>ZOOM::exception and subclasses
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ZOOM::exception and subclasses

The ZOOM::exception 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.

When any of the ZOOM methods fail, they respond by throwing an object of type exception or one of its subclasses. This most usually happens with the connection constructor, the various query constructors, the resultSet constructor (which is actually the searching method) and resultSet::getRecord().

The base class has this declaration: 

    class exception {
    public:
      exception (int code);
      int errcode () const;
      const char *errmsg () const;
    };

It has three concrete subclasses:

ZOOM::systemException

    class systemException: public exception {
    public:
      systemException ();
      int errcode () const;
      const char *errmsg () const;
    };

Represents a ``system error'', typically indicating that a system call failed - often in the low-level networking code that underlies Z39.50. errcode() returns the value that the system variable errno had at the time the exception was constructed; and errmsg() returns a human-readable error-message corresponidng to that error code.

ZOOM::bib1Exception

    class bib1Exception: public exception {
    public:
      bib1Exception (int errcode, const char *addinfo);
      int errcode () const;
      const char *errmsg () const;
      const char *addinfo () const;
    };

Represents an error condition communicated by a Z39.50 server. errcode() returns the BIB-1 diagnostic code of the error, and errmsg() a human-readable error message corresponding to that code. addinfo() returns any additional information associated with the error.

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 bib1Exception will be thrown in which errcode() returns 109, errmsg() returns the corresponding error message ``Database unavailable'' and addinfo() returns the name of the requested, but unavailable, database.

ZOOM::queryException

    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;
    };

This class represents an error in parsing a query into a form that a Z39.50 can understand. It must be created with the qtype parameter equal to one of the query-type constants, which can be retrieved via the errcode() method; errmsg() returns an error-message specifying which kind of query was malformed; and addinfo() returns a copy of the query itself (that is, the value of source with which the exception object was created.)

Revised Sample Program

Now we can revise the sample program from the introduction to catch exceptions and report any errors: 

    /* g++ -o zoom-c++-hw zoom-c++-hw.cpp -lyaz++ -lyaz */

    #include <iostream>
    #include <yaz++/zoom.h>

    using namespace ZOOM;

    int main(int argc, char **argv)
    {
        try {
            connection conn("z3950.loc.gov", 7090);
            conn.option("databaseName", "Voyager");
            conn.option("preferredRecordSyntax", "USMARC");
            resultSet rs(conn, prefixQuery("@attr 1=7 0253333490"));
            const record *rec = rs.getRecord(0);
            cout << rec->render() << endl;
        } catch (systemException &e) {
            cerr << "System error " <<
                e.errcode() << " (" << e.errmsg() << ")" << endl;
        } catch (bib1Exception &e) {
            cerr << "BIB-1 error " << 
                e.errcode() << " (" << e.errmsg() << "): " << e.addinfo() << endl;
        } catch (queryException &e) {
            cerr << "Query error " <<
                e.errcode() << " (" << e.errmsg() << "): " << e.addinfo() << endl;
        } catch (exception &e) {
            cerr << "Error " <<
                e.errcode() << " (" << e.errmsg() << ")" << endl;
        }
    }

The heart of this program is the same as in the original version, but it's now wrapped in a try block followed by several catch blocks which try to give helpful diagnostics if something goes wrong.

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 catch block is a belt-and-braces measure to be sure that nothing escapes us.

References

Because C does not support exceptions, ZOOM-C has no API element that corresponds directly with ZOOM-C++'s exception class and its subclasses. The closest thing is the ZOOM_connection_error function described in The Connections section of the documentation.

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