SkLazyPtr, mk. 2

src/core/SkLazyPtr.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkLazyPtr_DEFINED
+#define SkLazyPtr_DEFINED
+
+/** Declare a lazily-chosen static pointer (or array of pointers) of type F.
+ *
+ *  Example usage:
+ *
+ *  Foo* CreateFoo() { return SkNEW(Foo); }
+ *  Foo* GetSingletonFoo() {
+ *      SK_DECLARE_STATIC_LAZY_PTR(Foo, singleton, CreateFoo);  // Clean up with SkDELETE.
+ *      return singleton.get();
+ *  }
+ *
+ *  These macros take an optional void (*Destroy)(T*) at the end. If not given, we'll use SkDELETE.
+ *  This option is most useful when T doesn't have a public destructor.
+ *
+ *  void CustomCleanup(Foo* ptr) { ... }
+ *  Foo* GetSingletonFooWithCustomCleanup() {
+ *      SK_DECLARE_STATIC_LAZY_PTR(Foo, singleton, CreateFoo, CustomCleanup);
+ *      return singleton.get();
+ *  }
+ *
+ *  If you have a bunch of related static pointers of the same type, you can
+ *  declare an array of lazy pointers together:
+ *
+ *  Foo* CreateFoo(int i) { return ...; }
+ *  Foo* GetCachedFoo(Foo::Enum enumVal) {
+ *      SK_DECLARE_STATIC_LAZY_PTR_ARRAY(Foo, Foo::kEnumCount, cachedFoos, CreateFoo);
+ *      return cachedFoos[enumVal];
+ *  }
+ *
+ *
+ *  You can think of SK_DECLARE_STATIC_LAZY_PTR as a cheaper specialization of
+ *  SkOnce.  There is no mutex or extra storage used past the pointer itself.
+ *  In debug mode, each lazy pointer will be cleaned up at process exit so we
+ *  can check that we've not leaked or freed them early.
+ *
+ *  We may call Create more than once, but all threads will see the same pointer
+ *  returned from get().  Any extra calls to Create will be cleaned up.
+ *
+ *  These macros must be used in a global or function scope, not as a class member.
+ */
+
+#define SK_DECLARE_STATIC_LAZY_PTR(T, name, Create, ...) \
+    static Private::SkLazyPtr<T, Create, ##__VA_ARGS__> name
+
+#define SK_DECLARE_STATIC_LAZY_PTR_ARRAY(T, name, N, Create, ...) \
+    static Private::SkLazyPtrArray<T, N, Create, ##__VA_ARGS__> name
+
+
+
+// Everything below here is private implementation details.  Don't touch, don't even look.
+
+#include "SkDynamicAnnotations.h"
+#include "SkThread.h"
+#include "SkThreadPriv.h"
+
+// See FIXME below.
+class SkFontConfigInterface;
+class SkTypeface;
+
+namespace Private {
+
+template <typename T> void sk_delete(T* ptr) { SkDELETE(ptr); }
+
+// Set *dst to ptr if *dst is NULL.  Returns value of *dst, destroying ptr if not swapped in.
+// Issues the same memory barriers as sk_atomic_cas: acquire on failure, release on success.
+template <typename P, void (*Destroy)(P)>
+static P try_cas(void** dst, P ptr) {
+    P prev = (P)sk_atomic_cas(dst, NULL, ptr);
+
+    if (prev) {
+        // We need an acquire barrier before returning prev, which sk_atomic_cas provided.
+        Destroy(ptr);
+        return prev;
+    } else {
+        // We need a release barrier before returning ptr, which sk_atomic_cas provided.
+        return ptr;
+    }
+}
+
+// This has no constructor and must be zero-initalized (the macro above does this).
+template <typename T, T* (*Create)(), void (*Destroy)(T*) = sk_delete<T> >
+class SkLazyPtr {
+public:
+    T* get() {
+        // If fPtr has already been filled, we need an acquire barrier when loading it.
+        // If not, we need a release barrier when setting it.  try_cas will do that.
+        T* ptr = (T*)sk_acquire_load(&fPtr);
+        return ptr ? ptr : try_cas<T*, Destroy>(&fPtr, Create());
+    }
+
+#ifdef SK_DEBUG
+    // FIXME: We know we leak refs on some classes.  For now, let them leak.
+    void cleanup(SkFontConfigInterface*) {}
+    void cleanup(SkTypeface*) {}
+    template <typename U> void cleanup(U* ptr) { Destroy(ptr); }
+
+    ~SkLazyPtr() {
+        this->cleanup((T*)fPtr);
+        fPtr = NULL;
+    }
+#endif
+
+private:
+    void* fPtr;
+};
+
+// This has no constructor and must be zero-initalized (the macro above does this).
+template <typename T, int N, T* (*Create)(int), void (*Destroy)(T*) = sk_delete<T> >
+class SkLazyPtrArray {
+public:
+    T* operator[](int i) {
+        SkASSERT(i >= 0 && i < N);
+        // If fPtr has already been filled, we need an acquire barrier when loading it.
+        // If not, we need a release barrier when setting it.  try_cas will do that.
+        T* ptr = (T*)sk_acquire_load(&fArray[i]);
+        return ptr ? ptr : try_cas<T*, Destroy>(&fArray[i], Create(i));
+    }
+
+#ifdef SK_DEBUG
+    ~SkLazyPtrArray() {
+        for (int i = 0; i < N; i++) {
+            Destroy((T*)fArray[i]);
+            fArray[i] = NULL;
+        }
+    }
+#endif
+
+private:
+    void* fArray[N];
+};
+
+}  // namespace Private
+
+#endif//SkLazyPtr_DEFINED