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* 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.
#include <type_traits>
#include <utility>
#include "rtc_base/checks.h"
// 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.
// Its constructors are 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.
// Example use:
// void SomeFunction(rtc::FunctionView<int(int)> index_transform);
// ...
// SomeFunction([](int i) { return 2 * i + 1; });
// Note: FunctionView is tiny (essentially just two pointers) and trivially
// copyable, so it's probably cheaper to pass it by value than by const
// reference.
namespace rtc {
template <typename T>
class FunctionView; // Undefined.
template <typename RetT, typename... ArgT>
class FunctionView<RetT(ArgT...)> final {
// Constructor for lambdas and other callables; it accepts every type of
// argument except those noted in its enable_if call.
template <
typename F,
typename std::enable_if<
// Not for function pointers; we have another constructor for that
// below.
!std::is_function<typename std::remove_pointer<
typename std::remove_reference<F>::type>::type>::value &&
// Not for nullptr; we have another constructor for that below.
typename std::remove_cv<F>::type>::value &&
// Not for FunctionView objects; we have another constructor for that
// (the implicitly declared copy constructor).
typename std::remove_cv<typename std::remove_reference<
F>::type>::type>::value>::type* = nullptr>
FunctionView(F&& f)
: call_(CallVoidPtr<typename std::remove_reference<F>::type>) {
f_.void_ptr = &f;
// Constructor that accepts function pointers. If the argument is null, the
// result is an empty FunctionView.
template <
typename F,
typename std::enable_if<std::is_function<typename std::remove_pointer<
typename std::remove_reference<F>::type>::type>::value>::type* =
FunctionView(F&& f)
: call_(f ? CallFunPtr<typename std::remove_pointer<F>::type> : nullptr) {
f_.fun_ptr = reinterpret_cast<void (*)()>(f);
// Constructor that accepts nullptr. It creates an empty FunctionView.
template <typename F,
typename std::enable_if<std::is_same<
typename std::remove_cv<F>::type>::value>::type* = nullptr>
FunctionView(F&& f) : call_(nullptr) {}
// Default constructor. Creates an empty FunctionView.
FunctionView() : call_(nullptr) {}
RetT operator()(ArgT... args) const {
return call_(f_, std::forward<ArgT>(args)...);
// Returns true if we have a function, false if we don't (i.e., we're null).
explicit operator bool() const { return !!call_; }
union VoidUnion {
void* void_ptr;
void (*fun_ptr)();
template <typename F>
static RetT CallVoidPtr(VoidUnion vu, ArgT... args) {
return (*static_cast<F*>(vu.void_ptr))(std::forward<ArgT>(args)...);
template <typename F>
static RetT CallFunPtr(VoidUnion vu, ArgT... args) {
return (reinterpret_cast<typename std::add_pointer<F>::type>(vu.fun_ptr))(
// A pointer to the callable thing, with type information erased. It's a
// union because we have to use separate types depending on if the callable
// thing is a function pointer or something else.
VoidUnion f_;
// Pointer to a dispatch function that knows the type of the callable thing
// that's stored in f_, and how to call it. A FunctionView object is empty
// (null) iff call_ is null.
RetT (*call_)(VoidUnion, ArgT...);
} // namespace rtc