Constant expression string parameter in C++11 constexpr function

Question

I'm trying to make a constexpr function that converts a UUID string like "f6ece560-cc3b-459a-87f1-22331582216e" to a class something like this:

class UUID {
public:
      explicit UUID(uint8_t bytes[]); // Must be 16 byte array.

This is what I've got so far:

// Compile time hex conversion of a single character into a nibble (half-byte).
constexpr uint8_t hexToNibble(char a)
{
    // Does not work:
//  static_assert(a >= '0' && a <= '9' || a >= 'a' && a <= 'f' || a >= 'A' && a <= 'F', "Invalid hex character");
    return a >= '0' && a <= '9' ? (a - '0') :
           a >= 'a' && a <= 'f' ? (a - 'a' + 10) :
           a >= 'A' && a <= 'F' ? (a - 'A' + 10) : 0;
}

// Compile time hex conversion of two characters into a byte.
constexpr uint8_t hexToByte(char a, char b)
{
    return (hexToNibble(a) << 4) + hexToNibble(b);
}

// Compile time string length.
constexpr int strlenConst(const char* str)
{
    return *str ? 1 + strlenConst(str + 1) : 0;
}

// Convert a UUID string to an array of bytes.
// Example: "f6ece560-cc3b-459a-87f1-22331582216e".
constexpr std::array<uint8_t, 16> UUIDFromString(const char* str)
{
    // This does not work:
//  static_assert(strlenConst(str) == 36, "Invalid GUID length");

    return std::array<uint8_t, 16>{
        hexToByte(str[0], str[1]),
        hexToByte(str[2], str[3]),
        hexToByte(str[4], str[5]),
        hexToByte(str[6], str[7]),
        hexToByte(str[9], str[10]),
        hexToByte(str[11], str[12]),
        hexToByte(str[14], str[15]),
        hexToByte(str[16], str[17]),
        hexToByte(str[19], str[20]),
        hexToByte(str[21], str[22]),
        hexToByte(str[24], str[25]),
        hexToByte(str[26], str[27]),
        hexToByte(str[28], str[29]),
        hexToByte(str[30], str[31]),
        hexToByte(str[32], str[33]),
        hexToByte(str[34], str[35]),
    };
}

#define MAKE_UUID(var, str) \
    static_assert(strlenConst(str) == 36, "Invalid GUID length for " #var); \
    const UUID var(UUIDFromString(str).data());

// Works but doesn't check string length.
const UUID UUID_1(UUIDFromString("f6ece560-cc3b-459a-87f1-22331582216e").data());

// Checks string length but uses an ugly macro.
MAKE_UUID(UUID_2, "f6ece560-cc3b-459a-87f1-22331582216e")

As you can see there is a problem - it seems it is impossible to have function parameters that are constant expressions in a constexpr function, so you can't do static_asserts on the parameters, even if the value that was passed in was a constant expression.

So I have resorted to a macro to check the string length, and given up checking the characters.

Is there a way around this? Also how can I easily ensure that this function is actually evaluated at compile time?

Edit: This is not the same as C++11 - static_assert within constexpr function? - or at least the same answers don't work - see comments below.

Edit 2: Shafik's excellent answer works for the size issue, but not for checking the hex characters. As far as I can tell that is impossible - even if you use this...

// Compile time hex conversion of a single character into a nibble (half-byte).
template<char a>
constexpr uint8_t hexToNibble()
{
    static_assert(a >= '0' && a <= '9' || a >= 'a' && a <= 'f' || a >= 'A' && a <= 'F', "Invalid hex character");
    return a >= '0' && a <= '9' ? (a - '0') :
           a >= 'a' && a <= 'f' ? (a - 'a' + 10) :
           a >= 'A' && a <= 'F' ? (a - 'A' + 10) : 0;
}

// Compile time hex conversion of two characters into a byte.
template<char a, char b>
constexpr uint8_t hexToByte()
{
    return (hexToNibble<a>() << 4) + hexToNibble<b>();
}

This won't work:

// Convert a UUID string to an array of bytes.
// Example: "f6ece560-cc3b-459a-87f1-22331582216e".
template <size_t N>
constexpr std::array<uint8_t, 16> UUIDFromString(const char (&str)[N])
{
    // Note you have to include the null byte.
    static_assert(N == 37, "Invalid GUID length.");

    return std::array<uint8_t, 16>{
        hexToByte<str[0], str[1]>(),
        hexToByte<str[2], str[3]>(),

Because str[0] isn't a constant expression.

