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

Question

in this question:
template; Point<2, double>; Point<3, 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

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.)

Answer 2

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.