[#digit_to_int]
[section digit_to_int]

[h1 Synopsis]

  namespace util
  {
    template <class D>
    struct digit_to_int;
  }

This is a [link lazy_metafunction lazy template metafunction] that supports
[link currying currying].

[table Arguments
  [[Name] [Type]]
  [[`D`]  [[link boxed_value boxed] character value]]
]

[h1 Description]

Converts a boxed character containing a value in the range `['0'..'9']` to an
integer.

[h1 Return value]

It returns a [link boxed_value boxed] integer value.

[h1 Header]

  #include <boost/metaparse/util/digit_to_int.hpp>

[h1 Expression semantics]

For any `C` boxed character value in the range `['0'..'9']` the following
expressions are equivalent

  digit_to_int<>::apply<C>::type
  digit_to_int<C>::type
  digit_to_int_c<C::type::value>::type

[h1 Example]

  #include <boost/metaparse/util/digit_to_int.hpp>
  
  #include <type_traits>
  
  using namespace boost::metaparse;
  
  struct nullary_metafunction_returning_4
  {
    using type = std::integral_constant<char, '4'>;
  };
  
  static_assert(
    util::digit_to_int<std::integral_constant<char, '0'>>::type::value == 0,
    "it should convert a character to the corresponding integer value"
  );
  
  static_assert(
    util::digit_to_int<>::type
      ::apply<std::integral_constant<char, '7'>>::type::value == 7,
    "it should support currying"
  );
  
  static_assert(
    util::digit_to_int<nullary_metafunction_returning_4>::type::value == 4,
    "it should support lazy evaluation"
  );

[endsect]