Can Q_GLOBAL_STATIC replace KStaticDelete ?

Thiago Macieira thiago at
Mon Jan 8 17:41:55 GMT 2007

Olivier Goffart wrote:
>So can Q_GLOBAL_STATIC replace KStaticDeleter ?
>+ Q_GLOBAL_STATIC is thread safe while KStaticDeleter is not  (but could
>eventually be modified)

KStaticDeleter cannot be made thread-safe unless we're willing to code a 
pthreads implementation (not using Qt mutex and stuff, since those are 
C++ non-POD objects). Rinse and repeat for each thread implementation 
that KDE will support.

Also note that Q_GLOBAL_STATIC doesn't have a mutex, but an atomic 
test-and-set. The result is that it is faster than a mutex or even g++'s 
static initialiser guards. There isn't ever a contention case that 
requires kernel arbitration.

The side effect is that in a real crunch condition, the target object may 
be created twice (don't worry, one copy will be deleted; no leaks).

>- Q_GLOBAL_STATIC is not documented by Qt (at least i did not find the
>documentation),  is it a part on the public API ?

It's not part of the public API, but it's safe enough for use in KDE code. 
Same as Q_PRIVATE_SLOT that Simon suggested.

I also asked the TT maintainer to consider making QAtomic / QAtomicPointer 
public as well. There's still time before Qt 4.3 is out for those things 
to be documented.

>- KStaticDeleter support arrays, Q_GLOBAL_STATIC don't

You're probably better off using QVector, QList, QVarLengthArray or 
something else.

Yes, Q_GLOBAL_STATIC doesn't support arrays. Do we really need this?

>Should KStaticDeleter be fixed to be thread safe ?

See above. The only solution to make KStaticDeleter work without resorting 
to low-level stuff is to use Q_GLOBAL_STATIC in it :-)

>Or Should we remove KStaticDeleter in favor of Q_GLOBAL_STATIC ?

I'd say it's pretty obvious...

  Thiago Macieira  -  thiago (AT) - thiago (AT)
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