SkRefCnt.h

/*
 * Copyright 2006 The Android Open Source Project
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#ifndef SkRefCnt_DEFINED
#define SkRefCnt_DEFINED

#include "../private/SkTLogic.h"
#include "SkTypes.h"
#include <atomic>
#include <functional>
#include <memory>
#include <type_traits>
#include <utility>

class SK_API SkRefCntBase : SkNoncopyable {
public:
    SkRefCntBase() : fRefCnt(1) {}

    virtual ~SkRefCntBase() {
#ifdef SK_DEBUG
        SkASSERTF(getRefCnt() == 1, "fRefCnt was %d", getRefCnt());
        // illegal value, to catch us if we reuse after delete
        fRefCnt.store(0, std::memory_order_relaxed);
#endif
    }

#ifdef SK_DEBUG

    int32_t getRefCnt() const {
        return fRefCnt.load(std::memory_order_relaxed);
    }

    void validate() const {
        SkASSERT(getRefCnt() > 0);
    }
#endif

    bool unique() const {
        if (1 == fRefCnt.load(std::memory_order_acquire)) {
            // The acquire barrier is only really needed if we return true.  It
            // prevents code conditioned on the result of unique() from running
            // until previous owners are all totally done calling unref().
            return true;
        }
        return false;
    }

    void ref() const {
        SkASSERT(getRefCnt() > 0);
        // No barrier required.
        (void)fRefCnt.fetch_add(+1, std::memory_order_relaxed);
    }

    void unref() const {
        SkASSERT(getRefCnt() > 0);
        // A release here acts in place of all releases we "should" have been doing in ref().
        if (1 == fRefCnt.fetch_add(-1, std::memory_order_acq_rel)) {
            // Like unique(), the acquire is only needed on success, to make sure
            // code in internal_dispose() doesn't happen before the decrement.
            this->internal_dispose();
        }
    }

protected:
    void internal_dispose_restore_refcnt_to_1() const {
        SkASSERT(0 == getRefCnt());
        fRefCnt.store(1, std::memory_order_relaxed);
    }

private:
    virtual void internal_dispose() const {
        this->internal_dispose_restore_refcnt_to_1();
        delete this;
    }

    // The following friends are those which override internal_dispose()
    // and conditionally call SkRefCnt::internal_dispose().
    friend class SkWeakRefCnt;

    mutable std::atomic<int32_t> fRefCnt;

    typedef SkNoncopyable INHERITED;
};

#ifdef SK_REF_CNT_MIXIN_INCLUDE
// It is the responsibility of the following include to define the type SkRefCnt.
// This SkRefCnt should normally derive from SkRefCntBase.
#include SK_REF_CNT_MIXIN_INCLUDE
#else
class SK_API SkRefCnt : public SkRefCntBase {
    // "#include SK_REF_CNT_MIXIN_INCLUDE" doesn't work with this build system.
    #if defined(GOOGLE3)
    public:
        void deref() const { this->unref(); }
    #endif
};
#endif


#if defined(SK_BUILD_FOR_ANDROID_FRAMEWORK)
// This version heuristically detects data races, since those otherwise result
// in redundant reference count decrements, which are exceedingly
// difficult to debug.

#define SkRefCnt_SafeAssign(dst, src)   \
    do {                                \
        typedef typename std::remove_reference<decltype(dst)>::type \
                SkRefCntPtrT;  \
        SkRefCntPtrT old_dst = *const_cast<SkRefCntPtrT volatile *>(&dst); \
        if (src) src->ref();            \
        if (old_dst) old_dst->unref();          \
        if (old_dst != *const_cast<SkRefCntPtrT volatile *>(&dst)) { \
            SkDebugf("Detected racing Skia calls at %s:%d\n", \
                    __FILE__, __LINE__); \
        } \
        dst = src;                      \
    } while (0)

#else /* !SK_BUILD_FOR_ANDROID_FRAMEWORK */

#define SkRefCnt_SafeAssign(dst, src)   \
    do {                                \
        if (src) src->ref();            \
        if (dst) dst->unref();          \
        dst = src;                      \
    } while (0)

