# Chapter 19: Diagnostics

Chapter 19 deals with program diagnostics, such as exceptions and assertions. You know, all the things we wish weren't even necessary at all.

# Contents

- Adding data to exceptions
- Exception class hierarchy diagram
- Concept checkers

## 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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## Concept checkers

As part of their 3.3 release, SGI added some nifty macros which perform assertions on type properties. For example, the Standard requires that types passed as template parameters to `vector` be "Assignable" (which means what you think it means).

The concept checkers allow the source code for `vector` to declare

       __STL_CLASS_REQUIRES(_Tp, _Assignable);

inside the template. `_Tp` is the element type of the vector, and `_Assignable` is the concept to be checked (it is defined in some back-end header files). When you instantiate `vector<MyType>`, compile-time checking can be done on whether MyType meets the requirements for vectors.

This is an extension to the library. This documentation needs updating.

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Comments and suggestions are welcome, and may be sent to Phil Edwards or Gabriel Dos Reis.
$Id: howto.html,v 1.5 2000/12/03 23:47:47 jsm28 Exp $