Specialised constructor for Vector class - c++

Question

I'm trying to write a math vector class. A first version goes like :
template <typename T, unsigned int n>
struct Vector {
Vector<T, n>(T t = T()) // default
{
for (int i = 0; i < n; i++)
{
data[i] = t;
}
}
Vector<T, n>(const Vector<T, n> &aVector)
{
for (unsigned int i = 0; i < n; i++)
{
data[i] = aVector.data[i];
}
}
Vector<T, n>(const T arr[n])
{
for (unsigned int i = 0; i < n; i++)
{
data[i] = arr[i];
}
}
T& operator[](unsigned int i);
const T& operator[](unsigned int i) const;
Vector<T, n>& operator=(const Vector<T, n> &aVector);
void normalise();
T data[n];
};
I also have operators (+, *, etc.) declared outside the class, as well as a couple of other classes.
Here's the thing though. For vectors of length 2, 3, 4 I'd like to have a constructor (or a function) that takes x,y (for Vec2), x,y,z or x,y,z,w as parameters.
However, it seems that you can't make specialised constructors for this purpose. How should I proceed in that case ? Do I have to completely specialise three cases ? Wouldn't that imply that I have to rewrite chunks of code ?
I also have a similar Matrix class ( Matrix), and I'm pretty sure I'll need some constructors for rotation, translation, scaling, etc. I'm assuming I'll need to overcome a similar problem.
If you see
I also have operators (+, *, etc.) declared outside the class, as well as a couple of other functions (dot, cross, etc.).
Here's the thing though. For vectors of length 2, 3, 4 I'd like to have a constructor (or a function) that takes x,y (for Vec2), x,y,z or x,y,z,w as parameters.
However, it seems that you can't make specialised constructors for this purpose. How should I proceed in that case ? Do I have to completely specialise three cases ? Wouldn't that imply that I have to rewrite chunks of code ?
I also have a similar Matrix class ( Matrix), and I'm pretty sure I'll need some constructors for rotation, translation, scaling, etc. I'm assuming I'll need to overcome a similar problem.
If you see anything in the code that seems wrong to you, feel free to point it out by the way.
EDIT : In case I was not clear enough, the arrays are meant to be one-dimensional, and all of its components are of the same type. The specialisations are for arrays with 2, 3 and 4 elements.

Answer 1

You may utilize a variadic template:
#include <chrono>
#include <future>
#include <iostream>
#include <stdexcept>
template<typename T, unsigned int n>
struct Vector
{
// Note: We need x and y:
// The default (in the question) is a constructor taking a single argument.
template <typename ... Args>
Vector(T x, T y, Args ... args)
{
static_assert(sizeof ... (Args) == n - 2, "Invalid number of arguments");
auto initializer_list = { x, y, std::forward<Args>(args)... };
std::copy(initializer_list.begin(), initializer_list.end(), data);
}
T data[n];
};
template<typename T, unsigned int n>
void print(const Vector<T, n>& v) {
for(unsigned i = 0; i < n; ++i)
std::cout << v.data[i] << ' ';
std::cout << '\n';
}
int main()
{
Vector<int, 2> v2(1, 2);
Vector<int, 3> v3(1, 2, 3);
Vector<int, 4> v4(1, 2, 3, 4);
print(v2);
print(v3);
print(v4);
// Invalid number of arguments
// Vector<int, 3> e2(1, 2);
// Invalid number of arguments
// Vector<int, 3> e4(1, 2, 3, 4);
return 0;
}

Answer 2

Personally, I would declare different classes for each vector. Vec2<T1, T2>, Vec3<T1, T2, T3>, and Vec4<T1, T2, T3, T4>. Make each Vec* class publicly inherit a un-templated base class Vector, and then make a static function create in Vector with an overload for each Vec* type that will be responsible for creating them. Pseudo-example for Vec3:
template<T1, T2, T3> static std::shared_ptr<Vector> create(T1 a1, T2 a2, T3 a3)
{
return new Vec3<T1, T2, T3>(a1, a2, a3);
}
auto vec = Vector::create<int, int, int>(1, 2, 3);

Answer 3

variadic template and SFINAE may solve that.
but I think the simpler would be to inherit from (specialized) helper
template <typename T, unsigned int n>
struct VectorHelper<T, n>
{
T data[n];
};
template <typename T>
struct VectorHelper<T, 2>
{
VectorHelper(T x, T y) : {data[0] = x; data[1] = y;}
T data[2];
};
template <typename T>
struct VectorHelper<T, 3>
{
VectorHelper(T x, T y, T z) : {data[0] = x; data[1] = y; data[2] = z}
T data[3];
};
template <typename T, unsigned int n>
struct Vector : private VectorHelper<T, n>
{
using VectorHelper<T, n>::VectorHelper; // use base constructor(s)
// your previous implementation without `data` (as member of base)
};