Apache C++ Standard Library User's Guide

## 25.5 Modifying a Standard Facet's Behavior

At times you may need to replace a facet object in a locale by another kind of facet object. In the following example, let us derive from one of the standard facet classes, numpunct, and create a locale object in which the standard `std::numpunct` facet object is replaced by an instance of our new, derived facet class.

Here is the problem we want to solve. When you print Boolean values, you can choose between the numeric representation of the values `"true"` and `"false"`, or their alphanumeric representation.

    int main(int argc, char** argv)
    {
      bool any_arguments = (argc > 1);                           //1
      std::cout.setf(ios_base::boolalpha);                       //2
      std::cout << any_arguments << '\n';                        //3
      // ...
    }

`//1` | A variable of type `bool` is defined. Its initial value is the boolean value of the logical expression `(argc > 1)`, so the variable `any_arguments` contains the information, whether the program was called with or without arguments.
`//2` | The format flag `std::ios_base::boolalpha` is set in the predefined output stream `cout`. The effect is that the string representation of boolean values is printed, instead of their numerical representation `0` or `1`, which is the default representation.
`//3` | Here either the string `"true"` or the string `"false"` are printed.

Of course, the string representation depends on the language. Hence, the alphanumeric representation of boolean values is provided by a locale. It is the `std::numpunct` facet of a locale that describes the cultural conventions for numerical formatting. It contains services that return the string representations of the boolean values `true` and `false`.

This is the interface of facet numpunct:

    namespace std{
      template <class charT>
        class numpunct : public locale::facet {
          public:
            typedef charT                    char_type;
            typedef basic_string<charT> string_type;
            explicit numpunct(size_t refs = 0);
            char_type decimal_point()   const;
            char_type thousands_sep()   const;
            string    grouping()        const;
            string_type  truename()     const;
            string_type  falsename()    const;
            static locale::id id;
            // ...
        };
    }

Now let us replace this facet. To make it more exciting, let's use not only a different language, but also different words for `true` and `false`, such as `Yes!` and `No!`. For just using another language, we would not need a new facet; we would simply use the right native locale, and it would contain the right facet.

    template <class charT>                                         //1
    class change_bool_names
        : public std::numpunct_byname<charT>                       //2
    {
      public:
        typedef std::basic_string<charT> string_type;
        explicit change_bool_names (const char* name,              //3
                                    const charT* t,
                                    const charT* f, size_t refs=0)
          : std::numpunct_byname<charT> (name,refs),
            true_string(t), false_string(f) { }
       protected:
         virtual string_type do_truename() const {                  //4
           return true_string;
         }
         virtual string_type do_falsename() const {
           return false_string;
         }
       private:
         string_type true_string, false_string;
    };

`//1` | The new facet is a class template that takes the character type as a template parameter.
`//2` | The new facet is derived from the `std::numpunct_byname<charT>` facet. The byname facets read the respective locale information from the external representation of a C locale. The name provided to construct a byname facet is the name of a locale, as you would use it in a call to `setlocale()`.
`//3` | A constructor is provided that takes a locale name and the new values we want to display for the alpha versions of `true` and `false`. The fourth parameter, `refs`, controls the facet's lifetime, as described in an earlier section.
`//4` | The virtual member functions `do_truename()` and `do_falsename()` are reimplemented. They are called by the public member functions `truename()` and `falsename()`. See the Apache C++ Standard Library Reference Guide for further details.

Now let's create a German locale in which the `std::numpunct<char>` facet has been replaced by an object of our new derived facet type, as shown in Figure 11:

#### Figure 11: Replacing the std::numpunct<char> facet object

The code looks like this:

    void main(int argc, char** argv)
    {
      std::locale loc(std::locale("de_DE"),                        //1
        new change_bool_names<char>("de_DE","Ja.","Nein."));       //2
      std::cout.imbue(loc);                                        //3
      std::cout << "Argumente vorhanden? "  //Any arguments?
                << std::boolalpha << (argc > 1) << std::endl;      //4
    }

`//1` | A locale object is constructed with an instance of the new facet class. The locale object has all facet objects from a German locale object, except that the new facet object `change_bool_names` substitutes for the `std::numpunct` facet object`.`
`//2` | The new facet object takes all information from a German `numpunct` facet object, and replaces the default native names for `true` and `false` with the provided strings `"Ja."` ("Yes.") and `"Nein."` ("No.").
`//3` | The standard output stream `std::cout` is imbued with the newly created locale.
`//4` | The expression `(argc > 1)` yields a boolean value, which indicates whether the program was called with arguments. This boolean value's alphanumeric representation is printed to the standard output stream. The output might be: `Argument vorhanden? Ja.`