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<HTML>
<head><title>wxExprDatabase</title></head>
<BODY BGCOLOR=#FFFFFF>
<A NAME="wxexprdatabase"></A><CENTER>
<A HREF="wx.htm"><img align=center src="contents.gif" BORDER=0 ALT="Contents"></A> <A HREF="wx22.htm#classref"><img align=center src="up.gif" BORDER=0 ALT="Up"></A> <A HREF="wx86.htm#wxexpr"><img align=center src="back.gif" BORDER=0 ALT="Previous"></A> <A HREF="wx88.htm#wxfile"><img align=center src="forward.gif" BORDER=0 ALT="Next"></A> </CENTER><HR>
<H2>wxExprDatabase</H2>
<P>
The <B>wxExprDatabase</B> class represents a database, or list,
of Prolog-like expressions. Instances of this class are used for reading,
writing and creating data files.<P>
<B><FONT COLOR="#FF0000">Derived from</FONT></B><P>
<A HREF="wx127.htm#wxlist">wxList</A><BR>
<A HREF="wx158.htm#wxobject">wxObject</A><P>
<B><FONT COLOR="#FF0000">See also</FONT></B><P>
<A HREF="wx310.htm#exproverview">wxExpr overview</A>, <A HREF="wx86.htm#wxexpr">wxExpr</A><P>
<B><FONT COLOR="#FF0000">Members</FONT></B><P>
<A HREF="#wxexprdatabaseconstr">wxExprDatabase::wxExprDatabase</A><BR>
<A HREF="#topic360">wxExprDatabase::~wxExprDatabase</A><BR>
<A HREF="#wxexprdatabaseappend">wxExprDatabase::Append</A><BR>
<A HREF="#wxexprdatabasebeginfind">wxExprDatabase::BeginFind</A><BR>
<A HREF="#wxexprdatabasecleardatabase">wxExprDatabase::ClearDatabase</A><BR>
<A HREF="#wxexprdatabasefindclause">wxExprDatabase::FindClause</A><BR>
<A HREF="#wxexprdatabasefindclausebyfunctor">wxExprDatabase::FindClauseByFunctor</A><BR>
<A HREF="#wxexprdatabasegeterrorcount">wxExprDatabase::GetErrorCount</A><BR>
<A HREF="#wxexprdatabasehashfind">wxExprDatabase::HashFind</A><BR>
<A HREF="#wxexprdatabaseread">wxExprDatabase::Read</A><BR>
<A HREF="#wxexprdatabasereadfromstring">wxExprDatabase::ReadFromString</A><BR>
<A HREF="#wxexprdatabasewritelisp">wxExprDatabase::WriteLisp</A><BR>
<A HREF="#wxexprdatabasewrite">wxExprDatabase::Write</A><BR>
<P>
<HR>
<A NAME="wxexprdatabaseconstr"></A>
<H3>wxExprDatabase::wxExprDatabase</H3>
<P>
<B>void</B> <B>wxExprDatabase</B>(<B>proioErrorHandler</B><I> handler = 0</I>)<P>
Construct a new, unhashed database, with an optional error handler. The
error handler must be a function returning a bool and taking an integer and a string
argument. When an error occurs when reading or writing a database, this function is
called. The error is given as the first argument (currently one of WXEXPR_ERROR_GENERAL,
WXEXPR_ERROR_SYNTAX) and an error message is given as the second argument. If FALSE
is returned by the error handler, processing of the wxExpr operation stops.<P>
Another way of handling errors is simply to call <A HREF="wx87.htm#wxexprdatabasegeterrorcount">wxExprDatabase::GetErrorCount</A> after
the operation, to check whether errors have occurred, instead of installing an error handler.
If the error count is more than zero, <A HREF="wx87.htm#wxexprdatabasewrite">wxExprDatabase::Write</A> and
<A HREF="wx87.htm#wxexprdatabaseread">wxExprDatabase::Read</A> will return FALSE to
the application.<P>
For example:<P>
<PRE>
bool myErrorHandler(int err, chat *msg)
{
if (err == WXEXPR_ERROR_SYNTAX)
{
wxMessageBox(msg, "Syntax error");
}
return FALSE;
}
wxExprDatabase database(myErrorHandler);
</PRE>
<B></B> <B>wxExprDatabase</B>(<B>wxExprType</B><I> type</I>, <B>const wxString&</B><I>attribute</I>,
<B>int</B><I> size = 500</I>, <B>proioErrorHandler</B><I> handler = 0</I>)<P>
Construct a new database hashed on a combination of the clause functor and
a named attribute (often an integer identification).<P>
See above for an explanation of the error handler.<P>
<HR>
<A NAME="topic360"></A>
<H3>wxExprDatabase::~wxExprDatabase</H3>
<P>
<B></B> <B>~wxExprDatabase</B>()<P>
Delete the database and contents.<P>
<HR>
<A NAME="wxexprdatabaseappend"></A>
<H3>wxExprDatabase::Append</H3>
<P>
<B>void</B> <B>Append</B>(<B>wxExpr* </B><I>clause</I>)<P>
Append a clause to the end of the database. If the database is hashing,
the functor and a user-specified attribute will be hashed upon, giving the
option of random access in addition to linear traversal of the database.<P>
<HR>
<A NAME="wxexprdatabasebeginfind"></A>
<H3>wxExprDatabase::BeginFind</H3>
<P>
<B>void</B> <B>BeginFind</B>()<P>
Reset the current position to the start of the database. Subsequent
<A HREF="wx87.htm#wxexprdatabasefindclause">wxExprDatabase::FindClause</A> calls will move the pointer.<P>
<HR>
<A NAME="wxexprdatabasecleardatabase"></A>
<H3>wxExprDatabase::ClearDatabase</H3>
<P>
<B>void</B> <B>ClearDatabase</B>()<P>
Clears the contents of the database.<P>
<HR>
<A NAME="wxexprdatabasefindclause"></A>
<H3>wxExprDatabase::FindClause</H3>
<P>
Various ways of retrieving clauses from the database. A return
value of NULL indicates no (more) clauses matching the given criteria.
