Serialization - Archive Exceptions

###  | # Serialization ## Archive Exceptions

`unregistered_class`
`invalid_signature`
`unsupported_version`
`unsupported_class_version`
`pointer_conflict`
`incompatible_native_format`
`array_size_too_short`
`input_stream_error`
`output_stream_error`
`invalid_class_name`
`unregistered_class`
`multiple_code_instantiation`
`xml_archive_parsing_error`
`xml_archive_tag_mismatch`
`xml_archive_tag_name_error`

Archive operators can throw a `boost::archive_exception` object which can be caught by an application program. These exceptions are defined in the files archive_exception.hpp and basic_xml_archive.hpp.

    namespace boost {
    namespace archive {

    class archive_exception  : public std::exception
    {
    public:
        typedef enum {
            unregistered_class,     // attempt to serialize a pointer of
                                    // an unregistered class
            invalid_signature,      // first line of archive does not contain
                                    // expected string
            unsupported_version,    // archive created with library version subsequent
                                    // to this one
            pointer_conflict        // an attempt has been made to directly serialize
                                    // an object after having already serialized the same
                                    // object through a pointer.  Were this permitted,
                                    // the archive load would result in the creation
                                    // of an extraneous object.
            incompatible_native_format, // attempt to read native binary format
                                    // on incompatible platform
            array_size_too_short,   // array being loaded doesn't fit in array allocated
            input_stream_error      // error on stream input
            invalid_class_name,     // class name greater than the maximum permitted.
                                    // most likely a corrupted archive or an attempt
                                    // to insert virus via buffer overrun method.
            unregistered_cast,      // base - derived relationship not registered with
                                    // void_cast_register
            unsupported_class_version, // type saved with a version # greater than the
                                // one used by the program.  This indicates that the proggram
                                // needs to be rebuilt.
            multiple_code_instantiation, // code for implementing serialization for some
                                // type has been instantiated in more than one module.
            output_stream_error     // error on stream output
        } exception_code;
        exception_code code;
        archive_exception(exception_code c) : code(c) {}
        virtual const char *what( ) const throw();
    };

    class xml_archive_exception : public virtual archive_exception
    {
    public:
        typedef enum {
            xml_archive_parsing_error,  // archive doesn't contain expected data
    	xml_archive_tag_mismatch,   // start/end tag in archive doesn't match program
            xml_archive_tag_name_error  // tag name contains invalid characters

        } exception_code;
        xml_archive_exception(exception_code c){}
        virtual const char *what( ) const throw();
    };

    } // archive
    } // boost

### `unregistered_class`

An attempt has been made to serialize a polymorphic class through a pointer without either registering it or associating it with an export key. This can also occur when using a new archive whose class name has not been added to the system with the `BOOST_ARCHIVE_CUSTOM_ARCHIVE_TYPES` macro.

### `invalid_signature`

Archives are initiated with a known string. If this string is not found when the archive is opened, It is presumed that this file is not a valid archive and this exception is thrown.

### `unsupported_version`

This system records the current library version number to all archives created. Note that this is in no way related to version number of classes used by application programs. This refers to the version of the serialization system used to create the archive. Future versions of this serialization system will be able to identify archives created under a previous (i.e. this) system and alter the loading procedure accordingly. Hence, future enhancements to this serialization system should not obsolete any existing archive files. It is only necessary to increment this version number when the newer system creates archives incompatible in format with the current one.

Should it ever occur that an older program attempts to read newer archives whose format has changed, this exception is thrown.

### `unsupported_class_version`

An attempt has been made to load a class whose version has been incremented since the program was written. Suppose that a class has been assigned version number 3 and the program has been built and sent to third parties. Now suppose that the definition of that class has been altered, the version number has been incremented to 4 and new archives have been built. If one attempts to load these new archives with the original program, this exception will be thrown.

