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POSo why is i = ++i + 1 well-defined in C++11?
 

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  1. POSo why is i = ++i + 1 well-defined in C++11?
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    <p>I've seen the <a href="https://stackoverflow.com/q/1860461/150634">other</a> similar <a href="https://stackoverflow.com/q/4336860/150634">questions</a> and read the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#637" rel="nofollow noreferrer">defect</a> about it. But I still don't get it. Why is <code>i = ++i + 1</code> well-defined in C++11 when <code>i = i++ + 1</code> is not? How does the standard make this well defined?</p> <p>By my working out, I have the following <em>sequenced before</em> graph (where an arrow represents the sequenced before relationship and everything is a value computation unless otherwise specified):</p> <pre><code>i = ++i + 1 ^ | assignment (side effect on i) ^ ^ | | ☆i ++i + 1 || ^ i+=1 | ^ 1 | ★assignment (side effect on i) ^ ^ | | i 1 </code></pre> <p>I've marked a side effect on <code>i</code> with a black star and value computation of <code>i</code> with a white star. These appear to be unsequenced with respect to each other (according to my logic). And the standard says:</p> <blockquote> <p>If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.</p> </blockquote> <p>The explanation in the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#637" rel="nofollow noreferrer">defect report</a> didn't help me understand. What does the lvalue-to-rvalue conversion have to do with anything? What have I gotten wrong?</p>
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    1. USJoseph Mansfield
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    1. COYour value computation of "i" points to "ass". What is being value computed there? Not sure I understand that graph.
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      1. USJohannes Schaub - litb
    2. CO@JohannesSchaub-litb The left operand of the assignment. From the statement: "In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression."
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      1. USJoseph Mansfield
    3. CO@JohannesSchaub-litb Also, the only reason it points at "ass" is because doing diagonal lines in ASCII diagrams is a bitch. :D Imagine that they are branching out from the middle.
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