Alex Rivera | Logout

C++ member variable aliases?

Asked 2009-01-30T05:50:22.543
16

I'm pretty sure this is possible, because I'm pretty sure I've seen it done. I think it is awesome, but I will gladly accept answers along the lines of "this is a terrible idea because ____".

Say we have a basic struct.

struct vertex
{
    float x, y, z;
};

Now, I want to implement aliases on these variables.

vertex pos;
vertex col;
vertex arr;

pos.x = 0.0f; pos.y = 0.5f; pos.z = 1.0f;
col.r = 0.0f; col.g = 0.5f; col.b = 1.0f;
arr[0] = 0.0f; arr[1] = 0.5f; arr[2] = 1.0f;

Ideally the third syntax would be indistinguishable from an array. That is, if I sent arr as a reference parameter to a function expecting an array of floats into which it will store data (eg many of the OpenGL glGet functions), it would work fine.

What do you think? Possible? Possible but stupid?

c++ struct variables alias member
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2 Answers

25

What I would do is make accessors:

struct Vertex {
    float& r() { return values[0]; }
    float& g() { return values[1]; }
    float& b() { return values[2]; }

    float& x() { return values[0]; }
    float& y() { return values[1]; }
    float& z() { return values[2]; }

    float  operator [] (unsigned i) const { return this->values_[i]; }
    float& operator [] (unsigned i)       { return this->values_[i]; }
    operator float*() const { return this->values_; }

private:
    float[3] values_;
}
answered 2009-01-30T06:12:45.947
3

You can get this with a union as others have mentioned. Overloading color and position onto the same structure like this may not be a good idea ( for example, adding two colors usually means you want to saturate to 1.0, whereas adding vectors happens linearly ), but overlaying a float[] on top of them like that is perfectly fine and a well accepted means of interchanging data with GL/DirectX/etc.

I recommend you avoid referring to the same member by different aliases in the same function scope, though, because this will drive you into a nasty hardware stall called a load-hit-store. In particular, avoid this if you can:

vector foo; 
foo.x = 1.0f;
return foo[0] + foo[1];
answered 2009-01-30T06:05:20.733

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