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// Copyright The OpenTelemetry Authors
// SPDX-License-Identifier: Apache-2.0
#pragma once
#include <iosfwd>
#include <memory> // IWYU pragma: keep
#include <type_traits>
#include <utility>
#include "opentelemetry/version.h"
OPENTELEMETRY_BEGIN_NAMESPACE
namespace nostd
{
template <class Sig>
class function_ref; // IWYU pragma: keep
/**
* Non-owning function reference that can be used as a more performant
* replacement for std::function when ownership sematics aren't needed.
*
* See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0792r0.html
*
* Based off of https://stackoverflow.com/a/39087660/4447365
*/
template <class R, class... Args>
class function_ref<R(Args...)>
{
void *callable_ = nullptr;
R (*invoker_)(void *, Args...) = nullptr;
template <class F>
using FunctionPointer = decltype(std::addressof(std::declval<F &>()));
template <class F>
void BindTo(F &f) noexcept
{
callable_ = static_cast<void *>(std::addressof(f));
invoker_ = [](void *callable, Args... args) -> R {
return (*static_cast<FunctionPointer<F>>(callable))(std::forward<Args>(args)...);
};
}
template <class R_in, class... Args_in>
void BindTo(R_in (*f)(Args_in...)) noexcept
{
using F = decltype(f);
if (f == nullptr)
{
return BindTo(nullptr);
}
callable_ = reinterpret_cast<void *>(f);
invoker_ = [](void *callable, Args... args) -> R {
return (F(callable))(std::forward<Args>(args)...);
};
}
void BindTo(std::nullptr_t) noexcept
{
callable_ = nullptr;
invoker_ = nullptr;
}
public:
template <
class F,
typename std::enable_if<!std::is_same<function_ref, typename std::decay<F>::type>::value,
int>::type = 0,
typename std::enable_if<
#if (__cplusplus >= 201703L) || (defined(_MSVC_LANG) && (_MSVC_LANG > 201402))
// std::result_of deprecated in C++17, removed in C++20
std::is_convertible<typename std::invoke_result<F, Args...>::type, R>::value,
#else
// std::result_of since C++11
std::is_convertible<typename std::result_of<F &(Args...)>::type, R>::value,
#endif
int>::type = 0>
function_ref(F &&f)
{
BindTo(f); // not forward
}
function_ref(std::nullptr_t) {}
function_ref(const function_ref &) noexcept = default;
function_ref(function_ref &&) noexcept = default;
R operator()(Args... args) const { return invoker_(callable_, std::forward<Args>(args)...); }
explicit operator bool() const { return invoker_; }
};
} // namespace nostd
OPENTELEMETRY_END_NAMESPACE
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