Xcode - Unknown type name 'constexpr'

Xcode - Unknown type name 'constexpr'

I have this header file Hashing.h
#ifndef __Hashing__
#define __Hashing__
// This code is released under the terms of the "CC0" license. Full terms
and conditions
// can be found at: http://creativecommons.org/publicdomain/zero/1.0/
#include <iostream>
// NOTE: hashing algorithm used is FNV-1a
// FNV-1a constants
static constexpr unsigned long long basis = 14695981039346656037ULL;
static constexpr unsigned long long prime = 1099511628211ULL;
// compile-time hash helper function
constexpr unsigned long long hash_one(char c, const char* remain, unsigned
long long value)
{
return c == 0 ? value : hash_one(remain[0], remain + 1, (value ^ c) *
prime);
}
// compile-time hash
constexpr unsigned long long hash(const char* str)
{
return hash_one(str[0], str + 1, basis);
}
// run-time hash
unsigned long long hash_rt(const char* str)
{
unsigned long long hash = basis;
while (*str != 0) {
hash ^= str[0];
hash *= prime;
++str;
}
return hash;
}
// ensure that the constexpr value is really compile-time constant
template <unsigned long long NUM>
struct test_const
{
static const unsigned long long value = NUM;
};