c++ template test

// This snippet answers several answers about c++ template programming

#include <iostream>
#include <typeinfo>
#include <string>
#include <stdlib.h>

template < typename T >
T sq(T v) { return v*v;}

template < typename T, typename R>
R sq(T v) { return R(v*v);}

template <typename T, typename R>
R length3(const T * arr)
{
   return sq<T,R>(arr[0]) + sq<T,R>(arr[1]) + sq<T,R>(arr[2]);
}
template <typename T, typename R>
R length3(const T & v)
{
   return sq<T,R>(v[0]) + sq<T,R>(v[1]) + sq<T,R>(v[2]);
}

#include <cxxabi.h>

const std::string demangle(const char* name)
{
    int status = -4;

    char* res = abi::__cxa_demangle(name, NULL, NULL, &status);

    const char* const demangled_name = (status==0)?res:name;

    std::string ret_val(demangled_name);

    free(res);

    return ret_val;
}

template <typename T> struct IsRef {
  static bool const result = false;
};
template <typename T> struct IsRef<T&> {
  static bool const result = true;
};

// types with operator [] defined
template <typename T, typename R >
R distance3(const T & p1, const T & p2)
{
    std::cout << "distance T& " << demangle(typeid(T).name()) << '\n';

    R d[3];
    d[0] = p1[0] - p2[0];
    d[1] = p1[1] - p2[1];
    d[2] = p1[2] - p2[2];
    return length3<R,R>(&d[0]);
}

template <typename T, typename R >
R distance3(const T * p1, const T * p2)
{
    std::cout << "distance T* " << demangle(typeid(T).name()) << '\n';
    R d[3];
    d[0] = p1[0] - p2[0];
    d[1] = p1[1] - p2[1];
    d[2] = p1[2] - p2[2];
    return length3<R,R>(&d[0]);
}

template < class T >
struct Vec3
{
    typedef T value_type;
    Vec3(T x) { data[0] = data[1] = data[2] = x; }
    Vec3(T a, T b, T c) { data[0] = a; data[1] = b; data[2] = c; }

    T & operator[] (int i) { return data[i]; }
    const T & operator[] (int i) const { return data[i]; }
    T data[3];
};

typedef Vec3<float> Vec3f;
typedef Vec3<int> Vec3i;


int main(int argc, char* argv[])
{
    Vec3i v0(2,2,2), v1(4,4,4);

    signed char a[3] = {2,2,2};
    signed char b[3] = {-1,-1,-1};


    std::cout << distance3<Vec3i,float>(v0,v1) << '\n';
    std::cout << distance3<signed char*,int>(a, b) << '\n';

    return 0;
}