types |
| // are not comparable, reference to "==" would fail and the compiler will |
| // simply ignore this overload due to SFIANE. |
| // |
| // The final little trick used here is the reliance on operator comma inside |
| // the decltype() expression. The result of the expression is always |
| // std::true_type(). The expression on the left of comma is just evaluated and |
| // discarded. If it evaluates successfully (i.e. the type has operator==), the |
| // return value of the function is set to be std::true_value. If it fails, |
| // the whole function prototype is discarded and is not available in the |
| // IsEqualityComparableHelper<T> class. |
| // |
| // Here we use std::declval<U&>() to make sure we have operator==() that takes |
| // lvalue references to type U which is not necessarily default-constructible. |
| template <typename U> |
| static decltype((std::declval<U&>() == std::declval<U&>()), std::true_type()) |
| TriggerFunction(int dummy); |
| |
| // Finally, use the return type of the overload of TriggerFunction that |
| // matches the argument (int) to be aliased to type |type|. If T is |
| // comparable, there will be two overloads and the more specific (int) will |
| // be chosen which returns std::true_value. If the type is non-comparable, |
| // there will be only one version of TriggerFunction available which |
| // returns std::false_value. |
| using type = decltype(TriggerFunction<T>(0)); |
| }; |
| |
| // IsEqualityComparable<T> is simply a class that derives from either |
| // std::true_value, if type T is comparable, or from std::false_value, if the |
| // type is non-comparable. We just use |type| alias from |
| // IsEqualityComparableHelper<T> as the base class. |
| template <typename T> |
| struct IsEqualityComparable : IsEqualityComparableHelper<T>::type {}; |
| |
| // EqCompare() overload for non-comparable types. Always returns false. |
| template <typename T> |
| inline typename std::enable_if<!IsEqualityComparable<T>::value, bool>::type |
| EqCompare(const T& /* v1 */, const T& /* v2 */) { |
| return false; |
| } |
| |
| // EqCompare overload for comparable types. Calls operator==(v1, v2) to compare. |
| template <typename T> |
| inline typename std::enable_if<IsEqualityComparable<T>::value, bool>::type |
| EqCompare(const T& v1, const T& v2) { |
| return (v1 == v2); |
| } |
| |
| ////////////////////////////////////////////////////////////////////////////// |
| |
| class Buffer; // Forward declaration of data buffer container. |
| |
| // Abstract base class for contained variant data. |
| struct Data { |
| virtual ~Data() {} |
| // Returns the type tag (name) for the contained data. |
| virtual const char* GetTypeTag() const = 0; |
| // Copies the contained data to the output |buffer|. |
| virtual void CopyTo(Buffer* buffer) const = 0; |
| // Moves the contained data to the output |buffer|. |
| virtual void MoveTo(Buffer* buffer) = 0; |
| // Checks if the contained data is an integer type (not necessarily an 'int'). |
| virtual bool IsConvertibleToInteger() const = 0; |
| // Gets the contained integral value as an integer. |
| virtual intmax_t GetAsInteger() const = 0; |
| // Writes the contained value to the D-Bus message buffer. |
| virtual void AppendToDBusMessage(::dbus::MessageWriter* writer) const = 0; |
| // Compares if the two data containers have objects of the same value. |
| virtual bool CompareEqual(const Data* other_data) const = 0; |
| }; |
| |
| // Concrete implementation of variant data of type T. |
| template <typename T> |
| struct TypedData : public Data { |
| explicit TypedData(const T& value) : value_(value) {} |
| // NOLINTNEXTLINE(build/c++11) |
| explicit TypedData(T&& value) : value_(std::move(value)) {} |
| |
| const char* GetTypeTag() const override { return brillo::GetTypeTag<T>(); } |
