131 lines
4.6 KiB
C++
131 lines
4.6 KiB
C++
/*
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* Copyright 2016 The WebRTC Project Authors. All rights reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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#ifndef RTC_BASE_FUNCTION_VIEW_H_
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#define RTC_BASE_FUNCTION_VIEW_H_
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#include <type_traits>
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#include <utility>
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#include "rtc_base/checks.h"
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// Just like std::function, FunctionView will wrap any callable and hide its
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// actual type, exposing only its signature. But unlike std::function,
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// FunctionView doesn't own its callable---it just points to it. Thus, it's a
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// good choice mainly as a function argument when the callable argument will
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// not be called again once the function has returned.
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//
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// Its constructors are implicit, so that callers won't have to convert lambdas
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// and other callables to FunctionView<Blah(Blah, Blah)> explicitly. This is
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// safe because FunctionView is only a reference to the real callable.
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//
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// Example use:
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//
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// void SomeFunction(rtc::FunctionView<int(int)> index_transform);
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// ...
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// SomeFunction([](int i) { return 2 * i + 1; });
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//
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// Note: FunctionView is tiny (essentially just two pointers) and trivially
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// copyable, so it's probably cheaper to pass it by value than by const
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// reference.
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namespace rtc {
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template <typename T>
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class FunctionView; // Undefined.
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template <typename RetT, typename... ArgT>
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class FunctionView<RetT(ArgT...)> final {
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public:
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// Constructor for lambdas and other callables; it accepts every type of
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// argument except those noted in its enable_if call.
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template <
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typename F,
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typename std::enable_if<
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// Not for function pointers; we have another constructor for that
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// below.
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!std::is_function<typename std::remove_pointer<
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typename std::remove_reference<F>::type>::type>::value &&
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// Not for nullptr; we have another constructor for that below.
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!std::is_same<std::nullptr_t,
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typename std::remove_cv<F>::type>::value &&
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// Not for FunctionView objects; we have another constructor for that
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// (the implicitly declared copy constructor).
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!std::is_same<FunctionView,
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typename std::remove_cv<typename std::remove_reference<
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F>::type>::type>::value>::type* = nullptr>
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FunctionView(F&& f)
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: call_(CallVoidPtr<typename std::remove_reference<F>::type>) {
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f_.void_ptr = &f;
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}
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// Constructor that accepts function pointers. If the argument is null, the
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// result is an empty FunctionView.
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template <
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typename F,
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typename std::enable_if<std::is_function<typename std::remove_pointer<
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typename std::remove_reference<F>::type>::type>::value>::type* =
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nullptr>
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FunctionView(F&& f)
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: call_(f ? CallFunPtr<typename std::remove_pointer<F>::type> : nullptr) {
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f_.fun_ptr = reinterpret_cast<void (*)()>(f);
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}
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// Constructor that accepts nullptr. It creates an empty FunctionView.
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template <typename F,
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typename std::enable_if<std::is_same<
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std::nullptr_t,
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typename std::remove_cv<F>::type>::value>::type* = nullptr>
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FunctionView(F&& f) : call_(nullptr) {}
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// Default constructor. Creates an empty FunctionView.
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FunctionView() : call_(nullptr) {}
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RetT operator()(ArgT... args) const {
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RTC_DCHECK(call_);
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return call_(f_, std::forward<ArgT>(args)...);
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}
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// Returns true if we have a function, false if we don't (i.e., we're null).
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explicit operator bool() const { return !!call_; }
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private:
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union VoidUnion {
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void* void_ptr;
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void (*fun_ptr)();
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};
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template <typename F>
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static RetT CallVoidPtr(VoidUnion vu, ArgT... args) {
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return (*static_cast<F*>(vu.void_ptr))(std::forward<ArgT>(args)...);
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}
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template <typename F>
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static RetT CallFunPtr(VoidUnion vu, ArgT... args) {
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return (reinterpret_cast<typename std::add_pointer<F>::type>(vu.fun_ptr))(
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std::forward<ArgT>(args)...);
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}
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// A pointer to the callable thing, with type information erased. It's a
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// union because we have to use separate types depending on if the callable
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// thing is a function pointer or something else.
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VoidUnion f_;
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// Pointer to a dispatch function that knows the type of the callable thing
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// that's stored in f_, and how to call it. A FunctionView object is empty
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// (null) iff call_ is null.
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RetT (*call_)(VoidUnion, ArgT...);
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};
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} // namespace rtc
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#endif // RTC_BASE_FUNCTION_VIEW_H_
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