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Unified Diff: include/private/SkOncePtr.h

Issue 1301323006: SkOncePtr (Closed) Base URL: https://skia.googlesource.com/skia.git@master
Patch Set: drop destructor Created 5 years, 3 months ago
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Index: include/private/SkOncePtr.h
diff --git a/include/private/SkOncePtr.h b/include/private/SkOncePtr.h
new file mode 100644
index 0000000000000000000000000000000000000000..9af204bcb29381d97f8444165d10267ce79a460b
--- /dev/null
+++ b/include/private/SkOncePtr.h
@@ -0,0 +1,102 @@
+/*
+ * 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
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