template class: ctor against function -> new C++ standard

Question

Hi
in this question:
http://stackoverflow.com/questions/2779155/template-point2-double-point3-double
Dennis and Michael noticed the unreasonable foolishly implemented constructor.
They were right, I didn't consider this at that moment. But I found out that a constructor does not help very much for a template class like this one, instead a function is here much more convenient and safe

namespace point {

template < unsigned int dims, typename T >
struct Point {

    T X[ dims ];

    std::string str() {
        std::stringstream s;
        s << "{";
        for ( int i = 0; i < dims; ++i ) {
            s << " X" << i << ": " << X[ i ] << (( i < dims -1 )? " |": " ");
        }
        s  << "}";
        return s.str();
    }

    Point<dims, int> toint() {
        Point<dims, int> ret;
        std::copy( X, X+dims, ret.X );
        return ret;
    }
};

template < typename T >
Point< 2, T > Create( T X0, T X1 ) {
    Point< 2, T > ret;
    ret.X[ 0 ] = X0; ret.X[ 1 ] = X1;
    return ret;
}
template < typename T >
Point< 3, T > Create( T X0, T X1, T X2 ) {
    Point< 3, T > ret;
    ret.X[ 0 ] = X0; ret.X[ 1 ] = X1; ret.X[ 2 ] = X2;
    return ret;
}
template < typename T >
Point< 4, T > Create( T X0, T X1, T X2, T X3 ) {
    Point< 4, T > ret;
    ret.X[ 0 ] = X0; ret.X[ 1 ] = X1; ret.X[ 2 ] = X2; ret.X[ 3 ] = X3;
    return ret;
}
};
int main( void ) {
    using namespace point;
    Point< 2, double > p2d = point::Create( 12.3, 34.5 );
    Point< 3, double > p3d = point::Create( 12.3, 34.5, 56.7 );
    Point< 4, double > p4d = point::Create( 12.3, 34.5, 56.7, 78.9 );
    //Point< 3, double > p1d = point::Create( 12.3, 34.5 ); //no suitable user defined conversion exists

    //Point< 3, int > p1i = p4d.toint(); //no suitable user defined conversion exists
    Point< 2, int > p2i = p2d.toint();
    Point< 3, int > p3i = p3d.toint();
    Point< 4, int > p4i = p4d.toint();

    std::cout << p2d.str() << std::endl;
    std::cout << p3d.str() << std::endl;
    std::cout << p4d.str() << std::endl;
    std::cout << p2i.str() << std::endl;
    std::cout << p3i.str() << std::endl;
    std::cout << p4i.str() << std::endl;

    char c;
    std::cin >> c;
}  

has the new C++ standard any new improvements, language features or simplifications regarding this aspect of ctor of a template class?
what do you think about the implementation of the combination of namespace, stuct and Create function?
many thanks in advance
Oops

Answer 1

Yes, as Michael pointed out in his answer to your previous question, in C++0x you'll be able to use an initializer list to pass an arbitrary number of arguments to your ctor. In your case, the code would look something like:

template <int dims, class T>
class point { 
    T X[dims];
public:
    point(std::initializer_list<T> const &init) { 
        std::copy(init.begin(), init.begin()+dims, X);
    }
};

You could create a point object with this something like:

point<3, double> x{0.0, 0.0, 0.0};

Personally, I'm not sure I like the basic design very well though. In particular, I'd rather see X turned into an std::vector, and determine the number of dimensions strictly from the parameter list that was passed instead of having it as a template argument:

template <class T>
class point { 
    std::vector<T> X;
public:
    point(std::initializer_list<T> init) {
        std::copy(init.begin(), init.end(), std::back_inserter(X));
    }
};

This does have some trade-offs though -- points with a different number of dimensions are still the same type. For example, it basically asserts that it's reasonable to assign a 2D point to a 3D point, or vice versa.

Answer 2

Since the array is public, it is an option to omit the constructor and allow aggregate initialization (like boost::array<T, N> for example).

Point<2, int> p = {1, 2};

This is no worse than having to call a create function. (The create function might still be handy as a utility.)

---

In C++0x you will be able to have all sorts of coolness. For example, play with variadic templates, to have it checked at compile-time if the constructor is called with a right number of arguments. (The following could also check if the arguments ...U are all of type T, with some more metaprogramming fun, but it might not be absolutely necessary.)

//helper to copy variable amount of arguments into an array
namespace detail {

template <class T, class U>
void copy_variadic(T* p, U value)
{
    *p = value;
}

template <class T, class First, class ...Rest>
void copy_variadic(T* p, First var, Rest ...args)
{
    *p = var;
    copy_variadic(++p, args...);
}
} //detail

template < unsigned int dims, typename T >
struct Point {

    T X[ dims ];

    Point() : X{}
    {
    }

    template <class ...U>
    Point(U... args) {
        static_assert(sizeof...(args) == dims, "Too many or too few arguments to Point constructor");
        detail::copy_variadic(X, args...);
    }
    //...
};

(Actually, with some modifications - perfect forwarding - copy_variadic would make a nice addition to my collection of variadic-template utilities, if someone doesn't come and point out a significantly better way.)