On 2 January 2023 11:13:29 GMT, "Lénárd Szolnoki via Std-Proposals" <std-proposals_at_[hidden]> wrote:
>
>
>On 2 January 2023 10:06:52 GMT, "Lénárd Szolnoki via Std-Proposals" <std-proposals_at_[hidden]> wrote:
>>Hi,
>>
>>On 2 January 2023 03:22:31 GMT, Simon Scatton via Std-Proposals <std-proposals_at_[hidden]> 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
>--
>Std-Proposals mailing list
>Std-Proposals_at_[hidden]
>https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

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