The RWxmsg class serves as a base class for the various exception types that can be thrown and caught within the Essential Tools Module library, or from within Essential Tools Module client applications.

The Essential Tools Module uses the following hierarchy.

As shown in Figure 9, RWxmsg serves as a base class for all exception types. From this point, the three major types of exceptions are derived: external errors, internal errors, and stream buffer allocation errors. Let us describe each of these classes in greater detail.

RWxmsg provides a basic interface for exceptions, from which an exception string is always available. This exception string is obtained through the why method, and is represented as a const char*, single-byte character sequence.

RWExternalErr is derived from RWxmsg, and is used to report errors caused by external sources over which the library has no control. Currently, this class is used to report errors from RWFileManager and RWCollectable during file processing when an invalid file format is encountered.

RWStreamErr is an external error reported when invalid stream data is encountered. For example, this class is used to report errors from RWeistream when an invalid stream header is encountered.

RWFileErr is a type of external error reported when file IO operations fail. For example, RWBTreeOnDisk makes extensive use of this class to report file read, seek, and write errors.

RWxalloc is a type of RWxmsg that can be used to report buffer allocation errors.

RWInternalErr is a type of error that is used to report errors that occur within the Essential Tools Module. For example, RWDateTime uses this class to report operations on invalid date/time objects.

RWRegexErr is used to report regular expression compilation errors from RWTRegex<T>.

RWBoundsErr is used to report invalid indexes into buffers. For example RWCString may report this type of error when an invalid index is provided to the assertElement method.