SkOncePtr

include/private/SkOncePtr.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkOncePtr_DEFINED
+#define SkOncePtr_DEFINED
+
+#include "SkAtomics.h"
+
+template <typename T> class SkStaticOnce;
+
+// Use this to create a global static pointer that's intialized exactly once when you call get().
+#define SK_DECLARE_STATIC_ONCE_PTR(type, name) namespace {} static SkStaticOnce<type> name
+
+// Use this for a local or member pointer that's initialized exactly once when you call get().
+template <typename T>
+class SkOncePtr : SkNoncopyable {
+public:
+    SkOncePtr() { sk_bzero(this, sizeof(*this)); }
+
+    // SkOncePtr does not have a destructor and does not clean up the pointer.  But you may, e.g.
+    //   delete (T*)fOncePtr;
+    //   SkSafeUnref((T*)fOncePtr);
+    // etc.
+
+    template <typename F>
+    T* get(const F& f) const {
+        return fOnce.get(f);
+    }
+
+    operator T*() const {
+        return (T*)fOnce;
+    }
+
+private:
+    SkStaticOnce<T> fOnce;
+};
+
+/* TODO(mtklein): in next CL
+typedef SkStaticOnce<void> SkOnceFlag;
+#define SK_DECLARE_STATIC_ONCE(name) namespace {} static SkOnceFlag name
+
+template <typename F>
+inline void SkOnce(SkOnceFlag* once, const F& f) {
+    once->get([&]{ f(); return (void*)2; });
+}
+*/
+
+// Implementation details below here!  No peeking!
+
+template <typename T>
+class SkStaticOnce {
+public:
+    template <typename F>
+    T* get(const F& f) const {
+        uintptr_t state = fState.load(sk_memory_order_acquire);
+        if (state < 2) {
+            if (state == 0) {
+                // It looks like no one has tried to create our pointer yet.
+                // We try to claim that task by atomically swapping our state from '0' to '1'.
+                if (fState.compare_exchange(&state, 1, sk_memory_order_relaxed,
+                                                       sk_memory_order_relaxed)) {
+                    // We've claimed it.  Create our pointer and store it into fState.
+                    state = (uintptr_t)f();
+                    SkASSERT(state > 1);
+                    fState.store(state, sk_memory_order_release);
+                } else {
+                    // Someone else claimed it.
+                    // We fall through to the spin loop just below to wait for them to finish.
+                }
+            }
+
+            while (state == 1) {
+                // State '1' is our busy-but-not-done state.
+                // Some other thread has claimed the job of creating our pointer.
+                // We just need to wait for it to finish.
+                state = fState.load(sk_memory_order_acquire);
+            }
+
+            // We shouldn't be able to get here without having created our pointer.
+            SkASSERT(state > 1);
+        }
+        return (T*)state;
+    }
+
+    operator T*() const {
+        auto state = fState.load(sk_memory_order_acquire);
+        return state < 2 ? nullptr : (T*)state;
+        // TODO: If state == 1 spin until it's not?
+    }
+
+private:
+    // fState == 0 --> we have not created our ptr yet
+    // fState == 1 --> someone is in the middle of creating our ptr
+    // else        --> (T*)fState is our ptr
+    mutable SkAtomic<uintptr_t> fState;
+};
+
+#endif//SkOncePtr_DEFINED