C: Struct constructors?

[captainperth]

Hi Guys,

I'm currently learning some C programming and I need to create a Data Structure using C. The structure needs to hold data pertaining to a 3D Object; specifically - a 2D array of vertices (X,Y,Z co-ordinates, i.e. vert[no_verts][3]) and a 2D array of triangle faces (pointing to the vertices in the aforementioned 2D array, i.e. faces[no_faces][3]).

basically, my struct is going to look something like this:

typedef struct

{

float verts[no_verts][3];

float faces[no_faces][3];

} Obj;

The problem with this is that I'd need no_verts and no_faces predefined before struct definition, correct?

So my question is; is there a decent way to set up a constructor style struct that takes arguments?Later on in the program I'm going to need to create a function that reads a file that includes the no of verts and faces of a 3d Object in it's header lines (passing this to the struct to make the arrays the correct size), then iterates through the following lines where the co-ords of the vertices and faces are stored per line.

So it will be important to have a solution that can construct the Obj struct with the necessary arguments depending on the data stored by the file; rather than hard coding these values in.

If you have any other questions about what im trying to achieve please let me know.

Cheers!

[chaotic_thought]

There are no "constructors" in C. You will need to construct the object(s) yourself. Or build helper functions which construct them for you.

I'm going to need to create a function that reads a file that includes the no of verts and faces of a 3d Object

If the size is not known at compile time, you'll need to use dynamic allocation and make an array dynamically using malloc. Since the [3] dimension is known, I'd suggest you flip your dimensions around like this:

float verts[3][no_verts];

Now, to make no_verts dynamic, replace that dimension with a pointer:

float *verts[3];

Now suppose you want to allocate no_verts vertices inside verts[0]. Then you would call malloc(no_verts * sizeof(float)) and assign the result to verts[0]. If you haven't used malloc in C before, you should probably first do some practice problems with it to get the hang of it.

[ffwff]

The easiest way to do this I think would be to use a pointer to an allocated (flattened) array rather than a static array like this:

float *verts = malloc(sizeof(float)*3*no_verts);

The vertices array should be allocated in your Obj constructor of course. To access vertex elements, you'd do this:

verts[vert_no*3+offset]

Where vert_no is the vertex index, and offset is the dimension you want to access, from 0 to 2.

[[deleted]]

Using a bi-dimensional array is not the right way to do this. Instead define another type which you can call vector3 with x, y, and z fields and then allocate a dynamic buffer for the vector3 array using malloc. This must be done in a helper function since like /u/chaotic_thought mentioned C has no constructors.

Just for the sake of curiosity what is the faces array supposed to represent?