Actually what is happen if I am in the middle of changing a qvector, and then
copying the vector in an other thread?
--
Sent from cellphone, sorry for the typos
On January 22, 2016 18:34:46 Matthew Woehlke <mwoehlke.fl...@gmail.com> wrote:
> On 2016-01-22 13:13, Marc Mutz wrote:
>> On Friday 22 January 2016 17:44:40 Thiago Macieira wrote:
>>> On Friday 22 January 2016 11:14:47 Marc Mutz wrote:
>>>> On Friday 22 January 2016 09:57:00 Иван Комиссаров wrote:
>>>>> What
>>>>> i'm missing?
>>>>
>>>> You haven't done the exercise with the int first.
>>>
>>> Here's the exercise with int. This is thread-safe:
>>>
>>> int f()
>>> {
>>> return 1;
>>> }
>>>
>>> auto x = f();
>>> ++x;
>>>
>>> No matter how many threads call f(), all of them will get a value from f,
>>> can assign it to a variable and modify without caring what other threads
>>> do.
>>>
>>> Replace int with QMap or QString and you have the same behaviour.
>>
>> This part of the discussion was about copying a container. You return a new
>> instance instead. Returning a new instance does not copy, nor move, due to
>> NRVO.
>
> The same assertions would hold if Thiago had written:
>
> int f()
> {
> static int result = 1;
> return result;
> }
>
> ...or anything else for the body of f().
>
> And in fact, you're again attacking a straw man. The implementation of
> getMap() was not shown; for all you know, it too might have returned a
> new instance every time. (Probably not, but doesn't matter.)
>
> What matters is that the result is returned *by value*. It is thread
> safe because it is not a reference, it is a copy. Qt containers are
> likewise thread safe because they behave (in a thread-safe manner) *as
> if* an actual copy was made (even though the actual copy might not
> happen until some time later, if ever).
>
> --
> Matthew
>
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