constexpr ( C++11)

#include <iostream>
#include <stdexcept>

//  constexpr ++11  ,   
constexpr int factorial(int n)
{
    return n <= 1 ? 1 : (n * factorial(n - 1));
}

//  constexpr ++14      
#if __cplusplus >= 201402L
constexpr int factorial_cxx14(int n)
{
    int res = 1;
    while (n > 1)
        res *= n--;
    return res;
}
#endif // C++14

//  
class conststr
{
    const char* p;
    std::size_t sz;
public:
    template<std::size_t N>
    constexpr conststr(const char(&a)[N]): p(a), sz(N - 1) {}
    
    //  constexpr   ,  
    //  C++11,         ?:
    constexpr char operator[](std::size_t n) const
    {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
    
    constexpr std::size_t size() const { return sz; }
};

//  constexpr ++11        return
// (++14    )
constexpr std::size_t countlower(conststr s, std::size_t n = 0,
                                             std::size_t c = 0)
{
    return n == s.size() ? c :
        'a' <= s[n] && s[n] <= 'z' ? countlower(s, n + 1, c + 1) :
                                     countlower(s, n + 1, c);
}

//  ,      
template<int n>
struct constN
{
    constN() { std::cout << n << '\n'; }
};

int main()
{
    std::cout << "4! = " ;
    constN<factorial(4)> out1; //    

    volatile int k = 8; //    volatile
    std::cout << k << "! = " << factorial(k) << '\n'; //    

    std::cout << "    \", !\"  ";
    constN<countlower(", !")> out2; //    conststr

    constexpr int a[12] = {0, 1, 2, 3, 4, 5, 6, 7, 8};
    constexpr int length_a = sizeof(a)/sizeof(int); // std::size(a)  C++17,
                                                    // std::ssize(a)  C++20
    std::cout << "  " << length_a << "  : ";
    for (int i = 0; i < length_a; ++i)
        std::cout << a[i] << " ";
}