chromium/base/types/id_type.h

// Copyright 2016 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TYPES_ID_TYPE_H_
#define BASE_TYPES_ID_TYPE_H_

#include <cstdint>
#include <type_traits>

#include "base/ranges/algorithm.h"
#include "base/types/strong_alias.h"

namespace base {

// A specialization of StrongAlias for integer-based identifiers.
//
// 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
//             default/null values 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;
// - it can be used as a key in std::unordered_map (see StrongAlias::Hasher);
// - 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.
// - it can be used in IPC messages.
//
// IdType32<Foo> has the following differences from a bare int32_t:
// - it forces coercions to go through the explicit constructor and value()
//   getter;
// - it restricts the set of available operations (e.g. no multiplication);
// - it default-constructs to a null value and allows checking against the null
//   value via is_null method.
// - optionally it may have additional values that are all considered null.
template <typename TypeMarker,
          typename WrappedType,
          WrappedType kInvalidValue,
          WrappedType kFirstGeneratedId = kInvalidValue + 1,
          WrappedType... kExtraInvalidValues>
class IdType : public StrongAlias<TypeMarker, WrappedType> { … };

// Type aliases for convenience:
IdType32;
IdTypeU32;
IdType64;
IdTypeU64;

}  // namespace base

#endif  // BASE_TYPES_ID_TYPE_H_