This is fine with my compiler (Output is 3.000000), but I'm unsureCode:#include <stdio.h> typedef struct { double x, y, z; }vector; double *vector_iterator(vector *v, int axis) { switch (axis) { case 0: return (&v->x); case 1: return (&v->y); case 2: return (&v->z); } } int main(void) { vector v; v.x = 5; v.y = 3; v.z = 2; *(vector_iterator(&v, 0)) = 3; /* Note this */ printf("%f", v.x); return 0; }
about function call on the left side of = operator (lvalue ?)
Apart from this
If you did fall off the end of the function, you'd get a garbage pointer and a whole heap of trouble when you dereference it.Code:$ gcc -W -Wall -ansi -pedantic -O2 foo.c foo.c: In function `vector_iterator': foo.c:18: warning: control reaches end of non-void function
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
I understand what you are saying. I actually got this warning in many other programs when there was no return value specified at the end of the function. What is the solution or can I ignore this warning ?Originally Posted by Salem
Return a value like you're supposed to. Try NULL perhaps.
trying to store a value at a NULL address may result in undefined behaviour. I think before the assignment takes place using an assert statement may be helpful :
assert(vector_iterator(&v, axis) != NULL);
Of course it's undefined. That is the point. You're supposed to check if the pointer returned is NULL.
Asserting is another option, of course, but that seems a little heavy to stop the program dead in its tracks for this.
I don't know, I need to do some mathematical calculations in my project and I'm just debugging right now so that's why I'm using assertions.
>trying to store a value at a NULL address may result in undefined behaviour
You should know that, the control will never reach that stage unless the precondition within the function are not satisfied. Perhaps you have to include some error checking to make sure the return value is NULL. The logic remains perfect.
ssharish
Something like this
In debug, we'll drop into the debugger for analysis of the problem.Code:double *vector_iterator(vector *v, int axis) { switch (axis) { case 0: return (&v->x); case 1: return (&v->y); case 2: return (&v->z); } assert(0); // how did we pass the switch? return NULL; }
In release, at least it will fault in a predictable manner, rather than perhaps stumbling on for a while only to crash in some totally random place.
Or you could be brutal and call exit() instead of returning.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
One more "solution" would be to have "static double junk;" in function, and return &junk after switch... But, in rare cases is this satisfactory...