#endif


template <typename T> static inline T* SkRef(T* obj) {
    SkASSERT(obj);
    obj->ref();
    return obj;
}

template <typename T> static inline T* SkSafeRef(T* obj) {
    if (obj) {
        obj->ref();
    }
    return obj;
}

template <typename T> static inline void SkSafeUnref(T* obj) {
    if (obj) {
        obj->unref();
    }
}

template<typename T> static inline void SkSafeSetNull(T*& obj) {
    if (obj) {
        obj->unref();
        obj = nullptr;
    }
}


// This is a variant of SkRefCnt that's Not Virtual, so weighs 4 bytes instead of 8 or 16.
// There's only benefit to using this if the deriving class does not otherwise need a vtable.
template <typename Derived>
class SkNVRefCnt : SkNoncopyable {
public:
    SkNVRefCnt() : fRefCnt(1) {}
    ~SkNVRefCnt() { SkASSERTF(1 == getRefCnt(), "NVRefCnt was %d", getRefCnt()); }

    // Implementation is pretty much the same as SkRefCntBase. All required barriers are the same:
    //   - unique() needs acquire when it returns true, and no barrier if it returns false;
    //   - ref() doesn't need any barrier;
    //   - unref() needs a release barrier, and an acquire if it's going to call delete.

    bool unique() const { return 1 == fRefCnt.load(std::memory_order_acquire); }
    void ref() const { (void)fRefCnt.fetch_add(+1, std::memory_order_relaxed); }
    void  unref() const {
        if (1 == fRefCnt.fetch_add(-1, std::memory_order_acq_rel)) {
            // restore the 1 for our destructor's assert
            SkDEBUGCODE(fRefCnt.store(1, std::memory_order_relaxed));
            delete (const Derived*)this;
        }
    }
    void  deref() const { this->unref(); }

private:
    mutable std::atomic<int32_t> fRefCnt;
    int32_t getRefCnt() const {
        return fRefCnt.load(std::memory_order_relaxed);
    }
};


template <typename T> class sk_sp {
    using unspecified_bool_type = T* sk_sp::*;
public:
    using element_type = T;

    constexpr sk_sp() : fPtr(nullptr) {}
    constexpr sk_sp(std::nullptr_t) : fPtr(nullptr) {}

    sk_sp(const sk_sp<T>& that) : fPtr(SkSafeRef(that.get())) {}
    template <typename U, typename = skstd::enable_if_t<std::is_convertible<U*, T*>::value>>
    sk_sp(const sk_sp<U>& that) : fPtr(SkSafeRef(that.get())) {}

    sk_sp(sk_sp<T>&& that) : fPtr(that.release()) {}
    template <typename U, typename = skstd::enable_if_t<std::is_convertible<U*, T*>::value>>
    sk_sp(sk_sp<U>&& that) : fPtr(that.release()) {}

    explicit sk_sp(T* obj) : fPtr(obj) {}

    ~sk_sp() {
        SkSafeUnref(fPtr);
        SkDEBUGCODE(fPtr = nullptr);
    }

    sk_sp<T>& operator=(std::nullptr_t) { this->reset(); return *this; }

    sk_sp<T>& operator=(const sk_sp<T>& that) {
        this->reset(SkSafeRef(that.get()));
        return *this;
    }
    template <typename U, typename = skstd::enable_if_t<std::is_convertible<U*, T*>::value>>
    sk_sp<T>& operator=(const sk_sp<U>& that) {
        this->reset(SkSafeRef(that.get()));
        return *this;
    }

    sk_sp<T>& operator=(sk_sp<T>&& that) {
        this->reset(that.release());
        return *this;
    }
    template <typename U, typename = skstd::enable_if_t<std::is_convertible<U*, T*>::value>>
    sk_sp<T>& operator=(sk_sp<U>&& that) {
        this->reset(that.release());
        return *this;
    }

    T& operator*() const {
        SkASSERT(this->get() != nullptr);
        return *this->get();
    }

    // MSVC 2013 does not work correctly with explicit operator bool.
    // https://chromium-cpp.appspot.com/#core-blacklist
    // When explicit operator bool can be used, remove operator! and operator unspecified_bool_type.
    //explicit operator bool() const { return this->get() != nullptr; }
    operator unspecified_bool_type() const { return this->get() ? &sk_sp::fPtr : nullptr; }
    bool operator!() const { return this->get() == nullptr; }

