Hello,

Thank you Lénárd for your feedback and your extended documentation. Do you think I should get started on a draft?

Best regards,

—
Simon Scatton

>
> Le 2 janv. 2023 à 12:38, Lénárd Szolnoki via Std-Proposals <std-proposals_at_lists.isocpp.org> a écrit :
>
> 
>
>> On 2 January 2023 11:13:29 GMT, "Lénárd Szolnoki via Std-Proposals" <std-proposals_at_lists.isocpp.org> wrote:
>>
>>
>>> On 2 January 2023 10:06:52 GMT, "Lénárd Szolnoki via Std-Proposals" <std-proposals_at_lists.isocpp.org> wrote:
>>> Hi,
>>>
>>> On 2 January 2023 03:22:31 GMT, Simon Scatton via Std-Proposals <std-proposals_at_lists.isocpp.org> wrote:
>>>> Hello everyone,
>>>>
>>>> I am currently implementing a generic logger that should be thread-safe.
>>>> To make sure I rewrite as little code as possible, I am templating a class as
>>>> follows:
>>>>
>>>> template <typename Mutex>
>>>> class Output
>>>> {
>>>> public:
>>>> void write();
>>>> protected:
>>>> Mutex mutex_;
>>>> };
>>>>
>>>> And I have a "null mutex" that is a struct like this:
>>>> struct NullMutex
>>>> {
>>>> void lock() {}
>>>> void unlock() noexcept {}
>>>> bool try_lock() { return true; }
>>>> };
>>>>
>>>> So that for instance if I want a thread safe Output, I use this:
>>>> Output<std::mutex> A;
>>>> And if I want a non-thread safe Output I use this:
>>>> Output<NullMutex> B;
>>>>
>>>> The core of the write() function being something like this:
>>>>
>>>> template <typename Mutex>
>>>> void Output<Mutex>::write()
>>>> {
>>>> const std::lock_guard<Mutex> lock(mutex_);
>>>> // ...
>>>> }
>>>>
>>>> I have not found any C++ official resources about this.
>>>>
>>>> Has this been already addressed in C++ standard discussion around
>>>> mutexes ? Do you think adding such a "null mutex" would be useful?
>>>>
>>>> I would love to hear what you think about this.
>>>>
>>>> Kind regards,
>>>>
>>>
>>> Yes, it definitely makes sense. I saw a similar type used in Boost.ASIO:
>>>
>>> https://github.com/boostorg/asio/blob/develop/include/boost/asio/detail/null_mutex.hpp#L29
>>>
>>> For example it is used for implementing basic_channel (null mutex) and basic_concurrent_channel (some regular mutex), employing the same pattern you described.
>>>
>>> I think it might be worth adding to the standard library.
>>>
>>> Cheers,
>>> Lénárd
>>> --
>>> Std-Proposals mailing list
>>> Std-Proposals_at_[hidden]
>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
>> Apparently there is a public null_mutex class in Boost as well:
>>
>> https://www.boost.org/doc/libs/1_81_0/boost/thread/null_mutex.hpp
>>
>> It looks like a lot of boilerplate to satisfy the syntactic requirements of all the mutex concepts.
>>
>> Cheers,
>> Lénárd
>> --
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>
> Additional examples:
>
> spdlog: https://github.com/gabime/spdlog/blob/v1.x/include/spdlog/details/null_mutex.h#L12
>
> Intel TBB:
> https://github.com/oneapi-src/oneTBB/blob/v2021.8.0/include/oneapi/tbb/null_mutex.h#L31
>
> There might be several other examples with different names (noop_mutex maybe, or NullMutex), but I did not bother searching further.
>
> There is some variation in the implementations, but not in a way that would make them inherently incompatible.
>
> I like that Boost's implementation satisfies all the mutex concepts syntactically.
>
> spdlog makes the lock and unlock members const-qualified. I think that makes sense. Personally I would make every member function constexpr as well.
>
> A std::null_mutex could also be part of freestanding as well.
>
> Cheers,
> Lénárd
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