### `pointer_conflict`

To understand what this exception means consider the following scenario

    template<class Archive>
    void T::save(Archive &ar) const
    {
        const A * aptr = &a;
        ar << aptr;          // save an instance of object of class A through a pointer
        ...
        ar << a;             // save an instance of an object of class A
        assert(aptr == &a);  // this must be true
    }

    template<class Archive>
    void T::load(Archive &ar)
    {
        A * aptr;
        ar >> aptr;          // create and initialize a new instance of class A
        ...
        ar >> a;             // restore state of on object of class A
        assert(aptr == &a);  // this won't be true
    }

An object is saved first through a pointer then directly. Upon loading back in the same sequence, we first create an new object and load in its data. Then we load the data into another existing object. Where we started with one object during save, we have two objects after restore. In a more realistic situation, it could be very difficult to find this error. Fortunately, these situations can be detected when the archive is created. When this occurs, this exception is thrown.

### `incompatible_native_format`

The library currently supports char text, wide char text and native binary archive files. At the beginning of every archive, a signature is written indicating the type of archive. This exception is thrown when an attempt is made to read an archive written in a different format.

### `array_size_too_short`

An attempt has been made to read an array that is larger than the array size. This should only occur when the size of an array in code is reduced after an archive has already been created.

### `input_stream_error`
`output_stream_error`

An error has occured during stream input or ouput. Aside from the common situations such as a corrupted or truncated input file, there are several less obvious ones that sometimes occur.

This includes an attempt to read past the end of the file. Text files need a terminating new line character at the end of the file which is appended when the archive destructor is invoked. Be sure that an output archive on a stream is destroyed before opening an input archive on that same stream. That is, rather than using something like:

    std::stringstream ss;
    std::vector<V> v;
    boost::archive::text_oarchive oa(ss);
    oa << v;
    boost::archive::text_iarchive ia(ss);
    ia >> v;

use

    std::stringstream ss;
    std::vector<V> v;
    {
        boost::archive::text_oarchive oa(ss);
        oa << v;
    }
    {
        boost::archive::text_iarchive ia(ss);
        ia >> v;
    }

Another one is the passing of uninitialized data. In general, the behavior of the serialization library when passed uninitialized data is undefined. If it can be detected, it will invoke an assertion in debug builds. Otherwise, depending on the type of archive, it may pass through without incident or it may result in an archive with unexpected data in it. This, in turn, can result in the throwing of this exception.

### `invalid_class_name`

Class name length greater than the maximum permitted. Most likely cause is a corrupted archive or an attempt to insert a virus via the buffer overrun method.

### `unregistered_cast`

In order to support casting between pointers of base and derived classes at runtime, a collection of legitimate conversions is maintained by the system. Normally this collection is maintained without any explicit action on the part of the user of the library. However, there are special cases where this might have to be done explicitly and could be overlooked. This is described in Runtime Casting. This exception is thrown if an attempt is made to convert between two pointers whose relationship has not been registered,

### `multiple_code_instantiation`

This exception is thrown when it is detected that the serialization of the same type has been instantiated more than once. This might occur when serialization code is instantiated in both the mainline and one or more DLLS.

### `xml_archive_parsing_error`

The XML generated by the serialization process is intimately coupled to the C++ class structure, relationships between objects and the serialization specifications. If these become out of sync in any way, the XML may not map to the loading serialization and this exception might be thrown. This might occur for one of the following reasons:

- The archive has been edited outside the serialization system. This might be possible if only the data is changed and the XML attributes and nesting structure are left unaltered. But any other editing is likely to render the archive unreadable by the serialization library.
- The serialization has been altered and an archive generated by the old code is being read. That is, versioning has not been properly employed to properly deserialize previously created archives.

### `xml_archive_tag_mismatch`

This exception will be thrown if the start or end tag of an XML element doesn't match the name specified for the object in the program.

### `xml_archive_tag_name_error`

This exception will be thrown if the tag name contains invalid characters. Valid characters for an XML tag are: upper and lower case letters, digits, and the following punctuation: .(period), _(underscore), :(colon), and -(hyphen).

© Copyright Robert Ramey 2002-2004. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)