| void CopyTo(Buffer* buffer) const override; |
| void MoveTo(Buffer* buffer) override; |
| bool IsConvertibleToInteger() const override { |
| return std::is_integral<T>::value || std::is_enum<T>::value; |
| } |
| intmax_t GetAsInteger() const override { |
| intmax_t int_val = 0; |
| bool converted = TryConvert(value_, &int_val); |
| CHECK(converted) << "Unable to convert value of type '" |
| << GetUndecoratedTypeName<T>() << "' to integer"; |
| return int_val; |
| } |
| |
| template <typename U> |
| static typename std::enable_if<dbus_utils::IsTypeSupported<U>::value>::type |
| AppendValueHelper(::dbus::MessageWriter* writer, const U& value) { |
| brillo::dbus_utils::AppendValueToWriterAsVariant(writer, value); |
| } |
| template <typename U> |
| static typename std::enable_if<!dbus_utils::IsTypeSupported<U>::value>::type |
| AppendValueHelper(::dbus::MessageWriter* /* writer */, const U& /* value */) { |
| LOG(FATAL) << "Type '" << GetUndecoratedTypeName<U>() |
| << "' is not supported by D-Bus"; |
| } |
| |
| void AppendToDBusMessage(::dbus::MessageWriter* writer) const override { |
| return AppendValueHelper(writer, value_); |
| } |
| |
| bool CompareEqual(const Data* other_data) const override { |
| return EqCompare<T>(value_, |
| static_cast<const TypedData<T>*>(other_data)->value_); |
| } |
| |
| // Special methods to copy/move data of the same type |
| // without reallocating the buffer. |
| void FastAssign(const T& source) { value_ = source; } |
| // NOLINTNEXTLINE(build/c++11) |
| void FastAssign(T&& source) { value_ = std::move(source); } |
| |
| T value_; |
| }; |
| |
| // Buffer class that stores the contained variant data. |
| // To improve performance and reduce memory fragmentation, small variants |
| // are stored in pre-allocated memory buffers that are part of the Any class. |
| // If the memory requirements are larger than the set limit or the type is |
| // non-trivially copyable, then the contained class is allocated in a separate |
| // memory block and the pointer to that memory is contained within this memory |
| // buffer class. |
| class Buffer final { |
| public: |
| enum StorageType { kExternal, kContained }; |
| Buffer() : external_ptr_(nullptr), storage_(kExternal) {} |
| ~Buffer() { Clear(); } |
| |
| Buffer(const Buffer& rhs) : Buffer() { rhs.CopyTo(this); } |
| // NOLINTNEXTLINE(build/c++11) |
| Buffer(Buffer&& rhs) : Buffer() { rhs.MoveTo(this); } |
| Buffer& operator=(const Buffer& rhs) { |
| rhs.CopyTo(this); |
| return *this; |
| } |
| // NOLINTNEXTLINE(build/c++11) |
| Buffer& operator=(Buffer&& rhs) { |
| rhs.MoveTo(this); |
| return *this; |
| } |
| |
| // Returns the underlying pointer to contained data. Uses either the pointer |
| // or the raw data depending on |storage_| type. |
| inline Data* GetDataPtr() { |
| return (storage_ == kExternal) ? external_ptr_ |
| : reinterpret_cast<Data*>(contained_buffer_); |
| } |
| inline const Data* GetDataPtr() const { |
| return (storage_ == kExternal) |
| ? external_ptr_ |
| : reinterpret_cast<const Data*>(contained_buffer_); |
| } |
| |
| // Destroys the contained object (and frees memory if needed). |
| void Clear() { |
| Data* data = GetDataPtr(); |
| if (storage_ == kExternal) { |
| delete data; |
| } else { |
| // Call the destructor manually, since the object was constructed inline |
| // in the pre-allocated buffer. We still need to call the destructor |
| // to free any associated resources, but we can't call delete |data| here. |
| data->~Data(); |
| } |
| external_ptr_ = nullptr; |
| storage_ = kExternal; |
| } |
| |
| // Stores a value of type T. |
| template <typename T> |
| void Assign(T&& value) { // NOLINT(build/c++11) |
| using Type = typename std::decay<T>::type; |
