[binaryturtle]

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.

[pavon]

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.

[binaryturtle]

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.