Question

I am trying to write some code to optimize some Open GL functions for a program I’m writing, unfortunately, I am not exactly a C or C++ veteran, but that’s partially why I’m doing this project!

So I’m creating a struct to handle 3×3 matrices and I am defining the struct as follows:

#ifndef MATRIX3BY3_H
#define MATRIX3BY3_H
struct Matrix3by3
{
    float ix, jx, kx;
    float iy, jy, ky;
    float iz, jz, kz;

    Matrix3by3() {}

    Matrix3by3(const Matrix3by3 &matrix)
    {
        ix = matrix.ix;
        jx = matrix.jx;
        kx = matrix.kx;

        iy = matrix.iy;
        jy = matrix.jy;
        ky = matrix.ky;

        iz = matrix.iz;
        jz = matrix.jz;
        kz = matrix.kz;
    }

    Matrix3by3 (const float _ix, const float _jx, const float _kx,
                const float _iy, const float _jy, const float _ky,
                const float _iz, const float _jz, const float _kz) :
        ix(_ix), jx(_jx), kx(_kx),
        iy(_iy), jy(_jy), ky(_ky),
        iy(_iz), jx(_jz), kz(_kz) {}
    }; 
#endif

And I get the error (twice)

Expected specifier-qualifier-list
before ‘Matrix3by3’

On the line of the first constructor. I have tried to look around for answers for this, and it seems that it has to do with the compiler not knowing that this is a type. So I have tried the following, I’ll remove the innards for brevity:

typedef struct Matrix3by3 { ... };

---

struct Matrix3by3 { struct Matrix3by3() {} ... };

---

struct Matrix3by3 { ... } Matrix3by3;

---

typdef struct Matrix3by3;
struct Matrix3by3 { ... };

Which are all solutions that were suggested on blogs and articles that I saw for this error. I also saw that it may arise because of a circular dependency, but this file has no includes that include anything else, and I’ve even removed them just to be certain from time to time – no change.

I could write this in a objective-c class, I’m sure, but it will probably take a tiny bit more memory and cycles, and that’s exactly what I’m trying to avoid. The only thing I can think of left is some compiler/project setting that I have set by default that precludes my using this type of structure. Entirely possible, as I’m learning the language/environment.

Can any one provide some help?

Thanks!

Answer 1

C does not support constructors or member functions of structs. There is no way you will get this to compile as C or Objective-C. You need to compile this as C++ or Objective-C++, at which point it will almost compile: you have an error in your 3rd constructor, in that you’re attempting to initialize the members iy and jx multiple times. Once you fix those typos, it compiles just fine.