Answer 1

As AndyG points out the question C++11 - static_assert within constexpr function? tells us one way to to do this is to use non-type template arguments which have to be available at compile-time.

The issue with this solution is that the OP is using string literals which can not be bound to non-type arguments:

In particular, this implies that string literals, addresses of array elements, and addresses of non-static members cannot be used as template arguments to instantiate templates whose corresponding non-type template parameters are pointers to objects.

One work-around for this is not directly use the string literal but the use the property that is important to the problem which is the length of the array, like follows:

template <size_t N>
constexpr std::array<uint8_t, 16> UUIDFromString( const char (&str)[N])
{
  static_assert(N == 36, "Invalid GUID length");

  //....
}

Answer 2

If you want the static_asserts, you have no choice but template-parameterize the pertinent arguments.

It's been remarked that you can't bind a string literal to non-type template parameter, which is legalistically true, but you can do it, with a little circumlocution.

Here's a sketch:

#include <cstddef>
#include <array>

constexpr char const f6ece560_cc3b_459a_87f1_22331582216e[] =
    "f6ece560-cc3b-459a-87f1-22331582216e";

constexpr int strlenConst(const char* str)
{
    return *str ? 1 + strlenConst(str + 1) : 0;
}

template<char const * Hex>
struct UUID {
    static constexpr char const * hex = Hex;
    static constexpr std::size_t len = strlenConst(hex);
    static_assert(len == 36,"Invalid GUID length");
    template<char Ch>
    static constexpr uint8_t hexToNibble() {
        static_assert((Ch >= '0' && Ch <= '9') || (Ch >= 'a' && Ch <= 'f')
                    || (Ch >= 'A' && Ch <= 'F'), "Invalid hex character");
        return Ch >= '0' && Ch <= '9' ? (Ch - '0') :
               Ch >= 'a' && Ch <= 'f' ? (Ch - 'a' + 10) :
               Ch >= 'A' && Ch <= 'F' ? (Ch - 'A' + 10) : 0;
    }
    template<char First, char Second>
    static constexpr uint8_t hexToByte()
    {
        return (hexToNibble<First>() << 4) + hexToNibble<Second>();
    }
    static constexpr std::array<uint8_t, 16> get() {
        return std::array<uint8_t, 16>{{
            hexToByte<hex[0], hex[1]>(),
            hexToByte<hex[2], hex[3]>(),
            hexToByte<hex[4], hex[5]>(),
            hexToByte<hex[6], hex[7]>(),
            hexToByte<hex[9], hex[10]>(),
            hexToByte<hex[11], hex[12]>(),
            hexToByte<hex[14], hex[15]>(),
            hexToByte<hex[16], hex[17]>(),
            hexToByte<hex[19], hex[20]>(),
            hexToByte<hex[21], hex[22]>(),
            hexToByte<hex[24], hex[25]>(),
            hexToByte<hex[26], hex[27]>(),
            hexToByte<hex[28], hex[29]>(),
            hexToByte<hex[30], hex[31]>(),
            hexToByte<hex[32], hex[33]>(),
            hexToByte<hex[34], hex[35]>()
        }};
    }
};

#include <iostream>

using namespace std;

int main()
{
    using some_uuid_t = UUID<f6ece560_cc3b_459a_87f1_22331582216e>;
    cout << some_uuid_t::hex << endl;
    auto uuid = some_uuid_t::get();
    for (auto const & byte : uuid) {
        cout << int(byte);
    }
    cout << endl;
    return 0;
}

(gcc 4.9.2/clang 3.5.2 -std=c++11 -Wall -pedantic)