[wpiutil] Split function_ref to a separate header

wpiutil/src/main/native/include/wpi/function_ref.h

@@ -0,0 +1,65 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+//===- llvm/ADT/STLExtras.h - Useful STL related functions ------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef WPIUTIL_WPI_FUNCTION_REF_H_
+#define WPIUTIL_WPI_FUNCTION_REF_H_
+
+#include <stdint.h>
+
+#include <type_traits>
+#include <utility>
+
+namespace wpi {
+
+/// An efficient, type-erasing, non-owning reference to a callable. This is
+/// intended for use as the type of a function parameter that is not used
+/// after the function in question returns.
+///
+/// This class does not own the callable, so it is not in general safe to store
+/// a function_ref.
+template <typename Fn>
+class function_ref;
+
+template <typename Ret, typename... Params>
+class function_ref<Ret(Params...)> {
+  Ret (*callback)(intptr_t callable, Params... params) = nullptr;
+  intptr_t callable;
+
+  template <typename Callable>
+  static Ret callback_fn(intptr_t callable, Params... params) {
+    return (*reinterpret_cast<Callable*>(callable))(
+        std::forward<Params>(params)...);
+  }
+
+ public:
+  function_ref() = default;
+  function_ref(std::nullptr_t) {}
+
+  template <typename Callable>
+  function_ref(Callable&& callable,
+               typename std::enable_if<
+                   !std::is_same<typename std::remove_reference<Callable>::type,
+                                 function_ref>::value>::type* = nullptr)
+      : callback(callback_fn<typename std::remove_reference<Callable>::type>),
+        callable(reinterpret_cast<intptr_t>(&callable)) {}
+
+  Ret operator()(Params... params) const {
+    return callback(callable, std::forward<Params>(params)...);
+  }
+
+  explicit operator bool() const { return callback; }
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_FUNCTION_REF_H_