Add operator* and operator safe-bool to sk_sp.

include/core/SkRefCnt.h

Index: include/core/SkRefCnt.h
diff --git a/include/core/SkRefCnt.h b/include/core/SkRefCnt.h
index cea72cda66f9a6bdad350a4c398d67d654c56385..d9591c893210681f7e02de30f4c19ebb8aecc267 100644
--- a/include/core/SkRefCnt.h
+++ b/include/core/SkRefCnt.h
@@ -238,6 +238,7 @@ private:
  *  may have its ref/unref be thread-safe, but that is not assumed/imposed by sk_sp.
  */
 template <typename T> class sk_sp {
+    using unspecified_bool_type = T* sk_sp::*;

[mtklein, 2016/03/02 20:05:10]

  // safe bool idiom.  obsolete with explicit operator bool

[bungeman-skia, 2016/03/02 20:32:58]

Done.

 public:
     sk_sp() : fPtr(nullptr) {}
     sk_sp(std::nullptr_t) : fPtr(nullptr) {}
@@ -316,10 +317,16 @@ public:
     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; }

[mtklein, 2016/03/02 20:05:10]

 ... ? &fPtr : nullptr ...
?

[bungeman-skia, 2016/03/02 20:32:58]

So it turns out the type of '&fPtr' here is 'T*const*' but '&sk_sp::fPtr' is 'T*
sk_sp<T>::*' (which is what we actually want). The difference is that the first
is the address of the fPtr object in memory, the other is a field pointer which
may be used to find the fPtr field of sk_sp<T> objects (note that this means
that all instances of sk_sp<T> for a given T will share the same value with the
same type).

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