#include <string> 

class number_format 
{ 
  public: 
    number_format() 
      : What("Typumwandlung misslungen") 
    { 
    } 

    number_format(std::string what) 
      : What(what) 
    { 
    } 

    std::string what() 
    { 
      return What; 
    } 

  private: 
    std::string What; 
};