text: // Exercise 16.31 Solution: InvalidIDException.h
  #include <string>
  #include <stdexcept> // standard C++ library class runtime_error
  using namespace std;
  
  // class InvalidIDException definition
  class InvalidIDException : public runtime_error 
  {
  public:
     // constructor specifies error message
     InvalidIDException( const string &message ) 
        : runtime_error( message ) {}
  }; // end class InvalidIDException
  
  
  
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