[Qt-interest] Strange (as for me) QByteArray behavior

[Konrad Rosenbaum]

On Wednesday 08 December 2010, Paul Miller wrote:
> On 12/8/2010 2:31 AM, Thiago Macieira wrote:
> > On Wednesday, 8 de December de 2010 09:07:52 Konrad Rosenbaum wrote:
> >> On Wednesday 08 December 2010, Ivan Kharin wrote:
> >>>> QByteArray tmp = username.toAscii();
> >>> 
> >>> const QByteArray&tmp = username.toAscii(); // avoid copy: const ref
> >>> to function result == //   keep automatic variable intact until end
> >>> of scope
> >> 
> >> I don't think this can work reliably. C++ is not a garbage collected
> >> language, the reference becomes invalid immediately after the
> >> semicolon.
> > 
> > That's not correct. The above is perfectly valid C++.

I stand corrected.

> How does that work? Is the temporary QByteArray returned from toAscii()
> somehow kept around on the stack once the reference to it is made?

I just did some experiments - giving the non-reference return value of a 
function (or method) a name by referencing it seems to expand its scope 
beyond the expression it is used in.

You can imagine what the compiler does like this:

When it reaches the semicolon of the expression it tries to decide what to 
do about all the temporary objects it just created (return values of 
functions, methods and operators):
a) if the temporary value does not have a name - kill it
b) if it has a name (i.e. a reference that is visible beyond the semicolon, 
or it is a variable declared on this line) then expand its scope as far as 
that reference is visible

After that the normal scope checking kicks in: as soon as the value hits its 
new end of scope: kill the value.

Don't worry, you can still shoot yourself in the foot with references. They 
are not magical - merely convenient. ;-)

This whole scope-expansion only works if the reference gives a name to a 
value that was temporary and anonymous until then.


	Konrad
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