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copied!<p>It comes from the history of using integer values as booleans.</p> <p>If <code>x</code> is an <code>int</code>, but I am using it as a boolean as per <code>if(x)...</code> then incrementing will mean that whatever its truth value before the operation, it will have a truth-value of <code>true</code> after it (barring overflow).</p> <p>However, it's impossible to predict the result of <code>--</code> given knowledge only of the truth value of <code>x</code>, as it could result in <code>false</code> (if the integral value is 1) or <code>true</code> (if the integral value is anything else - notably this includes 0 [<code>false</code>] and 2 or more [<code>true</code>]).</p> <p>So as a short-hand <code>++</code> worked, and <code>--</code> didn't.</p> <p><code>++</code> is allowed on bools for compatibility with this, but its use is deprecated in the standard.</p> <hr> <p>This assumes that I <strong>only</strong> use <code>x</code> as an boolean, meaning that overflow can't happen until I've done <code>++</code> often enough to cause an overflow on it's own. Even with char as the type used and <code>CHAR_BITS</code> something low like 5, that's 32 times before this doesn't work any more (that's still argument enough for it being a bad practice, I'm not defending the practice, just explaining why it works) for a 32-bit <code>int</code> we of course would have to use <code>++</code> 2^32 times before this is an issue. With <code>--</code> though it will only result in <code>false</code> if I started with a value of 1 for <code>true</code>, or started with 0 and used <code>++</code> precisely once before.</p> <p>This is different if we start with a value that is just a few below 0. Indeed, in such a case we might want <code>++</code> to result in the <code>false</code> value eventually such as in:</p> <pre><code>int x = -5; while(++x) doSomething(x); </code></pre> <p>However, this example treats <code>x</code> as an <code>int</code> everywhere except the conditional, so it's equivalent to:</p> <pre><code>int x = -5; while(++x != 0) doSomething(x); </code></pre> <p>Which is different to only using <code>x</code> as a boolean.</p>

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