Unsequenced modification and access to parameter

Question

I'm using open source project (NSBKeyframeAnimation) for some of animations in my p roject. Here are example of methods that i'm using:

double NSBKeyframeAnimationFunctionEaseInQuad(double t,double b, double c, double d)
{
    return c*(t/=d)*t + b;
}

I have updated my Xcode to 5.0, and every method from this project started to show me warnings like this: "Unsequenced modification and access to 't' ". Should i rewrite all methods to objective-c or there's another approach to get rid of all these warnings?

Answer 1

The behavior of the expression c*(t/=d)*t + b is undefined, and you should fix it, e.g. to

t /= d;
return c*t*t + b;

See for example Undefined behavior and sequence points for a detailed explanation.

Answer 2

those warnings can be disabled

put this before the code triggering the warning

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunsequenced"

and this after that code

#pragma clang diagnostic pop

however, nothing guarantees that compilers will always handle this case gracefully. i came to this page because i got 50 of those warnings, in exactly the same source file.

i'm grateful for those functions, but the programmer should realize that trying to write everything on one line is very "1980's", when the compilers weren't nearly as optimized as today.

and when it actually matter to win a few processor cycles, we only had a few million, not the billions we have now.

i would always put readability first.