Re: [boost] [thread] RFC standard proposed mutex, read-write mutex, condition

From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2007-08-22 18:56:15

On Aug 22, 2007, at 6:31 PM, David Abrahams wrote:

>
> on Wed Aug 22 2007, Howard Hinnant <howard.hinnant-AT-gmail.com>  
> wrote:
>
>> This is a class that we're proposing can be used in two different  
>> ways:
>
> Who's "we?" Does this correspond to your proposal, or, say, Peter's
> suggested modification?

This is my proposal plus Peter's suggested modification.  Sorry for  
the confusion.

My original proposal was:

condition(); // All waits must reference the same mutex of type  
mutex_type, else condition_error thrown

Peter's suggested modification is:

condition(); // No mutex consistency checking

The motivation for the suggested change is not that it saves  
performance (it doesn't).  But that there may be valid use cases.

The other constructor remains unmodified:

explicit condition(mutex_type& m); // All waits must reference m, else  
condition_error thrown

>> Use the default constructor and this code is legal:
>>
>> mutex m1;
>> mutex m2;
>> condition<mutex> cv;
>> ...
>> unique_lock<mutex> lk(m1);
>> while (pred)
>>    cv.wait(lk);
>> ...
>> unique_lock<mutex> lk(m2);
>> while (pred)
>>    cv.wait(lk);
>>
>> Simply change the condition construction to:
>>
>> condition<mutex> cv(m1);
>>
>> And all of the sudden the above code is no longer legal.  If the code
>> is legal with one constructor, what is it that makes the code a logic
>> error **100% of the time**, instead of a fixable exceptional
>> circumstance when using the second constructor?
>
> I didn't understand any of that sentence after the word "constructor."
> Could you rephrase?

With the current proposal, as modified by Peter's suggestion above,  
waiting on a single cv with multiple mutexes (as long as the waits are  
not simultaneous) is legal if the condition is default constructed,  
and illegal if the condition is constructed with a mutex.

If a given bit of code waits on a single cv with multiple mutexes in a  
legal fashion when the condition is default constructed.  Why is that  
same code necessarily a logic error if the constructor is changed?  A  
contrived example (sorry, best I can do on short notice):  The user,  
via std::cin, selects a mutex to wait on.  He is supposed to pick the  
right one.  But he makes a mistake and picks the wrong one.  Logic  
error or exceptional runtime condition?  If a logic error, why is it  
not still a logic error if the condition is default constructed?   
Simply because we declare it to be so in the documentation?

-Howard