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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>6. ZOOM::exception and subclasses</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="YAZ++ User's Guide and Reference"><link rel="up" href="zoom.html" title="Chapter 3. ZOOM-C++"><link rel="prev" href="zoom-record.html" title="5. ZOOM::record"><link rel="next" href="api.html" title="Chapter 4. YAZ C++ API"></head><body><link rel="stylesheet" type="text/css" href="common/style1.css"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">6. <code class="literal">ZOOM::exception</code> and subclasses</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="zoom-record.html">Prev</a> </td><th width="60%" align="center">Chapter 3. ZOOM-C++</th><td width="20%" align="right"> <a accesskey="n" href="api.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="zoom-exception"></a>6. <code class="literal">ZOOM::exception</code> and subclasses</h2></div></div></div><p>
    The <code class="literal">ZOOM::exception</code> class is a virtual base
    class, representing a diagnostic generated by the ZOOM-C++ library
    or returned from a server.  Its subclasses represent particular
    kinds of error.
   </p><p>
    When any of the ZOOM methods fail, they respond by
    <code class="literal">throw</code>ing an object of type
    <code class="literal">exception</code> or one of its subclasses.  This most
    usually happens with the <code class="literal">connection</code> constructor,
    the various query constructors, the <code class="literal">resultSet</code>
    constructor (which is actually the searching method), and
    <code class="literal">resultSet::getRecord()</code>.
   </p><p>
    The base class has this declaration:
   </p><pre class="synopsis">
    class exception {
    public:
      exception (int code);
      int errcode () const;
      const char *errmsg () const;
    };
   </pre><p>
    It has three concrete subclasses:
   </p><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="zoom-systemException"></a>6.1. <code class="literal">ZOOM::systemException</code></h3></div></div></div><pre class="synopsis">
     class systemException: public exception {
     public:
       systemException ();
       int errcode () const;
       const char *errmsg () const;
     };
    </pre><p>
     Represents a ``system error'', typically indicating that a system
     call failed - often in the low-level networking code that
     underlies Z39.50.  <code class="literal">errcode()</code> returns the value
     that the system variable <code class="literal">errno</code> had at the time
     the exception was constructed; and <code class="literal">errmsg()</code>
     returns a human-readable error-message corresponding to that error
     code.
    </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="zoom-bib1Exception"></a>6.2. <code class="literal">ZOOM::bib1Exception</code></h3></div></div></div><pre class="synopsis">
     class bib1Exception: public exception {
     public:
       bib1Exception (int errcode, const char *addinfo);
       int errcode () const;
       const char *errmsg () const;
       const char *addinfo () const;
     };
    </pre><p>
     Represents an error condition communicated by a Z39.50 server.
     <code class="literal">errcode()</code> returns the BIB-1 diagnostic code of
     the error, and <code class="literal">errmsg()</code> a human-readable error
     message corresponding to that code.  <code class="literal">addinfo()</code>
     returns any additional information associated with the error.
    </p><p>
     For example, if a ZOOM application tries to search in the
     ``Voyager'' database of a server that does not have a database of
     that name, a <code class="literal">bib1Exception</code> will be thrown in
     which <code class="literal">errcode()</code> returns 109,
     <code class="literal">errmsg()</code> returns the corresponding error
     message ``Database unavailable'' and <code class="literal">addinfo()</code>
     returns the name of the requested, but unavailable, database.
    </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="zoom-queryException"></a>6.3. <code class="literal">ZOOM::queryException</code></h3></div></div></div><pre class="synopsis">
     class queryException: public exception {
     public:
       static const int PREFIX = 1;
       static const int CCL = 2;
       queryException (int qtype, const char *source);
       int errcode () const;
       const char *errmsg () const;
       const char *addinfo () const;
     };
    </pre><p>
     This class represents an error in parsing a query into a form that
     Z39.50 can understand.  It must be created with the
     <code class="literal">qtype</code> parameter equal to one of the query-type
     constants, which can be retrieved via the
     <code class="literal">errcode()</code> method; <code class="literal">errmsg()</code>
     returns an error-message specifying which kind of query was
     malformed; and <code class="literal">addinfo()</code> returns a copy of the
     query itself (that is, the value of <code class="literal">source</code> with
     which the exception object was created.)
    </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="revised-sample"></a>6.4. Revised Sample Program</h3></div></div></div><p>
     Now we can revise the sample program from the
     <a class="link" href="zoom.html#zoom-introduction" title="1. Introduction">introduction</a>
     to catch exceptions and report any errors:
    </p><pre class="programlisting">
    /* g++ -o zoom-c++-hw zoom-c++-hw.cpp -lzoompp -lyaz */

