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.