gecko-dev/gfx/2d/GenericRefCounted.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

// This header provides virtual, non-templated alternatives to MFBT's
// RefCounted<T>. It intentionally uses MFBT coding style with the intention of
// moving there should there be other use cases for it.

#ifndef MOZILLA_GENERICREFCOUNTED_H_
#define MOZILLA_GENERICREFCOUNTED_H_

#include <type_traits>

#include "mozilla/RefPtr.h"
#include "mozilla/RefCounted.h"

namespace mozilla {

/**
 * Common base class for GenericRefCounted and GenericAtomicRefCounted.
 *
 * Having this shared base class, common to both the atomic and non-atomic
 * cases, allows to have RefPtr's that don't care about whether the
 * objects they're managing have atomic refcounts or not.
 */
class GenericRefCountedBase {
 protected:
  virtual ~GenericRefCountedBase() = default;

 public:
  // AddRef() and Release() method names are for compatibility with nsRefPtr.
  virtual void AddRef() = 0;

  virtual void Release() = 0;

  // ref() and deref() method names are for compatibility with wtf::RefPtr.
  // No virtual keywords here: if a subclass wants to override the refcounting
  // mechanism, it is welcome to do so by overriding AddRef() and Release().
  void ref() { AddRef(); }
  void deref() { Release(); }

#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
  virtual const char* typeName() const = 0;
  virtual size_t typeSize() const = 0;
#endif
};

namespace detail {

template <RefCountAtomicity Atomicity>
class GenericRefCounted : public GenericRefCountedBase {
 protected:
  GenericRefCounted() : refCnt(0) {}

  virtual ~GenericRefCounted() { MOZ_ASSERT(refCnt == detail::DEAD); }

 public:
  virtual void AddRef() override {
    // Note: this method must be thread safe for GenericAtomicRefCounted.
    MOZ_ASSERT(int32_t(refCnt) >= 0);
#ifndef MOZ_REFCOUNTED_LEAK_CHECKING
    ++refCnt;
#else
    const char* type = typeName();
    uint32_t size = typeSize();
    const void* ptr = this;
    MozRefCountType cnt = ++refCnt;
    detail::RefCountLogger::logAddRef(ptr, cnt, type, size);
#endif
  }

  virtual void Release() override {
    // Note: this method must be thread safe for GenericAtomicRefCounted.
    MOZ_ASSERT(int32_t(refCnt) > 0);
#ifndef MOZ_REFCOUNTED_LEAK_CHECKING
    MozRefCountType cnt = --refCnt;
#else
    const char* type = typeName();
    const void* ptr = this;
    MozRefCountType cnt = --refCnt;
    // Note: it's not safe to touch |this| after decrementing the refcount,
    // except for below.
    detail::RefCountLogger::logRelease(ptr, cnt, type);
#endif
    if (0 == cnt) {
      // Because we have atomically decremented the refcount above, only
      // one thread can get a 0 count here, so as long as we can assume that
      // everything else in the system is accessing this object through
      // RefPtrs, it's safe to access |this| here.
#ifdef DEBUG
      refCnt = detail::DEAD;
#endif
      delete this;
    }
  }

  MozRefCountType refCount() const { return refCnt; }
  bool hasOneRef() const {
    MOZ_ASSERT(refCnt > 0);
    return refCnt == 1;
  }

 private:
  std::conditional_t<Atomicity == AtomicRefCount, Atomic<MozRefCountType>,
                     MozRefCountType>
      refCnt;
};

}  // namespace detail

/**
 * This reference-counting base class is virtual instead of
 * being templated, which is useful in cases where one needs
 * genericity at binary code level, but comes at the cost
 * of a moderate performance and size overhead, like anything virtual.
 */
class GenericRefCounted
    : public detail::GenericRefCounted<detail::NonAtomicRefCount> {};

/**
 * GenericAtomicRefCounted is like GenericRefCounted, with an atomically updated
 * reference counter.
 */
class GenericAtomicRefCounted
    : public detail::GenericRefCounted<detail::AtomicRefCount> {};

}  // namespace mozilla

#endif