Introducing gpu::CommandBufferId as a distinct, IdType<...>-based type.

content/common/id_type.h

-// Copyright 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef CONTENT_COMMON_ID_TYPE_H_
-#define CONTENT_COMMON_ID_TYPE_H_
-
-#include <stdint.h>
-#include <cstddef>
-#include <ostream>
-#include <type_traits>
-
-// IdType32<>, IdType64<>, etc. wrap an integer id in a custom, type-safe type.
-//
-// IdType32<Foo> is an alternative to int, for a class Foo with methods like:
-//
-//    int GetId() { return id_; };
-//    static Foo* FromId(int id) { return g_all_foos_by_id[id]; }
-//
-// Such methods are a standard means of safely referring to objects across
-// thread and process boundaries.  But if a nearby class Bar also represents
-// its IDs as a bare int, horrific mixups are possible -- one example, of many,
-// is http://crrev.com/365437.  IdType<> offers compile-time protection against
-// such mishaps, since IdType32<Foo> is incompatible with IdType32<Bar>, even
-// though both just compile down to an int32_t.
-//
-// Templates in this file:
-//   IdType32<T> / IdTypeU32<T>: Signed / unsigned 32-bit IDs
-//   IdType64<T> / IdTypeU64<T>: Signed / unsigned 64-bit IDs
-//   IdType<>: For when you need a different underlying type or
-//             a default/invalid value other than zero.
-//
-// IdType32<Foo> behaves just like an int32_t in the following aspects:
-// - it can be used as a key in std::map and/or std::unordered_map;
-// - it can be used as an argument to DCHECK_EQ or streamed to LOG(ERROR);
-// - it has the same memory footprint and runtime overhead as int32_t;
-// - it can be copied by memcpy.
-//
-// IdType32<Foo> has the following differences from a bare int32_t:
-// - it forces coercions to go through GetUnsafeValue and FromUnsafeValue;
-// - it restricts the set of available operations (i.e. no multiplication);
-// - it ensures initialization to zero and allows checking against
-//   default-initialized values via is_null method.
-
-namespace content {
-
-template <typename TypeMarker, typename WrappedType, WrappedType kInvalidValue>
-class IdType {
- public:
-  IdType() : value_(kInvalidValue) {}
-  bool is_null() const { return value_ == kInvalidValue; }
-
-  static IdType FromUnsafeValue(WrappedType value) { return IdType(value); }
-  WrappedType GetUnsafeValue() const { return value_; }
-
-  IdType(const IdType& other) = default;
-  IdType& operator=(const IdType& other) = default;
-
-  bool operator==(const IdType& other) const { return value_ == other.value_; }
-  bool operator!=(const IdType& other) const { return value_ != other.value_; }
-  bool operator<(const IdType& other) const { return value_ < other.value_; }
-
-  // Hasher to use in std::unordered_map, std::unordered_set, etc.
-  struct Hasher {
-    using argument_type = IdType;
-    using result_type = std::size_t;
-    result_type operator()(const argument_type& id) const {
-      return std::hash<WrappedType>()(id.GetUnsafeValue());
-    }
-  };
-
- protected:
-  explicit IdType(WrappedType val) : value_(val) {}
-
- private:
-  // In theory WrappedType could be any type that supports ==, <, <<, std::hash,
-  // etc., but to make things simpler (both for users and for maintainers) we
-  // explicitly restrict the design space to integers.  This means the users
-  // can safely assume that IdType is relatively small and cheap to copy
-  // and the maintainers don't have to worry about WrappedType being a complex
-  // type (i.e. std::string or std::pair or a move-only type).
-  using IntegralWrappedType =
-      typename std::enable_if<std::is_integral<WrappedType>::value,
-                              WrappedType>::type;
-  IntegralWrappedType value_;
-};
-
-// Type aliases for convenience:
-template <typename TypeMarker>
-using IdType32 = IdType<TypeMarker, int32_t, 0>;
-template <typename TypeMarker>
-using IdTypeU32 = IdType<TypeMarker, uint32_t, 0>;
-template <typename TypeMarker>
-using IdType64 = IdType<TypeMarker, int64_t, 0>;
-template <typename TypeMarker>
-using IdTypeU64 = IdType<TypeMarker, uint64_t, 0>;
-
-template <typename TypeMarker, typename WrappedType, WrappedType kInvalidValue>
-std::ostream& operator<<(
-    std::ostream& stream,
-    const IdType<TypeMarker, WrappedType, kInvalidValue>& id) {
-  return stream << id.GetUnsafeValue();
-}
-
-}  // namespace content
-
-#endif  // CONTENT_COMMON_ID_TYPE_H_