chromium/base/memory/scoped_refptr.h

// Copyright 2017 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_MEMORY_SCOPED_REFPTR_H_
#define BASE_MEMORY_SCOPED_REFPTR_H_

#include <stddef.h>

#include <compare>
#include <concepts>
#include <iosfwd>
#include <type_traits>
#include <utility>

#include "base/check.h"
#include "base/compiler_specific.h"
#include "base/memory/raw_ptr_exclusion.h"

template <class T>
class scoped_refptr;

namespace base {

template <class, typename>
class RefCounted;
template <class, typename>
class RefCountedThreadSafe;
template <class>
class RefCountedDeleteOnSequence;
class SequencedTaskRunner;

template <typename T>
scoped_refptr<T> AdoptRef(T* t);

namespace subtle {

enum AdoptRefTag { … };
enum StartRefCountFromZeroTag { … };
enum StartRefCountFromOneTag { … };

template <typename TagType>
struct RefCountPreferenceTagTraits;

template <>
struct RefCountPreferenceTagTraits<StartRefCountFromZeroTag> { … };

template <>
struct RefCountPreferenceTagTraits<StartRefCountFromOneTag> { … };

template <typename T, typename Tag = typename T::RefCountPreferenceTag>
constexpr Tag GetRefCountPreference() { … }

// scoped_refptr<T> is typically used with one of several RefCounted<T> base
// classes or with custom AddRef and Release methods. These overloads dispatch
// on which was used.

template <typename T, typename U, typename V>
constexpr bool IsRefCountPreferenceOverridden(const T*,
                                              const RefCounted<U, V>*) { … }

template <typename T, typename U, typename V>
constexpr bool IsRefCountPreferenceOverridden(
    const T*,
    const RefCountedThreadSafe<U, V>*) { … }

template <typename T, typename U>
constexpr bool IsRefCountPreferenceOverridden(
    const T*,
    const RefCountedDeleteOnSequence<U>*) { … }

constexpr bool IsRefCountPreferenceOverridden(...) { … }

template <typename T, typename U, typename V>
constexpr void AssertRefCountBaseMatches(const T*, const RefCounted<U, V>*) { … }

template <typename T, typename U, typename V>
constexpr void AssertRefCountBaseMatches(const T*,
                                         const RefCountedThreadSafe<U, V>*) { … }

template <typename T, typename U>
constexpr void AssertRefCountBaseMatches(const T*,
                                         const RefCountedDeleteOnSequence<U>*) { … }

constexpr void AssertRefCountBaseMatches(...) { … }

}  // namespace subtle

// Creates a scoped_refptr from a raw pointer without incrementing the reference
// count. Use this only for a newly created object whose reference count starts
// from 1 instead of 0.
template <typename T>
scoped_refptr<T> AdoptRef(T* obj) { … }

namespace subtle {

template <typename T>
scoped_refptr<T> AdoptRefIfNeeded(T* obj, StartRefCountFromZeroTag) { … }

template <typename T>
scoped_refptr<T> AdoptRefIfNeeded(T* obj, StartRefCountFromOneTag) { … }

}  // namespace subtle

// Constructs an instance of T, which is a ref counted type, and wraps the
// object into a scoped_refptr<T>.
template <typename T, typename... Args>
scoped_refptr<T> MakeRefCounted(Args&&... args) { … }

// Takes an instance of T, which is a ref counted type, and wraps the object
// into a scoped_refptr<T>.
template <typename T>
scoped_refptr<T> WrapRefCounted(T* t) { … }

}  // namespace base

//
// A smart pointer class for reference counted objects.  Use this class instead
// of calling AddRef and Release manually on a reference counted object to
// avoid common memory leaks caused by forgetting to Release an object
// reference.  Sample usage:
//
//   class MyFoo : public RefCounted<MyFoo> {
//    ...
//    private:
//     friend class RefCounted<MyFoo>;  // Allow destruction by RefCounted<>.
//     ~MyFoo();                        // Destructor must be private/protected.
//   };
//
//   void some_function() {
//     scoped_refptr<MyFoo> foo = MakeRefCounted<MyFoo>();
//     foo->Method(param);
//     // |foo| is released when this function returns
//   }
//
//   void some_other_function() {
//     scoped_refptr<MyFoo> foo = MakeRefCounted<MyFoo>();
//     ...
//     foo.reset();  // explicitly releases |foo|
//     ...
//     if (foo)
//       foo->Method(param);
//   }
//
// The above examples show how scoped_refptr<T> acts like a pointer to T.
// Given two scoped_refptr<T> classes, it is also possible to exchange
// references between the two objects, like so:
//
//   {
//     scoped_refptr<MyFoo> a = MakeRefCounted<MyFoo>();
//     scoped_refptr<MyFoo> b;
//
//     b.swap(a);
//     // now, |b| references the MyFoo object, and |a| references nullptr.
//   }
//
// To make both |a| and |b| in the above example reference the same MyFoo
// object, simply use the assignment operator:
//
//   {
//     scoped_refptr<MyFoo> a = MakeRefCounted<MyFoo>();
//     scoped_refptr<MyFoo> b;
//
//     b = a;
//     // now, |a| and |b| each own a reference to the same MyFoo object.
//   }
//
// Also see Chromium's ownership and calling conventions:
// https://chromium.googlesource.com/chromium/src/+/lkgr/styleguide/c++/c++.md#object-ownership-and-calling-conventions
// Specifically:
//   If the function (at least sometimes) takes a ref on a refcounted object,
//   declare the param as scoped_refptr<T>. The caller can decide whether it
//   wishes to transfer ownership (by calling std::move(t) when passing t) or
//   retain its ref (by simply passing t directly).
//   In other words, use scoped_refptr like you would a std::unique_ptr except
//   in the odd case where it's required to hold on to a ref while handing one
//   to another component (if a component merely needs to use t on the stack
//   without keeping a ref: pass t as a raw T*).
template <class T>
class TRIVIAL_ABI scoped_refptr { … };

template <typename T>
T* scoped_refptr<T>::release() { … }

// static
template <typename T>
void scoped_refptr<T>::AddRef(T* ptr) { … }

// static
template <typename T>
void scoped_refptr<T>::Release(T* ptr) { … }

template <typename T>
std::ostream& operator<<(std::ostream& out, const scoped_refptr<T>& p) { … }

template <typename T>
void swap(scoped_refptr<T>& lhs, scoped_refptr<T>& rhs) noexcept { … }

#endif  // BASE_MEMORY_SCOPED_REFPTR_H_