Using a C++ user-defined literal to initialise an array

Use a numeric literal operator template, with the signature:

template <char...>
result_type operator "" _x();

Also, since the data is known at compile-time, we might as well make everything constexpr. Note that we use std::array instead of C-style arrays:

#include <cstdint>
#include <array>
#include <vector>

// Constexpr hex parsing algorithm follows:
struct InvalidHexDigit {};
struct InvalidPrefix {};
struct OddLength {};

constexpr std::uint8_t hex_value(char c)
{
    if ('0' <= c && c <= '9') return c - '0';
    // This assumes ASCII:
    if ('A' <= c && c <= 'F') return c - 'A' + 10;
    if ('a' <= c && c <= 'f') return c - 'a' + 10;
    // In constexpr-land, this is a compile-time error if execution reaches it:
    // The weird `if (c == c)` is to work around gcc 8.2 erroring out here even though
    // execution doesn't reach it.
    if (c == c) throw InvalidHexDigit{};
}

constexpr std::uint8_t parse_single(char a, char b)
{
    return (hex_value(a) << 4) | hex_value(b);
}

template <typename Iter, typename Out>
constexpr auto parse_hex(Iter begin, Iter end, Out out)
{
    if (end - begin <= 2) throw InvalidPrefix{};
    if (begin[0] != '0' || begin[1] != 'x') throw InvalidPrefix{};
    if ((end - begin) % 2 != 0) throw OddLength{};

    begin += 2;

    while (begin != end)
    {
        *out = parse_single(*begin, *(begin + 1));
        begin += 2;
        ++out;
    }

    return out;
}

// Make this a template to defer evaluation until later        
template <char... cs>
struct HexByteArray {
    static constexpr auto to_array()
    {
        constexpr std::array<char, sizeof...(cs)> data{cs...};

        std::array<std::uint8_t, (sizeof...(cs) / 2 - 1)> result{};

        parse_hex(data.begin(), data.end(), result.begin());

        return result;
    }

    constexpr operator std::array<std::uint8_t, (sizeof...(cs) / 2)>() const 
    {
        return to_array();
    }

    operator std::vector<std::uint8_t>() const
    {
        constexpr auto tmp = to_array();

        return std::vector<std::uint8_t>{tmp.begin(), tmp.end()};
    }
};

template <char... cs>
constexpr auto operator"" _$()
{
    static_assert(sizeof...(cs) % 2 == 0, "Must be an even number of chars");
    return HexByteArray<cs...>{};
}

Demo

Example usage:

auto data_array = 0x6BC1BEE22E409F96E93D7E117393172A_$ .to_array();
std::vector<std::uint8_t> data_vector = 0x6BC1BEE22E409F96E93D7E117393172A_$;

---

As a side note, $ in an identifier is actually a gcc extension, so it's non-standard C++. Consider using a UDL other than _$.

This will make it

namespace detail{
template <std::size_t C> constexpr std::integral_constant<std::size_t, C> int_c{ };

template <char c>
class hex_decimal_t
{
    constexpr static std::uint8_t get_value() {
        constexpr std::uint8_t k = c - '0';
        if constexpr (k >= 0 && k <= 9) { return k; }
        else if constexpr (k >= 17 && k <= 22) { return k - 7;  }
        else if constexpr (k >= 49 && k <= 54) { return k - 39; }
        else { return std::uint8_t(-1); }
    }
public:
    static constexpr std::uint8_t value = get_value();
    constexpr operator auto() const{
        return value;
    }
};
template <char C> constexpr hex_decimal_t<C> hex_decimal{ };

template <bool B> using bool_type = std::integral_constant<bool, B>;

template <char... cs> struct is_valid_hex : std::false_type { };
template <char... cs> struct is_valid_hex<'0', 'x', cs...> : bool_type<((hex_decimal<cs> != std::uint8_t(-1)) && ...)>{};
template <char... cs> struct is_valid_hex<'0', 'X', cs...> : bool_type<((hex_decimal<cs> != std::uint8_t(-1)) && ...)>{};

