add is_character

[gpeterhoff]

is_character.hpp

//	Distributed under the Boost Software License Version 1.0 https://www.boost.org/LICENSE_1_0.txt
//	Copyright Gero Peterhoff

#ifndef BOOST_TYPE_TRAITS_IS_CHARACTER_HPP
#define BOOST_TYPE_TRAITS_IS_CHARACTER_HPP

#include <boost/type_traits/integral_constant.hpp>
#include <boost/type_traits/remove_cvref.hpp>



namespace boost
{
template <typename Type> struct is_character				: public false_type{};
template <typename Type> struct is_character<volatile const Type>	: public is_character<Type>{};
template <typename Type> struct is_character<volatile Type>		: public is_character<Type>{};
template <typename Type> struct is_character<const Type>		: public is_character<Type>{};

#if defined(__cpp_unicode_characters)
template <> struct is_character<char32_t>	: public true_type{};
template <> struct is_character<char16_t>	: public true_type{};
#endif

#if defined(__cpp_char8_t)
template <> struct is_character<char8_t>	: public true_type{};
#endif

template <> struct is_character<wchar_t>	: public true_type{};
template <> struct is_character<char>		: public true_type{};



#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template <typename Type> inline constexpr bool is_character_v = is_character<Type>::value;
#endif
}	//	boost

#endif	//	BOOST_TYPE_TRAITS_IS_CHARACTER_HPP

[pdimov]

Character types are a standard concept, defined in https://eel.is/c++draft/basic.fundamental#11, and the above looks like the correct implementation for it.

This trait is occasionally useful to constrain functions taking C strings, e.g. https://github.com/boostorg/nowide/blob/e040930b0986d5a4b89d2b9b109a6233c2baa831/include/boost/nowide/detail/is_string_container.hpp#L26