Sketch SkFunction

Let's start with baby steps in case some bot can't handle this.
I have left many TODOs, most of which I know how to do if this
looks feasible and useful.

BUG=skia:

Review URL: https://codereview.chromium.org/1049223003

1 files changed, 70 insertions, 0 deletions

diff --git a/src/core/SkFunction.h b/src/core/SkFunction.h
new file mode 100644
index 0000000000..9ec421e2a6
--- /dev/null
+++ b/src/core/SkFunction.h
@@ -0,0 +1,70 @@
+/*
+ * 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 SkFunction_DEFINED
+#define SkFunction_DEFINED
+
+// TODO: document
+
+#include "SkTypes.h"
+
+template <typename> class SkFunction;
+
+template <typename R, typename... Args>
+class SkFunction<R(Args...)> : SkNoncopyable {
+public:
+    explicit SkFunction(R (*fn)(Args...)) : fVTable(GetFunctionPointerVTable()) {
+        // We've been passed a function pointer.  We'll just store it.
+        fFunction = reinterpret_cast<void*>(fn);
+    }
+
+    template <typename Fn>
+    explicit SkFunction(Fn fn) : fVTable(GetVTable<Fn>()) {
+        // We've got a functor.  The basic thing we can always do is copy it onto the heap.
+        fFunction = SkNEW_ARGS(Fn, (fn));
+    }
+
+    ~SkFunction() { fVTable.fDelete(fFunction); }
+
+    R operator()(Args... args) { return fVTable.fCall(fFunction, args...); }
+
+private:
+    struct VTable {
+        R (*fCall)(void*, Args...);
+        void (*fDelete)(void*);
+    };
+
+    static const VTable& GetFunctionPointerVTable() {
+        static const VTable vtable = {
+            [](void* fn, Args... args) { return reinterpret_cast<R(*)(Args...)>(fn)(args...); },
+            [](void*) { /* Don't delete function pointers. */ },
+        };
+        return vtable;
+    }
+
+    template <typename Fn>
+    static const VTable& GetVTable() {
+        static const VTable vtable = {
+            [](void* fn, Args... args) { return (*static_cast<Fn*>(fn))(args...); },
+            [](void* fn) { SkDELETE(static_cast<Fn*>(fn)); },
+        };
+        return vtable;
+    }
+
+    void* fFunction;        // Either a function pointer, or a pointer to a functor.
+    const VTable& fVTable;  // How to call, delete (and one day copy, move) fFunction.
+};
+
+// TODO:
+//   - is it worth moving fCall out of the VTable into SkFunction itself to avoid the indirection?
+//   - should constructors be implicit?
+//   - make SkFunction copyable
+//   - emulate std::forward for moveable functors (e.g. lambdas)
+//   - forward args too?
+//   - implement small-object optimization to store functors inline
+
+#endif//SkFunction_DEFINED