Issue #2472 - Part 2: Add FunctionRef callable type

Prerequisite for top layer refactor.

FunctionRef is a generic callable type that can be initialized from any
compatible callable, suitable for use as a function argument for the duration of
the function call (and no longer).

mfbt/FunctionRef.h

@@ -0,0 +1,220 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A generic callable type that can be initialized from any compatible callable,
+ * suitable for use as a function argument for the duration of the function
+ * call (and no longer).
+ */
+
+#ifndef mozilla_FunctionRef_h
+#define mozilla_FunctionRef_h
+
+#include "mozilla/OperatorNewExtensions.h"  // mozilla::NotNull, ::operator new
+
+#include <cstddef>      // std::nullptr_t
+#include <type_traits>  // std::{declval,integral_constant}, std::is_{convertible,same,void}_v, std::{enable_if,remove_reference,remove_cv}_t
+
+// This concept and its implementation are substantially inspired by foonathan's prior art:
+//
+// https://foonathan.net/2017/01/function-ref-implementation/
+// https://github.com/foonathan/type_safe/blob/2017851053f8dd268372f1612865792c5c621570/include/type_safe/reference.hpp
+//
+// Note: this is C++ code voodoo. Be careful if you have to make any changes.
+
+namespace mozilla {
+
+namespace detail {
+
+// Template helper to determine if |Returned| is a return type compatible with
+// |Required|: if the former converts to the latter, or if |Required| is |void|
+// and nothing is returned.
+template <typename Returned, typename Required>
+using CompatibleReturnType =
+    std::integral_constant<bool, std::is_void_v<Required> ||
+                                 std::is_convertible_v<Returned, Required>>;
+
+// Template helper to check if |Func| called with |Params| arguments returns
+// a type compatible with |Ret|.
+template <typename Func, typename Ret, typename... Params>
+using EnableMatchingFunction = std::enable_if_t<
+    CompatibleReturnType<
+        decltype(std::declval<Func&>()(std::declval<Params>()...)), Ret>::value,
+        int>;
+
+struct MatchingFunctionPointerTag {};
+struct MatchingFunctorTag {};
+struct InvalidFunctorTag {};
+
+// Template helper to determine the proper way to store |Callable|: as function
+// pointer, as pointer to object, or unstorable.
+template <typename Callable, typename Ret, typename... Params>
+struct GetCallableTag {
+  // Match the case where |Callable| is a compatible function pointer or
+  // converts to one.  (|+obj| invokes such a conversion.)
+  template <typename T>
+  static MatchingFunctionPointerTag test(
+      int, T& obj, EnableMatchingFunction<decltype(+obj), Ret, Params...> = 0);
+
+  // Match the case where |Callable| is callable but can't be converted to a
+  // function pointer.  (|short| is a worse match for 0 than |int|, causing the
+  // function pointer match to be preferred if both apply.)
+  template <typename T>
+  static MatchingFunctorTag test(short, T& obj,
+                                 EnableMatchingFunction<T, Ret, Params...> = 0);
+
+  // Match all remaining cases.  (Any other match is preferred to an ellipsis
+  // match.)
+  static InvalidFunctorTag test(...);
+
+  using Type = decltype(test(0, std::declval<Callable&>()));
+};
+
+// If the callable is |nullptr|, |std::declval<std::nullptr_t&>()| will be an
+// error.  Provide a specialization for |nullptr| that will fail substitution.
+template <typename Ret, typename... Params>
+struct GetCallableTag<std::nullptr_t, Ret, Params...> {};
+
+template <typename Result, typename Callable, typename Ret, typename... Params>
+using EnableFunctionTag = std::enable_if_t<
+    std::is_same_v<typename GetCallableTag<Callable, Ret, Params...>::Type,
+                   Result>,
+    int>;
+
+}  // namespace detail
+
+/**
+ * An efficient, type-erasing, non-owning reference to a callable. It 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 in general it is unsafe to store a
+ * FunctionRef.
+ */
+template <typename Fn>
+class FunctionRef;
+
+template <typename Ret, typename... Params>
+class FunctionRef<Ret(Params...)> {
+  union Payload;
+
+  // |FunctionRef| stores an adaptor function pointer, determined by the
+  // arguments passed to the constructor.  That adaptor will perform the steps
+  // needed to invoke the callable passed at construction time.
+  using Adaptor = Ret (*)(const Payload& aPayload, Params... aParams);
+
+  // If |FunctionRef|'s callable can be stored as a function pointer, that
+  // function pointer is stored after being cast to this *different* function
+  // pointer type.  |mAdaptor| then casts back to the original type to call it.