| using DataType = TypedData<Type>; |
| Data* ptr = GetDataPtr(); |
| if (ptr && strcmp(ptr->GetTypeTag(), GetTypeTag<Type>()) == 0) { |
| // We assign the data to the variant container, which already |
| // has the data of the same type. Do fast copy/move with no memory |
| // reallocation. |
| DataType* typed_ptr = static_cast<DataType*>(ptr); |
| // NOLINTNEXTLINE(build/c++11) |
| typed_ptr->FastAssign(std::forward<T>(value)); |
| } else { |
| Clear(); |
| // TODO(avakulenko): [see crbug.com/379833] |
| // Unfortunately, GCC doesn't support std::is_trivially_copyable<T> yet, |
| // so using std::is_trivial instead, which is a bit more restrictive. |
| // Once GCC has support for is_trivially_copyable, update the following. |
| if (!std::is_trivial<Type>::value || |
| sizeof(DataType) > sizeof(contained_buffer_)) { |
| // If it is too big or not trivially copyable, allocate it separately. |
| // NOLINTNEXTLINE(build/c++11) |
| external_ptr_ = new DataType(std::forward<T>(value)); |
| storage_ = kExternal; |
| } else { |
| // Otherwise just use the pre-allocated buffer. |
| DataType* address = reinterpret_cast<DataType*>(contained_buffer_); |
| // Make sure we still call the copy/move constructor. |
| // Call the constructor manually by using placement 'new'. |
| // NOLINTNEXTLINE(build/c++11) |
| new (address) DataType(std::forward<T>(value)); |
| storage_ = kContained; |
| } |
| } |
| } |
| |
| // Helper methods to retrieve a reference to contained data. |
| // These assume that type checking has already been performed by Any |
| // so the type cast is valid and will succeed. |
| template <typename T> |
| const T& GetData() const { |
| using DataType = internal_details::TypedData<typename std::decay<T>::type>; |
| return static_cast<const DataType*>(GetDataPtr())->value_; |
| } |
| template <typename T> |
| T& GetData() { |
| using DataType = internal_details::TypedData<typename std::decay<T>::type>; |
| return static_cast<DataType*>(GetDataPtr())->value_; |
| } |
| |
| // Returns true if the buffer has no contained data. |
| bool IsEmpty() const { |
| return (storage_ == kExternal && external_ptr_ == nullptr); |
| } |
| |
| // Copies the data from the current buffer into the |destination|. |
| void CopyTo(Buffer* destination) const { |
| if (IsEmpty()) { |
| destination->Clear(); |
| } else { |
| GetDataPtr()->CopyTo(destination); |
| } |
| } |
| |
| // Moves the data from the current buffer into the |destination|. |
| void MoveTo(Buffer* destination) { |
| if (IsEmpty()) { |
| destination->Clear(); |
| } else { |
| if (storage_ == kExternal) { |
| destination->Clear(); |
| destination->storage_ = kExternal; |
| destination->external_ptr_ = external_ptr_; |
| external_ptr_ = nullptr; |
| } else { |
| GetDataPtr()->MoveTo(destination); |
| } |
| } |
| } |
| |
| union { |
| // |external_ptr_| is a pointer to a larger object allocated in |
| // a separate memory block. |
| Data* external_ptr_; |
| // |contained_buffer_| is a pre-allocated buffer for smaller/simple objects. |
| // Pre-allocate enough memory to store objects as big as "double". |
| unsigned char contained_buffer_[sizeof(TypedData<double>)]; |
| }; |
| // Depending on a value of |storage_|, either |external_ptr_| or |
| // |contained_buffer_| above is used to get a pointer to memory containing |
| // the variant data. |
| StorageType storage_; // Declare after the union to eliminate member padding. |
| }; |
| |
| template <typename T> |
| void TypedData<T>::CopyTo(Buffer* buffer) const { |
| buffer->Assign(value_); |
| } |
| template <typename T> |
| void TypedData<T>::MoveTo(Buffer* buffer) { |
| buffer->Assign(std::move(value_)); |
| } |
| |
| } // namespace internal_details |
| |
| } // namespace brillo |
| |
| #endif // LIBBRILLO_BRILLO_ANY_INTERNAL_IMPL_H_ |