4 floats to determine a plane?

Question

I am taking up a programming and asked to create a function for a certain problem. I was given this struct for a plane. However I can't make sense of this struct. How can 4 floats determine a plane in 3d space?

// plane in 3d space
struct Plane
{
    float A;
    float B;
    float C;
    float D;
};

Answer 1

It's quite possible to use 4 floats to define a plane, as David Peterson's comment shows. But, without some documentation, it's impossible to say what plane the four floats define. So, for example, the four floats $A$, $B$, $C$, $D$ could define the plane $$ Ax + By + Cz + D = 0 $$ or the plane $$ Ax + By + Cz = D $$ or the plane $$ Dx + Cy + Bz = A $$ or any number of other planes.

In general, a data structure doesn't mean anything, by itself -- you need some semantic information.