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. 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.