Move FunctionView from AudioCodingModule to the rtc namespace

webrtc/base/function_view.h

+/*
+ *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_FUNCTION_VIEW_H_
+#define WEBRTC_BASE_FUNCTION_VIEW_H_
+
+#include <type_traits>
+#include <utility>
+
+// Just like std::function, FunctionView will wrap any callable and hide its
+// actual type, exposing only its signature. But unlike std::function,
+// FunctionView doesn't own its callable---it just points to it. Thus, it's a
+// good choice mainly as a function argument when the callable argument will
+// not be called again once the function has returned.
+//
+// TODO(kwiberg): FunctionView doesn't work with function pointers, just with
+// lambdas. It's trivial to work around this by wrapping the function pointer
+// in a stateless lambda, but it's tedious so it'd be nice to not have to do
+// it.
+
+namespace rtc {
+
+template <typename T>
+class FunctionView;  // Undefined.
+
+template <typename RetT, typename... ArgT>
+class FunctionView<RetT(ArgT...)> final {
+ public:
+  // This constructor is implicit, so that callers won't have to convert
+  // lambdas and other callables to FunctionView<Blah(Blah, Blah)> explicitly.
+  // This is safe because FunctionView is only a reference to the real
+  // callable.
+  //
+  // We jump through some template metaprogramming hoops to ensure that this
+  // constructor does *not* accept FunctionView arguments. That way, copy
+  // construction, assignment, swap etc. will all do the obvious thing (because
+  // they use the implicitly-declared copy constructor and copy assignment),
+  // and we will never get a FunctionView object that points to another
+  // FunctionView.
+  template <typename F,
+            typename std::enable_if<!std::is_same<
+                FunctionView,
+                typename std::remove_cv<typename std::remove_reference<
+                    F>::type>::type>::value>::type* = nullptr>
+  FunctionView(F&& f)
+      : f_(&f), call_(Call<typename std::remove_reference<F>::type>) {}
+
+  RetT operator()(ArgT... args) const {
+    return call_(f_, std::forward<ArgT>(args)...);
+  }
+
+ private:
+  template <typename F>
+  static RetT Call(void* f, ArgT... args) {
+    return (*static_cast<F*>(f))(std::forward<ArgT>(args)...);
+  }
+  void* f_;
+  RetT (*call_)(void* f, ArgT... args);
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_FUNCTION_VIEW_H_