RE: Problem evaluating XPath expressions

[Will Sappington]

Dave, thank you very much for taking the time to do this.  I followed up
my post below with another one later last night about an idea I hit upon
that is essentially what you are saying below.  I'm using a static
member function, getInstance() that initializes the X libraries only on
the first call and then instantiates a Profile class with 'new' and
returns a pointer to the instance.  As with your method below, the X
library initialization occurs before the parserLiaison constructs as
member data and everyone is happy.  It appears to be working fine.  I'll
read your suggestions below more thoroughly for things I could do to
improve my implementation.

Thanks again for all your help,

-will

-----Original Message-----
From: David Bertoni [mailto:[EMAIL PROTECTED] 
Sent: Wednesday, February 14, 2007 4:28 PM
To: xalan-c-users@xml.apache.org
Subject: Re: Problem evaluating XPath expressions

Will Sappington wrote:
> Hi Dave,
> 
> I think I may have hit a dead end with this.  When I define 
> XalanSourceTreeParserLiaison _parserLiaison; as member data, the
program
> crashes with an assertion during construction of the Profile class.
The
> first line of the constructor doesn't even execute so the assertion is
> occurring in the construction of the member classes.  I stepped into
it
> and found that it occurs during construction of the parserLiaison when
> it calls XalanMemMgrs::getDefaultXercesMemMgr() which is what
> XALAN_DEFAULT_MEMMGR is #defined as.  After that call there is an
assert
> (ptr != 0) and that is where the crash occurs.  I'm assuming this is
> because neither of the ::initialize() methods have been called.  Of
> course there's no opportunity to do so prior to member data
construction
> so it would appear I'm screwed.  Is there a way to instantiate a
memory
> manager such that the getDefaultXercesMemMgr() call would be able to
> find it?

If you're going to do Xerces-C and Xalan-C initialization in the 
constructor of your class, you'll need to create a simple class that 
initializes Xerces-C and Xalan-C in the right order, and make that class
a 
member of your class, ensuring that it appears before any other Xerces-C
or 
Xalan-C class in the class definition.  Here's an example, in
pseudo-code:

class XercesXalanInit
{
public:

     XercesXalanInit()
     {
         if (++s_initCount == 1)
         {
             PlatformUtils::Initialize();
             XalanTransformer::initialize();
         }
     }

     ~XercesXalanInit()
     {
         if (--s_initCount == 0)
         {
             XalanTransformer::terminate();
             PlatformUtils::Terminate();
         }
     }

     static unsigned long   s_initCount;
};

XercesXalanInit::s_initCount = 0;

class myClass
{
public:

     myClass() :
         m_init(),
         m_liason()
     {
     }

     // all my public member functions here...

private:

     XercesXalanInit    m_init;

     XalanSourceTreeParserLiaison   m_liason;
};


Ultimately, it's much better if you initialize Xerces-C and Xalan-C
_once_, 
when your application starts, and not when each instance is created.
Also, 
you'll notice that I put some counting in the XercesXalanInit init
class, 
because you should only call them each once.  Finally, if your
application 
will be multi-threaded, you will either need to initialize the libraries

once, before any threads start, or make the counting in XercesXalanInit 
thread-safe.

Dave