On 6/1/23 00:20, Thiago Macieira wrote:
> On Wednesday, 31 May 2023 20:46:37 PDT Phil Bouchard wrote:
>>> How? It's no different than using a mutex, because it is using a mutex. So
>>> if it is not different, how is it better?
>>
>> Because it is abstracted thus done implicitly.
>
> And yet there's still a lock and the user must know that it is there. The
> normal code one would write
> if (!container.empty())
> container.push)back(1);
>
> is not safe.

Why? The mutex is recursive so it just gives priority to the current
thread for the scope of the condition.

But you might have problems for child threads:

if (!container.empty())
     std::thread
     (
         [&]()
         {
             container.push_back(1);
         }
     };

So we'll need "child-recursive" mutices.

> Is the code above meant to fail t compile? If so, how do you
> propose that happen? If not, how do you propose we change our teaching methods
> of thread-safe code so that developers will know this, instead of knowing the
> regular mutex API?
>
> And this of course becomes more complex when you have two or more containers.
> Here's the first part of an algorithm: given containers C1 and C2, if the first
> element in C1 isn't in C2, then remove it from there (pop_front) and append
> (push_bacl) to C2.
>
> If you realise what the second part will be, feel free to comment on it and
> explain how your API will ensure the user writes the proper code for that.

// imagine both containers are locked here for the scope of the condition:
if (find(C2.begin(), C2.end(), * C1.begin()) != C2.end())
{
     T value = C1.front();
     C1.pop_front();
     C2.push_back(value);
}

Again, where's the problem if both containers feature a recursive mutex?

Regarding teaching, this is higher-level programming so a new namespace
should encompass these new classes.

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