How to generate a constexpr version string from three integers (or perhaps a git/SVN commit/rev. string)?

Question

Say I have

constexpr const std::uint8_t major = 1;
constexpr const std::uint8_t minor = 10;
constexpr const std::uint8_t bugfix = 0;

and I want

constexpr const char* version_string(){ ... }

to return the equivalent of "1.10.0" in this example, how would I do it?

I assume I'll need both of these, in constexpr:

• integer to string conversion
• string concatenation

The problem is purely academic, and I see little to no use to actually have it constexpr other than "it's possible". I just can't see how this would pan out. I'm willing to accept C++1y solutions that work on GCC 4.9 and Clang 3.4/3.5.

I believe I have found nearly what I seek on some Japanese blogs:

• constexpr itoa
• constexpr strcat

I will see what I can do with these, and perhaps answer this self-declared interesting question myself when I'm satisfied with the result.

Answer 1

Here is a C++11 solution. It uses class templates with char... parameter pack to simulate strings:

#include <iostream>
#include <type_traits>

template <char... symbols>
struct String
{
    static constexpr char value[] = {symbols...};
};

template <char... symbols>
constexpr char String<symbols...>::value[];

template <typename, typename>
struct Concat;

template <char... symbols1, char... symbols2>
struct Concat<String<symbols1...>, String<symbols2...>>
{
    using type = String<symbols1..., symbols2...>;
};

template <typename...>
struct Concatenate;

template <typename S, typename... Strings>
struct Concatenate<S, Strings...>
{
    using type = typename Concat<S, typename Concatenate<Strings...>::type>::type;
};

template <>
struct Concatenate<>
{
    using type = String<>;
};

template <std::size_t N>
struct NumberToString
{
    using type = typename Concat
        <
            typename std::conditional<(N >= 10), typename NumberToString<N / 10>::type, String<>>::type,
            String<'0' + N % 10>
        >::type;
};

template <>
struct NumberToString<0>
{
    using type = String<'0'>;
};

constexpr const std::uint8_t major = 1;
constexpr const std::uint8_t minor = 10;
constexpr const std::uint8_t bugfix = 0;

using VersionString = Concatenate
    <
        NumberToString<major>::type,
        String<'.'>,
        NumberToString<minor>::type,
        String<'.'>,
        NumberToString<bugfix>::type
    >::type;

constexpr const char* version_string = VersionString::value;

int main()
{
    std::cout << version_string << std::endl;
}

See live example.