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// Copyright 2021 Hans Dembinski
//
// Distributed under the Boost Software License, version 1.0.
// (See accompanying file LICENSE_1_0.txt
// or copy at http://www.boost.org/LICENSE_1_0.txt)
#ifndef BOOST_HISTOGRAM_DETAIL_ATOMIC_NUMBER_HPP
#define BOOST_HISTOGRAM_DETAIL_ATOMIC_NUMBER_HPP
#include <atomic>
#include <boost/histogram/detail/priority.hpp>
#include <type_traits>
namespace boost {
namespace histogram {
namespace detail {
// copyable arithmetic type with thread-safe operator++ and operator+=
template <class T>
struct atomic_number : std::atomic<T> {
static_assert(std::is_arithmetic<T>(), "");
using base_t = std::atomic<T>;
using std::atomic<T>::atomic;
atomic_number() noexcept = default;
atomic_number(const atomic_number& o) noexcept : std::atomic<T>{o.load()} {}
atomic_number& operator=(const atomic_number& o) noexcept {
this->store(o.load());
return *this;
}
// not defined for float
atomic_number& operator++() noexcept {
increment_impl(static_cast<base_t&>(*this), priority<1>{});
return *this;
}
// operator is not defined for floating point before C++20
atomic_number& operator+=(const T& x) noexcept {
add_impl(static_cast<base_t&>(*this), x, priority<1>{});
return *this;
}
// not thread-safe
atomic_number& operator*=(const T& x) noexcept {
this->store(this->load() * x);
return *this;
}
// not thread-safe
atomic_number& operator/=(const T& x) noexcept {
this->store(this->load() / x);
return *this;
}
private:
// for integral types
template <class U = T>
auto increment_impl(std::atomic<U>& a, priority<1>) noexcept -> decltype(++a) {
return ++a;
}
// fallback implementation for floating point types
template <class U = T>
void increment_impl(std::atomic<U>&, priority<0>) noexcept {
this->operator+=(static_cast<U>(1));
}
// always available for integral types, in C++20 also available for float
template <class U = T>
static auto add_impl(std::atomic<U>& a, const U& x, priority<1>) noexcept
-> decltype(a += x) {
return a += x;
}
// pre-C++20 fallback implementation for floating point
template <class U = T>
static void add_impl(std::atomic<U>& a, const U& x, priority<0>) noexcept {
U expected = a.load();
// if another tread changed `expected` in the meantime, compare_exchange returns
// false and updates `expected`; we then loop and try to update again;
// see https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange
while (!a.compare_exchange_weak(expected, expected + x))
;
}
};
} // namespace detail
} // namespace histogram
} // namespace boost
#endif
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