std::exception::what (3) - Linux Man Pages
virtual const char* what() const throw(); (until C++11)
virtual const char* what() const noexcept; (since C++11)
Returns the explanatory string.
Pointer to a null-terminated string with explanatory information. The pointer is guaranteed to be valid at least until the exception object from which it is obtained is destroyed, or until a non-const member function on the exception object is called.