Calling the functions repeatedly retrieves more matching clauses, if any.<P>
<B>wxExpr* </B> <B>FindClause</B>(<B>long</B><I> id</I>)<P>
Find a clause based on the special "id'' attribute.<P>
<B>wxExpr* </B> <B>FindClause</B>(<B>const wxString&</B><I> attribute</I>, <B>const wxString&</B><I> value</I>)<P>
Find a clause which has the given attribute set to the given string or word value.<P>
<B>wxExpr*</B> <B>FindClause</B>(<B>const wxString&</B><I> attribute</I>, <B>long</B><I> value</I>)<P>
Find a clause which has the given attribute set to the given integer value.<P>
<B>wxExpr*</B> <B>FindClause</B>(<B>const wxString&</B><I> attribute</I>, <B>float</B><I> value</I>)<P>
Find a clause which has the given attribute set to the given floating point value.<P>
<HR>
<A NAME="wxexprdatabasefindclausebyfunctor"></A>
<H3>wxExprDatabase::FindClauseByFunctor</H3>
<P>
<B>wxExpr*</B> <B>FindClauseByFunctor</B>(<B>const wxString&</B><I> functor</I>)<P>
Find the next clause with the specified functor.<P>
<HR>
<A NAME="wxexprdatabasegeterrorcount"></A>
<H3>wxExprDatabase::GetErrorCount</H3>
<P>
<B>int</B> <B>GetErrorCount</B>() <B>const</B><P>
Returns the number of errors encountered during the last read or write operation.<P>
<HR>
<A NAME="wxexprdatabasehashfind"></A>
<H3>wxExprDatabase::HashFind</H3>
<P>
<B>wxExpr*</B> <B>HashFind</B>(<B>const wxString&</B><I> functor</I>, <B>long</B><I> value</I>) <B>const</B><P>
Finds the clause with the given functor and with the attribute specified
in the database constructor having the given integer value.<P>
For example,<P>
<PRE>
// Hash on a combination of functor and integer "id" attribute when reading in
wxExprDatabase db(wxExprInteger, "id");
// Read it in
db.ReadProlog("data");
// Retrieve a clause with specified functor and id
wxExpr *clause = db.HashFind("node", 24);
</PRE>
This would retrieve a clause which is written: <TT>node(id = 24, ..., )</TT>.<P>
<B>wxExpr*</B> <B>HashFind</B>(<B>const wxString&</B><I> functor</I>, <B>const wxString&</B><I> value</I>)<P>
Finds the clause with the given functor and with the attribute specified
in the database constructor having the given string value.<P>
<HR>
<A NAME="wxexprdatabaseread"></A>
<H3>wxExprDatabase::Read</H3>
<P>
<B>bool</B> <B>Read</B>(<B>const wxString&</B><I> filename</I>)<P>
Reads in the given file, returning TRUE if successful.<P>
<HR>
<A NAME="wxexprdatabasereadfromstring"></A>
<H3>wxExprDatabase::ReadFromString</H3>
<P>
<B>bool</B> <B>ReadFromString</B>(<B>const wxString&</B><I> buffer</I>)<P>
Reads a Prolog database from the given string buffer, returning TRUE if
successful.<P>
<HR>
<A NAME="wxexprdatabasewritelisp"></A>
<H3>wxExprDatabase::WriteLisp</H3>
<P>
<B>bool</B> <B>WriteLisp</B>(<B>ostream& </B><I>stream</I>)<P>
Writes the database as a LISP-format file.<P>
<HR>
<A NAME="wxexprdatabasewrite"></A>
<H3>wxExprDatabase::Write</H3>
<P>
<B>bool</B> <B>Write</B>(<B>ostream& </B><I>stream</I>)<P>
<B>bool</B> <B>Write</B>(<B>const wxString&</B><I> filename</I>)<P>
Writes the database as a Prolog-format file.<P>
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