Comment by paulftw | Hacker News Reader

Comment by paulftw | Hacker News Reader

While I get the overall message I'm not sure last question is that unpredictable: I++ + ++I If postfix is executed first it returns 0 but prefix returns 2 and the result is 2. If prefix is executed first it returns 1 and then postfix also returns 1, result of the addition is again 2. What else is ambiguous about this snippet? I can't think of a parse tree that'd evaluate addition before any of the increments - that'd be a syntax error. If the code were (++i + i++) there could be alternative interpretations, but again ++ requires lvalue...

The order of evaluation in such an expression is entirely unspecified. There are points in C programs that separate a "before" and an "after". The semicolon is a typical such point. There are others. Now between two adjacent points anything goes.

In this particular example the evaluations and effects are allowed to be interleaved:

    pre    = I + 1;      // evaluating the pre-increment
    post   = I;          // evaluating the post-increment
    I++;                 // effect of pre-increment
    I++;                 // effect of post-increment
    return pre + post;   // evaluating the sum

Simply put, pre and post-increment are allowed to occur "simultaneously".

what you are saying may be correct, but you are "overloading" the term "order of evaluation" to encompass atomicity (atomicity of operations like i++); is that kosher?

Indeed I am. What I want to convey is, between 2 sequence points, C is actually non-strict. Any effect is like unsafeInterleaveIO from Haskell. Which is why I rarely mix up effects together or with a computation in a single instruction.

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