Question

Is it possible to rename the variables of a struct when using typedef?

typedef struct {
    float x;
    float y;
    float z;
} Vector;

typedef Vector {
    float r;
    float g;
    float b;
} Color;

Answer 1

Presumably you mean some sort of typedef that would let you say: “a Color is the same as a Vector, but with the names x, y and z changed to r, g and b, respectively”?

If so, the answer is no, C doesn’t provide that (nor can I think of anything it provides that would be a reasonably close substitute).

Edit: you can, however, have a function operate on either one pretty easily:

typedef union { 
    Color c;
    Vector v;
} cv;

int my_func(cv *value);

C guarantees that the initial sequence of items with the same types in the same order (which, in this case is all of them) can be worked with via either selector equally. In other words, if you put a Vector into your cv and pass it to a function, the function can work with those three items as either r, g and b or as x, y and z without any problem (e.g., if it writes to g, that will change y, if you view it as a Vector instead of a Color).

Edit: C99, §6.5.2.3/5:

One special guarantee is made in order to simplify the use of unions: if a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the complete type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.

In this case, all the members of c and v have the same types in the same order, so this rule applies to the entirety of either member. Rereading things, however, this only allows you to “inspect” the common initial sequence, which probably doesn’t include writing to it (though I find it hard to imagine how you’d make reading from any member work without making writing work as well).