[Plplot-devel] C++ class constructor question

[Alan W. I. <ir...@be...>]

Hi Phil:

I have noticed the style of communications you are using between various
classes typically involves private variables such as

PLMemoryMap  m_outputMemoryMap;

for wxPLDevice, a default constructors for
PLMemoryMap that essentially sets everything relevant to invalid
values, and no explicit use of the constructor that does valid
initialization for PLMemoryMap from within wxPLDevice.  (Instead an
explicit create method is called to do the required valid
initialization.)

Would there be anything wrong with dropping that private variable,
dropping the default invalid constructor, dropping the create method,
creating an instance of PLMemoryMap (with correct explicit valid
constructor with a mustExist argument to decide if you are going to
reuse external memory locations from a previously initialized
instance), and using the methods of that instance when you need them?

That latter simplified approach is the one I have tentatively adopted
for the new PLTwoSemaphores class I have just implemented (see commit
cd13d65).  The (only) constructor checks that the argument values
corresponding to the addresses of the two semaphores are non-NULL, and
then the internal private variables of the instance that correspond to
those two addresses are updated.  If mustExist is true, nothing more
is done, but if mustExist is false, then sem_init is called for each
of those pointers. The (only) destructor for the PLTwoSemaphores class
calls sem_destroy for each of the (internal) pointers and sets those
internal pointers to NULL.  And all other methods of PLTwoSemaphores
that use those internal pointers check that they are non-NULL.

So far the two-semaphores implementation is incomplete so I have only
tested the new PLTwoSemaphores class with a build (with the g++
compiler finding no issue with it).  Do you anticipate I will be
running into trouble later on with the above simplified constructor
approach once I start to use PLTwoSemaphores class instances (e.g., in
the arguments for the currently empty PLMemoryMap::sendBytes and
PLMemoryMap::receiveBytes methods?

If it turns out you do believe the current simplified PLTwoSemaphores
constructor approach will work OK, should we also switch to that
simplified approach everywhere else (e.g., to handle wxPLDevice's use
of PLMemoryMap)?

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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Linux-powered Science
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