[Interest] QStringList, takeFirst and order of execution

[Thiago Macieira]

On segunda-feira, 11 de novembro de 2013 13:37:20, Samuel Gaist wrote:
> Hi,
> 
> I have been experiencing a strange behavior with QStringList and the order
> of execution of takeFirst.

> qDebug() << lineData.takeFirst() << lineData.takeFirst();

The problem is not takeFirst.

> behaves differently on two computers.

Correct. You have undefined behaviour.

Both calls to takeFirst() are in the same "sequence-point". That means the 
compiler is allowed to execute in any order.

It would be the same if you had written:

	f(lineData.takeFirst(), lineData.takeFirst());

> qDebug() << lineData.takeFirst();
> qDebug() << lineData.takeFirst();
> 
> The order is correct on both of them.
> 
> What could explain the difference ?

In the second case, there are two sequence points (they are separated by a ;). 

In C++11-speak, the first line of the second example is guaranteed to "happens-
before" the second line. In the first example, there's no "happens-before", 
therefore any one can happen before the other, or after.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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