bin2dec

namespace util
{
    template <typename Ty, char N, char... Others>
    struct bin2dec {
        static_assert(N == '1' || N == '0', "Invaild binary number");
        static constexpr Ty value = ( static_cast<Ty>(N - '0') << sizeof...(Others) ) + bin2dec<Ty, Others...>::value;
    };

    template <typename Ty, char N>
    struct bin2dec<Ty, N> {
        static_assert(N == '1' || N == '0', "Invaild binary number");
        static constexpr Ty value = static_cast<Ty>(N - '0');
    };
}

template <char... Nums>
constexpr int operator "" _b() {
    return util::bin2dec<int, Nums...>::value;
}

int main() {
    static_assert(0_b == 0, "");
    static_assert(101010_b == 42, "");
    static_assert(1111111_b == (1 << 7) - 1, "");
    static_assert(11111111111111111111111111111111_b == -1, "");
}