atomic_word_cxx11.h

/*    Copyright 2014 10gen Inc.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

#pragma once

#if !defined(MONGO_HAVE_CXX11_ATOMICS)
#error "Cannot use atomic_word_cxx11.h without C++11 <atomic> support"
#endif

// This file is a bit unusual. Most other source files in this codebase assume that C++11
// things are only usable if __cplusplus >= 201103L. However, we have made an explicit decision
// to use <atomic> when available, even if full C++11 conformance is not advertised. As a
// result, we unconditionally include <atomic>, but guard all other C++11 features under
// __cplusplus >= 201103L so that we can work on platforms that don't yet offer those features, but
// do offer <atomic>

#include <atomic>

#if __cplusplus >= 201103L
#include <type_traits>
#endif

#include <boost/static_assert.hpp>

#include "mongo/base/disallow_copying.h"

namespace mongo {

template <typename _WordType>
class AtomicWord {
#if __cplusplus < 201103L
    // AtomicWords are not copyable in C++03.
    MONGO_DISALLOW_COPYING(AtomicWord);
#endif

public:
    typedef _WordType WordType;

    explicit AtomicWord(WordType value = WordType(0)) : _value(value) {}

#if __cplusplus >= 201103L
    // In C++11, AtomicWords are not copyable or movable.
    AtomicWord(const AtomicWord&) = delete;
    AtomicWord& operator=(const AtomicWord&) = delete;
    AtomicWord(AtomicWord&&) = delete;
    AtomicWord& operator=(AtomicWord&&) = delete;
#endif

    WordType load() const {
        return _value.load();
    }

    WordType loadRelaxed() const {
        return _value.load(std::memory_order_relaxed);
    }

    void store(WordType newValue) {
        return _value.store(newValue);
    }

    WordType swap(WordType newValue) {
        return _value.exchange(newValue);
    }

    WordType compareAndSwap(WordType expected, WordType newValue) {
        // NOTE: Subtle: compare_exchange mutates its first argument.
        _value.compare_exchange_strong(expected, newValue);
        return expected;
    }

    WordType fetchAndAdd(WordType increment) {
        return _value.fetch_add(increment);
    }

    WordType fetchAndSubtract(WordType decrement) {
        return _value.fetch_sub(decrement);
    }

    WordType addAndFetch(WordType increment) {
        return fetchAndAdd(increment) + increment;
    }

    WordType subtractAndFetch(WordType decrement) {
        return fetchAndSubtract(decrement) - decrement;
    }

private:
    std::atomic<WordType> _value;
};

#if __cplusplus >= 201103L
#define _ATOMIC_WORD_DECLARE(NAME, WTYPE)                       \
    typedef class AtomicWord<WTYPE> NAME;                       \
    namespace {                                                 \
    BOOST_STATIC_ASSERT(sizeof(NAME) == sizeof(WTYPE));         \
    BOOST_STATIC_ASSERT(std::is_standard_layout<WTYPE>::value); \
    }  // namespace
#else
#define _ATOMIC_WORD_DECLARE(NAME, WTYPE)               \
    typedef class AtomicWord<WTYPE> NAME;               \
    namespace {                                         \
    BOOST_STATIC_ASSERT(sizeof(NAME) == sizeof(WTYPE)); \
    }  // namespace
#endif

_ATOMIC_WORD_DECLARE(AtomicUInt32, unsigned)
_ATOMIC_WORD_DECLARE(AtomicUInt64, unsigned long long)
_ATOMIC_WORD_DECLARE(AtomicInt32, int)
_ATOMIC_WORD_DECLARE(AtomicInt64, long long)
#undef _ATOMIC_WORD_DECLARE

}  // namespace mongo