Constructor and call argument forwarding for SkFunction.

src/core/SkFunction.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 SkFunction_DEFINED
 #define SkFunction_DEFINED
 
 // TODO: document
 
 #include "SkTypes.h"
 #include "SkTLogic.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, SK_WHEN_C((sizeof(Fn) > sizeof(void*)), void*) = nullptr)
             : fVTable(GetOutlineVTable<Fn>()) {
         // We've got a functor larger than a pointer.  We've go to copy it onto the heap.
-        fFunction = SkNEW_ARGS(Fn, (fn));
+        fFunction = SkNEW_ARGS(Fn, (Forward(fn)));
     }
 
     template <typename Fn>
     explicit SkFunction(Fn fn, SK_WHEN_C((sizeof(Fn) <= sizeof(void*)), void*) = nullptr)
             : fVTable(GetInlineVTable<Fn>()) {
         // We've got a functor that fits in a pointer.  We copy it right inline.
-        SkNEW_PLACEMENT_ARGS(&fFunction, Fn, (fn));
+        fFunction = NULL;  // Quiets a (spurious) warning that fFunction might be uninitialized.
+        SkNEW_PLACEMENT_ARGS(&fFunction, Fn, (Forward(fn)));
     }
 
     ~SkFunction() { fVTable.fCleanUp(fFunction); }
 
-    R operator()(Args... args) { return fVTable.fCall(fFunction, args...); }
+    R operator()(Args... args) { return fVTable.fCall(fFunction, Forward(args)...); }
 
 private:
+    // ~= std::forward.  This moves its argument if possible, falling back to a copy if not.
+    template <typename T> static T&& Forward(T& v) { return (T&&)v; }
+
     struct VTable {
         R (*fCall)(void*, Args...);
         void (*fCleanUp)(void*);
     };
 
     // Used when fFunction is a function pointer of type R(*)(Args...).
     static const VTable& GetFunctionPointerVTable() {
         static const VTable vtable = {
-            [](void* fn, Args... args) { return reinterpret_cast<R(*)(Args...)>(fn)(args...); },
+            [](void* fn, Args... args) {
+                return reinterpret_cast<R(*)(Args...)>(fn)(Forward(args)...);
+            },
             [](void*) { /* Nothing to clean up for function pointers. */ }
         };
         return vtable;
     }
 
     // Used when fFunction is a pointer to a functor of type Fn on the heap (we own it).
     template <typename Fn>
     static const VTable& GetOutlineVTable() {
         static const VTable vtable = {
-            [](void* fn, Args... args) { return (*static_cast<Fn*>(fn))(args...); },
+            [](void* fn, Args... args) { return (*static_cast<Fn*>(fn))(Forward(args)...); },
             [](void* fn) { SkDELETE(static_cast<Fn*>(fn)); },
         };
         return vtable;
     }
 
     // Used when fFunction _is_ a functor of type Fn, not a pointer to the functor.
     template <typename Fn>
     static const VTable& GetInlineVTable() {
         static const VTable vtable = {
             [](void* fn, Args... args) {
-                union { void* p; Fn f; } pun = { fn };
-                return pun.f(args...);
+                union { void** p; Fn* f; } pun = { &fn };
+                return (*pun.f)(Forward(args)...);
             },
             [](void* fn) {
-                union { void* p; Fn f; } pun = { fn };
-                pun.f.~Fn();
+                union { void** p; Fn* f; } pun = { &fn };
+                (*pun.f).~Fn();
                 (void)(pun.f);   // Otherwise, when ~Fn() is trivial, MSVC complains pun is unused.
             }
         };
         return vtable;
     }
 
 
     void* fFunction;        // A function pointer, a pointer to a functor, or an inlined 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?
 
 #endif//SkFunction_DEFINED