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      ISO C++

      library

      All exception objects are defined in one of the standard header

      files: exception,

      stdexcept, new, and

      typeinfo.

      The base exception object is exception,

      located in exception. This object has no

      string member.

      Derived from this are several classes that may have a

      string member: a full hierarchy can be

      found in the source documentation.

      Full API details.

      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 hierarchy:

   struct My_Exception : public std::runtime_error

   {

     public:

       My_Exception (const string& whatarg)

           : std::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

   };

    In 1999, SGI added concept checkers to their

    implementation of the STL: code which checked the template

    parameters of instantiated pieces of the STL, in order to insure

    that the parameters being used met the requirements of the

    standard.  For example, the Standard requires that types passed as

    template parameters to vector be

    "Assignable" (which means what you think it means).  The

    checking was done during compilation, and none of the code was

    executed at runtime.

     Unfortunately, the size of the compiler files grew significantly

     as a result.  The checking code itself was cumbersome.  And bugs

     were found in it on more than one occasion.

     The primary author of the checking code, Jeremy Siek, had already

     started work on a replacement implementation.  The new code has been

     formally reviewed and accepted into

   the

   Boost libraries, and we are pleased to incorporate it into the

   GNU C++ library.

   The new version imposes a much smaller space overhead on the generated

   object file.  The checks are also cleaner and easier to read and

   understand.

   They are off by default for all versions of GCC.

   They can be enabled at configure time with

   --enable-concept-checks.

   You can enable them on a per-translation-unit basis with

     -D_GLIBCXX_CONCEPT_CHECKS.

   Please note that the upcoming C++ standard has first-class

   support for template parameter constraints based on concepts in the core

   language. This will obviate the need for the library-simulated concept

   checking described above.