C++11 const string length, gcc 4.7+ and clang 3.5+

const_str_len.hpp

// https://godbolt.org/z/uu4TMP

// API: constexpr size_t const_strlen( "foo" );
// Much more readable version here: https://gist.github.com/kaidokert/dfc08be8e75a3fc650d3daf8e89c3fe9 
// but that doesn't work with GCC before version 7
#include <utility> //std::forward
#include <cstddef> //std::size_t

namespace detail {
    template<size_t COUNTER, typename T>
    struct recursive_array {};

    template<size_t COUNTER, size_t N> // General case, Counter=0...N-1
    struct recursive_array<COUNTER, const char (&)[N]> {
        constexpr static size_t check(const char(&x)[N]) {
            return x[COUNTER] == '\0' ? COUNTER :
                   recursive_array<COUNTER + 1, const char(&)[N]>::check(x);
        }
    };

    template<size_t N> // Termination case, COUNTER = N
    struct recursive_array<N, const char (&)[N]> {
        constexpr static size_t check(const char(&)[N]) {
            return N;
        }
    };

    template<typename T>
    struct len_helper {
        constexpr static size_t len(const char *x) {
            return *x == '\0' ? 0 : 1 + len(x + 1);
        }
    };

    template<>
    struct len_helper<std::nullptr_t> {
        constexpr static size_t len(std::nullptr_t) {
            return 0;
        }
    };

    template<size_t N>
    struct len_helper<const char (&)[N]> {
        constexpr static size_t len(const char (&x)[N]) {
            return recursive_array<0, const char (&)[N]>::check(x);
        }
    };
} // namespace detail

template<typename T>
constexpr size_t const_strlen(T&& x) {
    return detail::len_helper< T >::len(std::forward< T >(x)  );
}


int main() {
    constexpr const char ce_const_arr_empty[] = "";
    constexpr const char ce_const_arr[] = "foo";
    constexpr char ce_arr[] = "foo";
    constexpr const char * ptr = "foo";
    constexpr const char real_long[] =
            "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vestibulum sit amet erat accumsan, "
            "pellentesque libero id, pulvinar diam. Cras ullamcorper velit vitae tortor ultricies, at semper mi consequat. "
            "Vestibulum pretium aliquam sapien, sit amet"
#if !(__clang_major__ < 4)
            " placerat libero. Fusce in tempus risus. Lorem ipsum dolor sit amet, "
            "consectetur adipiscing elit. Aenean lobortis, ante maximus tempor ullamcorper, tortor sem iaculis purus, dignissim "
            "consectetur massa leo a mauris. Aliquam aliquam bibendum eros eget viverra."
#endif
            ;

    SASSERT_EQ( const_stringlen(nullptr), 0);
    SASSERT_EQ( const_stringlen(""), 0);
    SASSERT_EQ( const_stringlen("\0" "bar"), 0);
    SASSERT_EQ( const_stringlen("f\0" "ba"), 1);
    SASSERT_EQ( const_stringlen("fo\0" "b"), 2);
    SASSERT_EQ( const_stringlen((const char*)"\0" "bar"), 0);
    SASSERT_EQ( const_stringlen(ce_const_arr_empty) ,0);
    SASSERT_EQ( const_stringlen(ptr) ,3);
    SASSERT_EQ( const_stringlen(ce_const_arr) ,3);
    SASSERT_EQ( const_stringlen(ce_arr) ,3);
#if (__clang_major__ < 4) //old clang defaults to 256 recursion depth
    SASSERT_EQ( const_stringlen(real_long) ,245);
#else //gcc defaults to 512
    SASSERT_EQ( const_stringlen(real_long) ,504);
#endif
}