C++ Compile time conversion of a number to a string

compiletime-strings.hpp

/*!
    This file contains code to convert unsigned integers to strings
    at compile-time, and then modify those strings include more
    information.
    
    One use case is to convert library MAJOR, MINOR, and PATCH
    numbers into a custom string (see below).
    
    Other types like signed integers and floats can be added
    with template specializition on the converter struct.
    
    Inspiration for this code came from these two stackoverflow posts:
    https://stackoverflow.com/questions/6713420/c-convert-integer-to-string-at-compile-time
    https://stackoverflow.com/questions/26568920/how-do-you-implement-compile-time-string-conversion-functions
*/

/*!
    Returns the number of digits in n
*/
constexpr std::size_t num_digits(std::size_t n)
{
    return n <  10 ? 1 : num_digits(n / 10) + 1;
}

/*!
    Type container of characters. All of the characters that make
    up the string are stored in the template parameters.
    
    The str member variable will then be a compile-time
    created string consisting of the characters in the
    template parameter pack.
*/
template<char... Chars>
struct char_list {
    const char str[sizeof...(Chars)] = { Chars... };
};

/*!
    Partial template specialization to append characters
    onto the template paramters of a char_list
*/
template<class List, char C>
struct append_char;

template<template<char...> class List, char... Cs, char C>
struct append_char<List<Cs...>, C> {
    using type = char_list<Cs..., C>;
};

/*!
    Partial template specialization to combine the parameters
    in two char_lists into one.
*/
template< class List1, class List2>
struct combine_strings;

template<template<char...> class List1, char... Cs1,
        template<char...> class List2, char... Cs2>
struct combine_strings<List1<Cs1...>, List2<Cs2...>> {
    using type = char_list<Cs1..., Cs2...>;
};

/*!
    This struct takes an unsigned integer as a template argument
    and separates the digits in the unsigned integer into
    separate characters.
*/
template<std::size_t D, std::size_t N, char... Chars>
struct converter {
    using type = typename converter<D - 1, N / 10, '0' + N % 10, Chars...>::type;
};

/*!
    Base case for integer conversion: Only 1 digit left in the
    unsigned integer;
*/
template<std::size_t N, char... Chars>
struct converter<1, N, Chars...> {
    using type = char_list<'0' + N, Chars...>;
};

/* Convenience using alias templates */

template<class List, char C>
using append_char_t = typename append_char<List, C>::type;

template< class List1, class List2>
using combine_strings_t = typename combine_strings<List1, List2>::type;

template<std::size_t N>
using convert_t = typename converter<numDigits(N), N>::type;

/*!
    Convenience function that's meant to be given a
    char_list as a template argument;
*/
template<typename List>
static constexpr const char* make_string() {
    static List charList;
    return charList.str;
}

/*!
    Example of how a custom version string could be produced.
    This function builds a string at compile time using three
    integers defined in the MyLib_VERSION_* macros.
    
    \return "0.1.0"
*/
const char* GetVersionString()
{
    using major = append_char_t<convert_t<MyLib_VERSION_MAJOR>, '.'>;
    using minor = append_char_t<convert_t<MyLib_VERSION_MINOR>, '.'>;
    using patch = convert_t<MyLib_VERSION_PATCH>;

    using VersionString = combine_strings_t<combine_strings_t<major, minor>, patch>;

    return make_string<VersionString>();
}