[Development] Qt 5 types under consideration for deprecation / removal in Qt 6

[Allan Sandfeld Jensen]

On Friday, 23 August 2019 14:51:36 CEST Mutz, Marc via Development wrote:
> On 2019-08-23 14:31, Allan Sandfeld Jensen wrote:
> [...]
> 
> > condition_variable_any::_M_mutex
> 
> I didn't say _any :)
> 
> > The std::conditional_variable has the same problem QWaitCondition has.
> 
> I don't think so:
> 
> 
> https://code.woboq.org/gcc/libstdc++-v3/include/std/condition_variable.html#
> 166
> 
> takes the mutex from the lock.
> 
For waiting but not waking. I did note the libstdc++ non-any implementation 
doesn't lock for notifying, but it remains a fundamental requirement that you 
need to lock to modify the waiting queue, so I looked deeper:

So yeah, okay glibc does it, see https://code.woboq.org/userspace/glibc/nptl/
pthread_cond_signal.c.html#47 and https://code.woboq.org/userspace/glibc/nptl/
pthread_cond_common.c.html#254

It is just inherent to this kind of design that an extra lock is necessary. So 
std::conditional_variable_any and QWaitCondition has two extras, where the 
std::conditional_variable only has one extra mutex overhead.

Oh well. Conditions are not exactly the most efficient of multi-threading 
primitives in any case.

'Allan