Re: [boost] [boost thread rework] New version uploaded to BoostVault

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-09-05 12:10:53

From: "Anthony Williams"

> "Peter Dimov" <pdimov_at_[hidden]> writes:
>
>> Anthony Williams wrote:
>>> Also, the semaphore version runs the function once, even if it
>>> throws an exception. The mutex version runs the function repeatedly
>>> if it throws an exception. The docs say that the function isn't
>>> allowed to throw, so this is a moot point.
>>
>> I like the signature of call_once:
>>
>>     template<typename Function>
>>     void call_once(Function f, once_flag& flag)
>>
>> but you need to fix the above behavior. The function is considered "run"
>> only when it returns without an exception. So your semaphore version runs 
>> it
>> zero times when an exception is thrown.
>
> void f()
> {
>    throw "something";
> }
>
> int main()
> {
>    try
>    {
>        f();
>    }
>    catch(...){}
> }
>
> How many times has f() been *called*? One, or none? I find your concept of 
> a
> function only having run if it exits without an exception odd.

It has been called once, but it didn't run to completion, and hence, its 
postconditions haven't been satisfied. Therefore, if a subsequent call_once 
returns successfully, as if nothing happened, the code will proceed under 
the assumption that the postconditions are met, and will break in various 
ways.

Local statics retry construction on exception for the same reason.