SkTCopyOnFirstWrite

R=reed@google.com
Review URL: https://codereview.appspot.com/6650047

git-svn-id: http://skia.googlecode.com/svn/trunk@5905 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 54 insertions, 0 deletions

diff --git a/include/core/SkTLazy.h b/include/core/SkTLazy.h
index cf2d943829..cd1e8f2b9c 100644
--- a/include/core/SkTLazy.h
+++ b/include/core/SkTLazy.h
@@ -88,5 +88,59 @@ private:
     char fStorage[sizeof(T)];
 };
 
+/**
+ * A helper built on top of SkTLazy to do copy-on-first-write. The object is initialized
+ * with a const pointer but provides a non-const pointer accessor. The first time the
+ * accessor is called (if ever) the object is cloned.
+ *
+ * In the following example at most one copy of constThing is made:
+ *
+ * SkTCopyOnFirstWrite<Thing> thing(&constThing);
+ * ...
+ * function_that_takes_a_const_thing_ptr(thing); // constThing is passed
+ * ...
+ * if (need_to_modify_thing()) {
+ *    thing.writable()->modifyMe(); // makes a copy of constThing
+ * }
+ * ...
+ * x = thing->readSomething();
+ * ...
+ * if (need_to_modify_thing_now()) {
+ *    thing.writable()->changeMe(); // makes a copy of constThing if we didn't call modifyMe()
+ * }
+ *
+ * consume_a_thing(thing); // could be constThing or a modified copy.
+ */
+template <typename T>
+class SkTCopyOnFirstWrite {
+public:
+    SkTCopyOnFirstWrite(const T& initial) : fObj(&initial) {}
+
+    /**
+     * Returns a writable T*. The first time this is called the initial object is cloned.
+     */
+    T* writable() {
+        if (!fLazy.isValid()) {
+            fLazy.set(*fObj);
+            fObj = fLazy.get();
+        }
+        return const_cast<T*>(fObj);
+    }
+
+    /**
+     * Operators for treating this as though it were a const pointer.
+     */
+
+    const T *operator->() const { return fObj; }
+
+    operator const T*() const { return fObj; }
+
+    const T& operator *() const { return *fObj; }
+
+private:
+    const T*    fObj;
+    SkTLazy<T>  fLazy;
+};
+
 #endif