How do I generate an integer from a string literal at compile-time?

Question

In C++, is it possible to generate an integer from a string literal using only compile-time facilities ?

For instance, if all we have is the literal "6", is there some way to use it as a template argument, like std::array<GET_INTEGER("6")> a; ?

I know about constexpr-based techniques, such as :

template <int N> constexpr char get_char(const char s[N], int n) {
  return s[n];
}

However constexpr isn't ready yet on most compilers, so I'm looking for solutions using probably macros and TMP.

It's just for experimentation, so crazy ideas are welcome.

Answer 1

Apparently gcc allows "abcd"[3] be interpreted as 'd', which allows this to work (at least on g++-4.6 and 4.7):

#include <boost/preprocessor/repetition/enum.hpp>

template <const char... characters>
struct GetIntegerTemplate;

template <const char head, const char... rest>
struct GetIntegerTemplate<head, rest...>
{
    typedef GetIntegerTemplate<rest...> Prev;
    enum
    {
        power = Prev::power * 10,
        value = (head - '0') * Prev::power + Prev::value
    };
};

template <>
struct GetIntegerTemplate<>
{
    enum
    {
        power = 1,
        value = 0
    };
};

#define GET_NTH_CHARACTER(z, n, data) data[n]
#define GET_INTEGER(length, the_string) GetIntegerTemplate<BOOST_PP_ENUM(length, GET_NTH_CHARACTER, the_string)>::value

int main()
{
    static_assert(GET_INTEGER(7, "1234567") == 1234567, "oops");
}

But it won't compile on clang, which says "non-type template argument of type 'const char' is not an integral constant expression".

---

What it really does is to break down the string literal "1234567" into a list of character literals '1', '2', '3', '4', '5', '6', '7'. The instantiation

GetIntegerTemplate<'1', '2', '3', '4', '5', '6', '7'>::value

is then invoked to turn the list into an integer 1234567. The string → char literal step may involve non-standard behavior which may not work outside of g++ (i.e. worse than constexpr ☺), but that GetIntegerTemplate<...>::value is portable.

Answer 2

_{(Reposting from another answer of mine)}

If you don't mind changing your conceptual definition of 'string literal' from e.g.
"425897" to '4258','97', then you can use Boost.MPL's boost::mpl::string<> to accomplish this:

#include <cstddef>
#include <boost/type_traits/is_integral.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/type_traits/is_signed.hpp>
#include <boost/mpl/and.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/mpl/char.hpp>
#include <boost/mpl/contains.hpp>
#include <boost/mpl/end.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/find_if.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/front.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/integral_c.hpp>
#include <boost/mpl/minus.hpp>
#include <boost/mpl/negate.hpp>
#include <boost/mpl/next.hpp>
#include <boost/mpl/not.hpp>
#include <boost/mpl/pair.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/plus.hpp>
#include <boost/mpl/pop_front.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/reverse_fold.hpp>
#include <boost/mpl/size_t.hpp>
#include <boost/mpl/string.hpp>
#include <boost/mpl/times.hpp>
#include <boost/mpl/vector.hpp>

namespace details
{
    namespace mpl = boost::mpl;

    typedef mpl::vector10<
        mpl::char_<'0'>, mpl::char_<'1'>, mpl::char_<'2'>, mpl::char_<'3'>,
        mpl::char_<'4'>, mpl::char_<'5'>, mpl::char_<'6'>, mpl::char_<'7'>,
        mpl::char_<'8'>, mpl::char_<'9'>
    > valid_chars_t;

    template<typename IntegralT, typename PowerT>
    struct power_of_10;

    template<typename IntegralT, std::size_t Power>
    struct power_of_10<IntegralT, mpl::size_t<Power> > : mpl::times<
        power_of_10<IntegralT, mpl::size_t<Power - 1u> >,
        mpl::integral_c<IntegralT, 10>
    > { };

    template<typename IntegralT>
    struct power_of_10<IntegralT, mpl::size_t<1u> >
        : mpl::integral_c<IntegralT, 10>
    { };

    template<typename IntegralT>
    struct power_of_10<IntegralT, mpl::size_t<0u> >
        : mpl::integral_c<IntegralT, 1>
    { };

    template<typename IntegralT, typename StringT>
    struct is_negative : mpl::and_<
        boost::is_signed<IntegralT>,
        boost::is_same<
            typename mpl::front<StringT>::type,
            mpl::char_<'-'>
        >
    > { };

    template<typename IntegralT, typename StringT>
    struct extract_actual_string : mpl::eval_if<
        is_negative<IntegralT, StringT>,
        mpl::pop_front<StringT>,
        mpl::identity<StringT>
    > { };

    template<typename ExtractedStringT>
    struct check_valid_characters : boost::is_same<
        typename mpl::find_if<
            ExtractedStringT,
            mpl::not_<mpl::contains<valid_chars_t, mpl::_> >
        >::type,
        typename mpl::end<ExtractedStringT>::type
    > { };

    template<typename ExtractedStringT>
    struct pair_digit_with_power : mpl::first<
        typename mpl::reverse_fold<
            ExtractedStringT,
            mpl::pair<mpl::vector0<>, mpl::size_t<0> >,
            mpl::pair<
                mpl::push_back<
                    mpl::first<mpl::_1>,
                    mpl::pair<mpl::_2, mpl::second<mpl::_1> >
                >,
                mpl::next<mpl::second<mpl::_1> >
            >
        >::type
    > { };

    template<typename IntegralT, typename ExtractedStringT>
    struct accumulate_digits : mpl::fold<
        typename pair_digit_with_power<ExtractedStringT>::type,
        mpl::integral_c<IntegralT, 0>,
        mpl::plus<
            mpl::_1,
            mpl::times<
                mpl::minus<mpl::first<mpl::_2>, mpl::char_<'0'> >,
                power_of_10<IntegralT, mpl::second<mpl::_2> >
            >
        >
    > { };

    template<typename IntegralT, typename StringT>
    class string_to_integral_impl
    {
        BOOST_MPL_ASSERT((boost::is_integral<IntegralT>));

        typedef typename extract_actual_string<
            IntegralT,
            StringT
        >::type ExtractedStringT;
        BOOST_MPL_ASSERT((check_valid_characters<ExtractedStringT>));

        typedef typename accumulate_digits<
            IntegralT,
            ExtractedStringT
        >::type ValueT;

    public:
        typedef typename mpl::eval_if<
            is_negative<IntegralT, StringT>,
            mpl::negate<ValueT>,
            mpl::identity<ValueT>
        >::type type;
    };
}

template<typename IntegralT, typename StringT>
struct string_to_integral2
    : details::string_to_integral_impl<IntegralT, StringT>::type
{ };

template<typename IntegralT, int C0, int C1 = 0, int C2 = 0,
    int C3 = 0, int C4 = 0, int C5 = 0, int C6 = 0, int C7 = 0>
struct string_to_integral : string_to_integral2<
    IntegralT,
    boost::mpl::string<C0, C1, C2, C3, C4, C5, C6, C7>
> { };

Usage would look like:

int i = string_to_integral<int, '4258','97'>::value;
// or
typedef boost::mpl::string<'4258','97'> str_t;
unsigned j = string_to_integral2<unsigned, str_t>::value;

Support for negative numbers is implemented, support for overflow detection is not (but your compiler will probably give a warning).