# Chapter 19: Diagnostics

Chapter 19 deals with program diagnostics, such as exceptions and assertions.

# Contents

- Adding data to exceptions
- Exception class hierarchy diagram

## Adding data to exceptions

The standard exception classes carry with them a single string as data (usually describing what went wrong or where the 'throw' took place). It's good to remember that you can add your own data to these exceptions when extending the heirarchy:

       using std::runtime_error;
       struct My_Exception : public runtime_error
       {
         public:
           My_Exception (const string& whatarg)
               : runtime_error(whatarg), e(errno), id(GetDataBaseID()) { }
           int  errno_at_time_of_throw() const { return e; }
           DBID id_of_thing_that_threw() const { return id; }
         protected:
           int    e;
           DBID   id;     // some user-defined type
       };

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## Exception class hierarchy diagram

The diagram is in PDF, or at least it will be once it gets finished.

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Comments and suggestions are welcome, and may be sent to Phil Edwards or Gabriel Dos Reis.
$Id: howto.html,v 1.4 1999/12/15 16:57:06 pme Exp $