+  // ([expr.reinterpret.cast]p6 guarantees that A->B->A function pointer casts
+  // produce the original function pointer value.)  An outlandish signature is
+  // used to emphasize that the exact function pointer type doesn't matter.
+  using FuncPtr = Payload***** (*)(Payload*****);
+
+  /**
+   * An adaptor function (used by this class's function call operator) that
+   * invokes the callable in |mPayload|, forwarding arguments and converting
+   * return type as needed.
+   */
+  const Adaptor mAdaptor;
+
+  /** Storage for the wrapped callable value. */
+  union Payload {
+    // This arm is used if |FunctionRef| is passed a compatible function pointer
+    // or a lambda/callable that converts to a compatible function pointer.
+    FuncPtr mFuncPtr;
+
+    // This arm is used if |FunctionRef| is passed some other callable or
+    // |nullptr|.
+    void* mObject;
+  } mPayload;
+
+  template <typename RealFuncPtr>
+  static Ret CallFunctionPointer(const Payload& aPayload,
+                                 Params... aParams) noexcept {
+    auto func = reinterpret_cast<RealFuncPtr>(aPayload.mFuncPtr);
+    return static_cast<Ret>(func(static_cast<Params>(aParams)...));
+  }
+
+  template <typename Ret2, typename... Params2>
+  FunctionRef(detail::MatchingFunctionPointerTag, Ret2 (*aFuncPtr)(Params2...))
+      : mAdaptor(&CallFunctionPointer<Ret2 (*)(Params2...)>) {
+    ::new (KnownNotNull, &mPayload.mFuncPtr)
+        FuncPtr(reinterpret_cast<FuncPtr>(aFuncPtr));
+  }
+
+ public:
+  /**
+   * Construct a |FunctionRef| that's like a null function pointer that can't be
+   * called.
+   */
+  MOZ_IMPLICIT FunctionRef(std::nullptr_t) noexcept : mAdaptor(nullptr) {
+    // This is technically unnecessary, but it seems best to always initialize
+    // a union arm.
+    ::new (KnownNotNull, &mPayload.mObject) void*(nullptr);
+  }
+
+  FunctionRef() : FunctionRef(nullptr) {}
+
+  /**
+   * Constructs a |FunctionRef| from an object callable with |Params| arguments,
+   * that returns a type convertible to |Ret|, where the callable isn't
+   * convertible to function pointer (often because it contains some internal
+   * state).  For example:
+   *
+   *   int x = 5;
+   *   auto doSideEffect = [&x]{ x++; }; // state is captured reference to |x|
+   *   FunctionRef<void()> f(doSideEffect);
+   */
+  template <
+      typename Callable,
+      typename = detail::EnableFunctionTag<detail::MatchingFunctorTag, Callable,
+                                           Ret, Params...>,
+      typename std::enable_if_t<!std::is_same_v<
+          typename std::remove_reference_t<typename std::remove_cv_t<Callable>>,
+          FunctionRef>>* = nullptr>
+  MOZ_IMPLICIT FunctionRef(Callable& aCallable) noexcept
+      : mAdaptor([](const Payload& aPayload, Params... aParams) {
+          auto& func = *static_cast<Callable*>(aPayload.mObject);
+          return static_cast<Ret>(func(static_cast<Params>(aParams)...));
+        }) {
+    ::new (KnownNotNull, &mPayload.mObject) void*(&aCallable);
+  }
+
+  /**
+   * Constructs a |FunctionRef| from an value callable with |Params| arguments,
+   * that returns a type convertible to |Ret|, where the callable is stateless
+   * and is (or is convertible to) a function pointer.  For example:
+   *
+   *   // Exact match
+   *   double twice(double d) { return d * 2; }
+   *   FunctionRef<double(double)> func1(&twice);
+   *
+   *   // Compatible match
+   *   float thrice(long double d) { return static_cast<float>(d) * 3; }
+   *   FunctionRef<double(double)> func2(&thrice);
+   *
+   *   // Non-generic lambdas that don't capture anything have a conversion
+   *   // function to the appropriate function pointer type.
+   *   FunctionRef<int(double)> f([](long double){ return 'c'; });
+   */
+  template <typename Callable,
+            typename = detail::EnableFunctionTag<
+                detail::MatchingFunctionPointerTag, Callable, Ret, Params...>>
+  MOZ_IMPLICIT FunctionRef(const Callable& aCallable) noexcept
+      : FunctionRef(detail::MatchingFunctionPointerTag{}, +aCallable) {}
+
+  /** Call the callable stored in this with the given arguments. */
+  Ret operator()(Params... params) const {
+    return mAdaptor(mPayload, static_cast<Params>(params)...);
+  }
+
+  /** Return true iff this wasn't created from |nullptr|. */
+  explicit operator bool() const noexcept { return mAdaptor != nullptr; }
+};
+
+} /* namespace mozilla */
+
+#endif /* mozilla_FunctionRef_h */

mfbt/moz.build

@@ -42,6 +42,7 @@ EXPORTS.mozilla = [
     'FastBernoulliTrial.h',
     'FloatingPoint.h',
     'Function.h',
+    'FunctionRef.h',
     'GuardObjects.h',
     'HashFunctions.h',
     'IndexSequence.h',