    #include &lt;iostream&gt;
    #include &lt;yazpp/zoom.h&gt;

    using namespace ZOOM;

    int main(int argc, char **argv)
    {
        try {
            connection conn("lx2.loc.gov", 210);
            conn.option("databaseName", "LCDB");
            conn.option("preferredRecordSyntax", "USMARC");
            resultSet rs(conn, prefixQuery("@attr 1=7 0253333490"));
            const record *rec = rs.getRecord(0);
            cout &lt;&lt; rec-&gt;render() &lt;&lt; endl;
        } catch (systemException &amp;e) {
            cerr &lt;&lt; "System error " &lt;&lt;
                e.errcode() &lt;&lt; " (" &lt;&lt; e.errmsg() &lt;&lt; ")" &lt;&lt; endl;
        } catch (bib1Exception &amp;e) {
            cerr &lt;&lt; "BIB-1 error " &lt;&lt; 
                e.errcode() &lt;&lt; " (" &lt;&lt; e.errmsg() &lt;&lt; "): " &lt;&lt; e.addinfo() &lt;&lt; endl;
        } catch (queryException &amp;e) {
            cerr &lt;&lt; "Query error " &lt;&lt;
                e.errcode() &lt;&lt; " (" &lt;&lt; e.errmsg() &lt;&lt; "): " &lt;&lt; e.addinfo() &lt;&lt; endl;
        } catch (exception &amp;e) {
            cerr &lt;&lt; "Error " &lt;&lt;
                e.errcode() &lt;&lt; " (" &lt;&lt; e.errmsg() &lt;&lt; ")" &lt;&lt; endl;
        }
    }
    </pre><p>
     The heart of this program is the same as in the original version,
     but it's now wrapped in a <code class="literal">try</code> block followed by
     several <code class="literal">catch</code> blocks which try to give helpful
     diagnostics if something goes wrong.
    </p><p>
     The first such block diagnoses system-level errors such as memory
     exhaustion or a network connection being broken by a server's
     untimely death; the second catches errors at the Z39.50 level,
     such as a server's report that it can't provide records in USMARC
     syntax; the third is there in case there's something wrong with
     the syntax of the query (although in this case it's correct); and
     finally, the last <code class="literal">catch</code> block is a
     belt-and-braces measure to be sure that nothing escapes us.
    </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="exception.references"></a>6.5. References</h3></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
       <a class="ulink" href="http://zoom.z3950.org/api/zoom-current.html#3.7" target="_top">Section 3.7 (Exception) of the ZOOM Abstract API</a>
      </p></li><li class="listitem"><p>
       <a class="ulink" href="http://www.loc.gov/z3950/agency/defns/bib1diag.html" target="_top">Bib-1 Diagnostics</a> on the
       <a class="ulink" href="http://www.loc.gov/z3950/agency/" target="_top">Z39.50 Maintenance Agency</a> site.
      </p></li></ul></div><p>
     Because C does not support exceptions, ZOOM-C has no API element
     that corresponds directly with ZOOM-C++'s
     <code class="literal">exception</code> class and its subclasses.  The
     closest thing is the <code class="literal">ZOOM_connection_error</code>
     function described in the
     <a class="ulink" href="http://www.indexdata.com/yaz/doc/zoom.html#zoom.connections" target="_top">Connections section</a> of the documentation.
    </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="zoom-record.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="zoom.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="api.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5. <code class="literal">ZOOM::record</code> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 4. YAZ C++ API</td></tr></table></div></body></html>

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