Comment by binaryturtle | Hacker News Reader

Comment by binaryturtle | Hacker News Reader

Parent Full thread

It's defined. And called "operator precedence", both post/pre-increment have a higher precedence than the single "+".

At least according to this: https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Exp...

I think the main confusion here comes from the fact that "a" is just a value, not a pointer, where it matters when the value/address which the pointer points at is accessed (before of after the increment of the pointer's own 'value').

Anyway… my C skills are rusty. Maybe I get it wrong. :) In any case I always would use brackets to avoid any ambiguity in constructs like this.

Nope. Order of evaluation and operator precedence are completely unrelated. They should have been defined to be the same, but instead order of evaluation was left undefined. So if you write ++a + a++, operator precedence means this will be interpreted as (++a) + (a++), not say ++(a + a)++, but it is up to the compiler whether to execute ++a or a++ first, rather than executing them left to right.

Sometimes it helps to test. Which I just did. :-)

Actually the compiler (at least clang) warns about this:

    $ gcc -W -Wall test.c -o test
    test.c:8:7: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
            a = a++ + ++a;
                 ^    ~~
    1 warning generated.

The undefined behaviour stems from the fact that "a" is modified multiple times between the "sequence points" (so it's irrelevant to the actual problem that this happens with ++, --, pre-, or, post-, or in which order) We only can modify the variable safely once on the right side without entering bizarro world.

A construct like this certainly can be confusing.

Reply on news.ycombinator.com