3D顶点类或结构

Question

I am writing a small program for learning C++ and 3D.

I have already written a vertex class with usefull methods. (like Dot,Cross, etc...)

class cVector {
    ...
    float x, y, z;
    ...
    float dot(cVector& v);      
    cVector cross(cVector& v);
    ...
}

Now I realize OpenGL expects buffers where elements are more like a struct (VBO).

struct sVector {
    float x, y, z;
}

So my vertex class is no longer useless, because if i want to manipulate data in the buffer :
1 - I need to extract data of elements in the buffer.
2 - Create a temporary instance of vertex class with the data.
3 - Use vertex class method. (Dot, cross, etc...)
4 - Put the data back to the buffer.
It's not very efficient :(.

I wonder if I should not use a struct to organize my vectors and create global functions that take a pointer to a struct as an argument. I could handle data buffers more efficiently (just moving pointer) but I feel i would lose the "convenient power" of C++.

In every 3D C++ source code i ever see, all use class for vertex but i dont understand how they can manipulate large amount of vertex in a "struct like" buffer.

Can you help me to understand ? What is the best approach ?

Answer 1

The most common approach in a language like C++ is actually neither of these things.

You are more likely to encounter the following:

struct Vector3 {
  union {
    struct {
      float x,y,z;
    };
    float v [3];
  };

  ...

  Vector3 (float x_, float y_, float z_) : x (x_), y (y_), z (z_) { };

  float Norm      (void) { return sqrt ((x * x) + (y * y) + (z * z)); }
  void  Normalize (void) {
    float norm = Norm ();

    v [0] /= norm;
    v [1] /= norm;
    v [2] /= norm;
  }
};

The reason for this is because using anonymous unions and structs, you can treat the data as either an array of floats ( v [...] ) or reference the individual components by their name ( x , y , z ) without a lot of muss or fuss. You get the best of both worlds by using the language more intelligently.

As for the difference between a struct and a class in this particular case, there is none from the perspective of memory representation. The only real difference between a class and a struct in C++ is the default access; struct has public access by default.

When GL needs to access the object's internal memory, you would accomplish this by passing it the pointer: Vector3::v or the individual components, depending on the particular function.

For instance:

Vector3 vec   (1.0f, 2.0f, 3.0f);
---------------------------------

glVertex3fv   (vec.v);

    and

glVertex3f    (vec.x, vec.y, vec.z);

    are equivalent

---

On a side-note, anonymous structures are a non-standard extension to C++ but supported virtually everywhere. In the case that you have a compiler that does not support them, you may have to qualify access to x , y , and z by giving the struct a name.

struct Vector3 {
  union {
    struct {
      float x,y,z;
    } s;
    float v [3];
  };
};

If you write your struct this way, then:

Vector3 vec;
assert (vec.v [0] == vec.s.x);

It is messier to have to qualify x that way (using an anonymous struct you can use vec.x ).

Answer 2

There is exactly one difference between struct and class : For class the default scope is private , while for struct it is public .

So

class cVector {
    ...
    float x, y, z; // data
    ...
    float dot(cVector& v); // just a function
    cVector cross(cVector& v); // just a function
    ...
}

and

struct sVector {
   float x, y, z; // data
}

have exactly the same memory layout (given that x,y,z are the only members variables of cVector ).

You can use &v.x to get a pointer to (x,y,z) for OpenGL, eg glVertex3f(&v.x); .

You can even do the following to get a pointer to a continuous sequence of vertices for usage with OpenGL:

std::vector<cVector> vertices(100);
const float* data = &(vertices[0].x);