Add element_type, swap, operators, fix reset on sk_sp.

include/core/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/SkAtomics.h"
 #include "../private/SkUniquePtr.h"
 #include "SkTypes.h"
+#include <functional>
 #include <utility>
 
 /** \class SkRefCntBase
 
     SkRefCntBase is the base class for objects that may be shared by multiple
     objects. When an existing owner wants to share a reference, it calls ref().
     When an owner wants to release its reference, it calls unref(). When the
     shared object's reference count goes to zero as the result of an unref()
     call, its (virtual) destructor is called. It is an error for the
     destructor to be called explicitly (or via the object going out of scope on
@@ skipping 210 matching lines @@
  *  Shared pointer class to wrap classes that support a ref()/unref() interface.
  *
  *  This can be used for classes inheriting from SkRefCnt, but it also works for other
  *  classes that match the interface, but have different internal choices: e.g. the hosted class
  *  may have its ref/unref be thread-safe, but that is not assumed/imposed by sk_sp.
  */
 template <typename T> class sk_sp {
     /** Supports safe bool idiom. Obsolete with explicit operator bool. */
     using unspecified_bool_type = T* sk_sp::*;
 public:
+    using element_type = T;
+
     sk_sp() : fPtr(nullptr) {}
     sk_sp(std::nullptr_t) : fPtr(nullptr) {}
 
     /**
      *  Shares the underlying object by calling ref(), so that both the argument and the newly
      *  created sk_sp both have a reference to it.
      */
     sk_sp(const sk_sp<T>& that) : fPtr(SkSafeRef(that.get())) {}
-    template <typename U,
-              typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
+    template <typename U, typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
     sk_sp(const sk_sp<U>& that) : fPtr(SkSafeRef(that.get())) {}
 
     /**
      *  Move the underlying object from the argument to the newly created sk_sp. Afterwards only
      *  the new sk_sp will have a reference to the object, and the argument will point to null.
      *  No call to ref() or unref() will be made.
      */
     sk_sp(sk_sp<T>&& that) : fPtr(that.release()) {}
-    template <typename U,
-              typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
+    template <typename U, typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
     sk_sp(sk_sp<U>&& that) : fPtr(that.release()) {}
 
     /**
      *  Adopt the bare pointer into the newly created sk_sp.
      *  No call to ref() or unref() will be made.
      */
     explicit sk_sp(T* obj) : fPtr(obj) {}
 
     /**
      *  Calls unref() on the underlying object pointer.
      */
     ~sk_sp() {
         SkSafeUnref(fPtr);
+        SkDEBUGCODE(fPtr = nullptr);
     }
 
     sk_sp<T>& operator=(std::nullptr_t) { this->reset(); return *this; }
 
     /**
      *  Shares the underlying object referenced by the argument by calling ref() on it. If this
      *  sk_sp previously had a reference to an object (i.e. not null) it will call unref() on that
      *  object.
      */
     sk_sp<T>& operator=(const sk_sp<T>& that) {
         this->reset(SkSafeRef(that.get()));
         return *this;
     }
-    template <typename U,
-              typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
+    template <typename U, typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
     sk_sp<T>& operator=(const sk_sp<U>& that) {
         this->reset(SkSafeRef(that.get()));
         return *this;
     }
 
     /**
      *  Move the underlying object from the argument to the sk_sp. If the sk_sp previously held
      *  a reference to another object, unref() will be called on that object. No call to ref()
      *  will be made.
      */
     sk_sp<T>& operator=(sk_sp<T>&& that) {
         this->reset(that.release());
         return *this;
     }
-    template <typename U,
-              typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
+    template <typename U, typename = skstd::enable_if_t<skstd::is_convertible<U*, T*>::value>>
     sk_sp<T>& operator=(sk_sp<U>&& that) {
         this->reset(that.release());
         return *this;
     }
 
-    bool operator==(std::nullptr_t) const { return this->get() == nullptr; }
-    bool operator!=(std::nullptr_t) const { return this->get() != nullptr; }
-
-    template <typename U>
-    bool operator==(const sk_sp<U>& that) const { return this->get() == that.get(); }
-    template <typename U>
-    bool operator!=(const sk_sp<U>& that) const { return this->get() != that.get(); }
-
     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; }
 
     /**
      *  Adopt the new bare pointer, and call unref() on any previously held object (if not null).
      *  No call to ref() will be made.
      */
     void reset(T* ptr = nullptr) {

[bungeman-skia, 2016/03/07 20:07:54]

At the very least, Chromium went through a lot of work to get scoped_ptr to do
this, and doing so uncovered bugs (see
https://bugs.chromium.org/p/chromium/issues/detail?id=176091). I believe it
would be one less point of contention with Chromium to do things this way.

-        SkSafeUnref(fPtr);
+        // 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);
     }
 
     /**
      *  Return the bare pointer, and set the internal object pointer to nullptr.
      *  The caller must assume ownership of the object, and manage its reference count directly.
      *  No call to unref() will be made.
      */
     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)...));
 }
 
 #endif