Uses of C comma operator [duplicate]

Answer 1

I often use it to run a static initializer function in some cpp files, to avoid lazy initalization problems with classic singletons:

void* s_static_pointer = 0;

void init() {
    configureLib(); 
    s_static_pointer = calculateFancyStuff(x,y,z);
    regptr(s_static_pointer);
}

bool s_init = init(), true; // just run init() before anything else

Foo::Foo() {
  s_static_pointer->doStuff(); // works properly
}

Answer 2

I think generally C's comma is not a good style to use simply because it's so very very easy to miss - either by someone else trying to read/understand/fix your code, or you yourself a month down the line. Outside of variable declarations and for loops, of course, where it is idiomatic.

You can use it, for example, to pack multiple statements into a ternary operator (?:), ala:

int x = some_bool ? printf("WTF"), 5 : fprintf(stderr, "No, really, WTF"), 117;

but my gods, why?!? (I've seen it used in this way in real code, but don't have access to it to show unfortunately)