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    <p>This is a result of <a href="http://en.wikipedia.org/wiki/Short-circuit_evaluation" rel="nofollow noreferrer">short-circuit evaluation</a>.</p> <p>The expression <code>++x</code> evaluates to <code>2</code>, and the compiler knows that <code>2 || anything</code> always evaluates to <code>1</code> ("true") no matter what <code>anything</code> is. Therefore it does not proceed to evaluate <code>anything</code> and the values of <code>y</code> and <code>z</code> do not change.</p> <p>If you try with </p> <pre><code>x=-1; y=z=1; </code></pre> <p>You will see that <code>y</code> and <code>z</code> <em>will</em> be incremented, because the compiler has to evaluate the right hand side of the OR to determine the result of the expression.</p> <p><strong>Edit:</strong> asaerl answered your follow-up question in the comments first so I 'll just expand on his correct answer a little.</p> <p>Operator precedence determines how the parts that make up an expression bind together. Because AND has higher precedence than OR, the compiler knows that you wrote</p> <pre><code>++x || (++y &amp;&amp; ++z) </code></pre> <p>instead of</p> <pre><code>(++x || ++y) &amp;&amp; ++z </code></pre> <p>This leaves it tasked to do an OR between <code>++x</code> and <code>++y &amp;&amp; ++z</code>. At this point it would normally be free to select if it would "prefer" to evaluate one or the other expression first -- as per the standard -- and you would not normally be able to depend on the specific order. This order has nothing to do with operator precedence.</p> <p>However, specifically for <code>||</code> and <code>&amp;&amp;</code> the standard demands that evaluation will <em>always</em> proceed from left to right so that short-circuiting can work and developers can <em>depend</em> on the rhs expression not being evaluated if the result of evaluating the lhs tells.</p>
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    1. COThanks that helped, but doesn't the order of precedence come into play here? Meaning the ++y && ++z should be evaluated first, before the || ?
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    2. CONo. precedence is order in parsing, not order of evaluating. BTW, if `++z` was been evaluated, it was UB. (changing `z` twice)
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