template <std::size_t... Is>
constexpr auto expand_over(std::index_sequence<0, Is...>)
{
    return [](auto&& f) -> decltype(auto) {
        return decltype(f)(f)(int_c<Is>...);
    };
}

template <class T,class... F>
constexpr auto select(T, F&&... f) {
    return std::get<T{}>(std::forward_as_tuple(std::forward<F>(f)...));
}
}

template <char... ds>
constexpr auto operator "" _H()
{
    static_assert(detail::is_valid_hex<ds...>{} || sizeof...(ds) < 3, "Not a valid hex number");
    static_assert(!(sizeof...(ds) > 3 && sizeof...(ds) & 0x1), "Hex string must have even length");

    constexpr int Sz = sizeof...(ds);

    constexpr auto expand = detail::select(detail::int_c<(Sz > 3)>,
        [] { return detail::expand_over(std::make_index_sequence<2>{}); },
        [] { return detail::expand_over(std::make_index_sequence<Sz/2>{}); }
    )();

    if constexpr (Sz <= 3) {
        return expand([](auto... Is) {
            constexpr std::array digs{ds...};
            return std::array { (detail::hex_decimal<digs[2 * Is]>)... };
        });
    } else {
        return expand([](auto... Is) {
            constexpr std::array digs{ds...};
            return std::array { ((detail::hex_decimal<digs[2 * Is]> << 4) | detail::hex_decimal<digs[2 * Is + 1]>)... };
        });
    }
}

---

constexpr auto arr = 0x070A16B46B4D4144F79BDD9DD04A287C_H;
static_assert(arr.size() == 16);
static_assert(std::get<0>(arr) == 0x7);
static_assert(std::get<arr.size() - 1>(arr) == 0x7C);

Live demo

A completely compile-time static_assert version based on @Justin's answer.

The udl operator returns std::array directly. You can simply define other udl operators that return std::tuple<std::integral_constant<char, c>...> or even vector using the HexArrayBuilder implementation class.

This is c++11 version. The static_assert that checks character validity can be written in c++17 way like the commented line.

#include <array>
#include <type_traits>
#include <tuple>

struct HexArrayHelper {
    static constexpr bool valid(char c) { return ('0' <= c && c <= '9') || ('A' <= c && c <= 'F') || ('a' <= c && c <= 'f'); }
    static constexpr char hex_value(char c) {
        return ('0' <= c && c <= '9') ? c - '0'
             : ('A' <= c && c <= 'F') ? c - 'A' + 10
             : c - 'a' + 10;
    }
    static constexpr char build(char a, char b) {
        return (hex_value(a) << 4) + hex_value(b);
    };
};

template <char... cs>
struct HexArray {
    static constexpr std::array<char, sizeof...(cs)> to_array() { return {cs...}; }
    static constexpr std::tuple<std::integral_constant<char, cs>...> to_tuple() { return {}; }
};

template <typename T, char... cs>
struct HexArrayBuilder : T {};

template <char... built, char a, char b, char... cs>
struct HexArrayBuilder<HexArray<built...>, a, b, cs...> : HexArrayBuilder<HexArray<built..., HexArrayHelper::build(a, b)>, cs...> {
    static_assert(HexArrayHelper::valid(a) && HexArrayHelper::valid(b), "Invalid hex character");
};

 template <char zero, char x, char... cs>
struct HexByteArray : HexArrayBuilder<HexArray<>, cs...> {
    static_assert(zero == '0' && (x == 'x' || x == 'X'), "Invalid prefix");
    // static_assert(std::conjunction<std::bool_constant<HexArrayHelper::valid(cs)>...>::value, "Invalid hex character");
};

template <char... cs>
constexpr auto operator"" _hexarr() -> std::array<char, sizeof...(cs) / 2 - 1> {
    static_assert(sizeof...(cs) % 2 == 0 && sizeof...(cs) >= 2, "Must be an even number of chars");
    return HexByteArray<cs...>::to_array();
}

auto x = 0X1102030405060708abcdef_hexarr;