Convert class to fixed-length float array - c++

Question

How can I convert the following class to a fixed-length float array?
class Vertex
{
public:
Vertex( float x = 0,
float y = 0,
float z = 0)
: x(x), y(y), z(z) {}
float x, y, z;
};
For example, I would like to use it like so:
Vertex v(0, 1, 0);
float arr[3] = v; // How to convert here?
Thanks!
Edit:
I should have added some background information before posting this question.
The reason why I'm using C-style arrays is because I want to combine the high level vertex objects into a vertex array for rendering with OpenGL, which as far as I know requires a collection of raw arrays (float[3]) or structs.
For that purpose I think user2079303's answer is the best option. However, if a more elegant solution exists that would be even better. :)

Answer 1

#include <iostream>
#include <array>
using namespace std;
class Vertex
{
public:
Vertex( float x = 0,
float y = 0,
float z = 0)
: x(x), y(y), z(z) {}
operator array<float, 3>() const {
return {x,y,z};
}
/* See #user2079303's answer for a compile-time check of the array dimension */
void fillArray(float arr[3]) {
arr[0] = x;
arr[1] = y;
arr[2] = z;
}
float x, y, z;
};
int main() {
Vertex v(1,1.4,2);
array<float, 3> arr = v;
float arr2[3];
v.fillArray(arr2);
for (int i = 0; i < 3; i++) {
cout << arr[i] << " " << arr2[i] << endl;
}
return 0;
}
Live Demo
std::array is as efficient as using a C-style array, no performance is lost. You can also use std::vector instead.
You can't just return and copy an array, even in C. That's why if you absolutely want to use a C array, you have to have a function like fillArray.

Answer 2

A class cannot be convertible to a (raw) array, because the cast operator would have to return an array, which is not allowed in c++. Furthermore, arrays cannot be copy-initialized anyway.
What you can do is define an array, and pass it to a function that populates the array according to the contents of an object:
void Vertex::fill_arr(float (&arr)[3]) {
arr[0] = x;
arr[1] = y;
arr[2] = z;
}
// usage
Vertex v(1, 2, 3);
float arr[3];
v.fill_arr(arr);
Another option is to use std::array which can be returned and copy-initialized normally.

Answer 3

You have many options, and it which you choose depends a lot on context. Here are four different ways to "convert" your vertex:
class Vertex
{
public:
Vertex(float x = 0,
float y = 0,
float z = 0)
: x(x), y(y), z(z) {}
operator array<float, 3> () const {
return {x, y, z};
}
array<float, 3> array_copy() const {
return {x, y, z};
}
unique_ptr<float[]> c_array_copy() const {
return unique_ptr<float[]>(new float[3]{ x, y, z });
}
void copy_into(float in[3]) const {
in[0] = x;
in[1] = y;
in[2] = z;
}
float x, y, z;
};
First, you can just cast the class using the () operator:
cout << "Direct access" << endl;
auto as_array = (array<float, 3>)vertex;
cout << as_array[0] << as_array[1] << as_array[2] << endl;
Second, you can let copy semantics to some work for you with array_copy():
cout << "Get a copy" << endl;
auto as_copy = vertex.array_copy();
cout << as_copy[0] << as_copy[1] << as_copy[2] << endl;
Third, you can get a more c-style array copy with a unique pointer to a dynamically allocated array:
cout << "Get a c-style copy" << endl;
auto as_c_copy = vertex.c_array_copy();
cout << as_c_copy[0] << as_c_copy[1] << as_c_copy[2] << endl;
Finally, you can copy into an out-parameter:
cout << "Copy onto an out-parameter" << endl;
float copied[3];
vertex.copy_into(copied);
cout << copied[0] << copied[1] << copied[2] << endl;
As I say, which you choose really depends on context and performance requirements.
Note that in no case can you just return a c-style array.

Answer 4

You can define a conversion operator to construct your array. Also I would suggest using a std::array instead of a raw array.
#include <array>
class Vertex
{
public:
Vertex(float x = 0.0f, float y = 0.0f, float z = 0.0f)
: x(x), y(y), z(z)
{}
float x;
float y;
float z;
operator const std::array<float, 3>() const
{
return {x, y, z};
}
};
int main()
{
Vertex v(0.0f, 1.0f, 0.0f);
std::array<float, 3> arr = v;
}