Constants and Constant Expressions in C++11

// file: constarray.cpp
#include <iostream>
#include <ostream>

template <class ElemType, int N>
class ConstArray  
{
    const ElemType a[N];
public:    
    template <class ... T> constexpr ConstArray(T ... p):a {p...} {}
    constexpr int size() { return N;}
    constexpr ElemType operator[](int i) 
           { return (i < 0 || i >= N ? throw "ERROR: out of range": a[i]);}    
};

constexpr ConstArray<double, 3> a{1.0, 2.5, 3.0};
constexpr ConstArray<char, 4>s{'a','b','c','d'};

int main()
{    
    try
    {
        std::cout << "a.size(): " << a.size() << std::endl;
        std::cout << "a[2]: " << a[2] << std::endl;
        std::cout << "s[3]: " << s[3] << std::endl;               
        std::cout << "s[4]: " << s[4] << std::endl;               
    }
    catch (const char* error)
    {
        std::cout << error << std::endl;
    }
    return 0;
}