    T* get() const { return fPtr; }
    T* operator->() const { return fPtr; }

    void reset(T* ptr = nullptr) {
        // Calling fPtr->unref() may call this->~() or this->reset(T*).
        // http://wg21.cmeerw.net/lwg/issue998
        // http://wg21.cmeerw.net/lwg/issue2262
        T* oldPtr = fPtr;
        fPtr = ptr;
        SkSafeUnref(oldPtr);
    }

    T* SK_WARN_UNUSED_RESULT release() {
        T* ptr = fPtr;
        fPtr = nullptr;
        return ptr;
    }

    void swap(sk_sp<T>& that) /*noexcept*/ {
        using std::swap;
        swap(fPtr, that.fPtr);
    }

private:
    T*  fPtr;
};

template <typename T> inline void swap(sk_sp<T>& a, sk_sp<T>& b) /*noexcept*/ {
    a.swap(b);
}

template <typename T, typename U> inline bool operator==(const sk_sp<T>& a, const sk_sp<U>& b) {
    return a.get() == b.get();
}
template <typename T> inline bool operator==(const sk_sp<T>& a, std::nullptr_t) /*noexcept*/ {
    return !a;
}
template <typename T> inline bool operator==(std::nullptr_t, const sk_sp<T>& b) /*noexcept*/ {
    return !b;
}

template <typename T, typename U> inline bool operator!=(const sk_sp<T>& a, const sk_sp<U>& b) {
    return a.get() != b.get();
}
template <typename T> inline bool operator!=(const sk_sp<T>& a, std::nullptr_t) /*noexcept*/ {
    return static_cast<bool>(a);
}
template <typename T> inline bool operator!=(std::nullptr_t, const sk_sp<T>& b) /*noexcept*/ {
    return static_cast<bool>(b);
}

template <typename T, typename U> inline bool operator<(const sk_sp<T>& a, const sk_sp<U>& b) {
    // Provide defined total order on sk_sp.
    // http://wg21.cmeerw.net/lwg/issue1297
    // http://wg21.cmeerw.net/lwg/issue1401 .
    return std::less<skstd::common_type_t<T*, U*>>()(a.get(), b.get());
}
template <typename T> inline bool operator<(const sk_sp<T>& a, std::nullptr_t) {
    return std::less<T*>()(a.get(), nullptr);
}
template <typename T> inline bool operator<(std::nullptr_t, const sk_sp<T>& b) {
    return std::less<T*>()(nullptr, b.get());
}

template <typename T, typename U> inline bool operator<=(const sk_sp<T>& a, const sk_sp<U>& b) {
    return !(b < a);
}
template <typename T> inline bool operator<=(const sk_sp<T>& a, std::nullptr_t) {
    return !(nullptr < a);
}
template <typename T> inline bool operator<=(std::nullptr_t, const sk_sp<T>& b) {
    return !(b < nullptr);
}

template <typename T, typename U> inline bool operator>(const sk_sp<T>& a, const sk_sp<U>& b) {
    return b < a;
}
template <typename T> inline bool operator>(const sk_sp<T>& a, std::nullptr_t) {
    return nullptr < a;
}
template <typename T> inline bool operator>(std::nullptr_t, const sk_sp<T>& b) {
    return b < nullptr;
}

template <typename T, typename U> inline bool operator>=(const sk_sp<T>& a, const sk_sp<U>& b) {
    return !(a < b);
}
template <typename T> inline bool operator>=(const sk_sp<T>& a, std::nullptr_t) {
    return !(a < nullptr);
}
template <typename T> inline bool operator>=(std::nullptr_t, const sk_sp<T>& b) {
    return !(nullptr < b);
}

template <typename T, typename... Args>
sk_sp<T> sk_make_sp(Args&&... args) {
    return sk_sp<T>(new T(std::forward<Args>(args)...));
}

/*
 *  Returns a sk_sp wrapping the provided ptr AND calls ref on it (if not null).
 *
 *  This is different than the semantics of the constructor for sk_sp, which just wraps the ptr,
 *  effectively "adopting" it.
 */
template <typename T> sk_sp<T> sk_ref_sp(T* obj) {
    return sk_sp<T>(SkSafeRef(obj));
}

#endif