RuntimeError.h

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#ifndef __RUNTIMEERROR__
#define __RUNTIMEERROR__

#include <sstream>
#include <stdexcept>

class RuntimeError: public std::runtime_error
{
  public:
    RuntimeError( ): std::runtime_error( "" ), m_firstWhat( true ) { }
    virtual ~RuntimeError( ) throw ( ) { }

    const char * what( ) const throw( )
    {
      if( m_firstWhat )
      {
        std::ostringstream o;
        outputWhat( o );
        m_what = o.str( );
        m_firstWhat = false;
      }
      return m_what.c_str( );
    }

  protected:
    virtual void outputWhat( std::ostream & o ) const =0;

  private:
    mutable bool m_firstWhat;  
    mutable std::string m_what;  
};

inline std::ostream& operator<<( std::ostream& left, const RuntimeError& right )
{
  return left << right.what( );
}

#endif