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    <p>This is pure nasty. First we have to look at the declaration of swap:</p> <pre><code>template&lt;class T&gt; void swap(T &amp;left, T &amp;right); </code></pre> <p>Now, <code>operator[]()</code> on <code>bitset</code> has two overloads:</p> <pre><code>bool operator[](size_type _Pos) const; reference operator[](size_type _Pos); </code></pre> <p>Here <code>reference</code> is <code>bitset::reference</code>, a nested class in <code>bitset</code> that effectively acts as a proxy reference to the one of the underlying bits. What it encapsulates is the <code>bitset</code> and a position in the <code>bitset</code>. Because of the declaration of <code>swap</code>, the second overload is chosen and we are swapping two <code>bitset::reference</code>s. Now here's where it gets nasty. Let's look at a typical implementation of swap:</p> <pre><code>template class&lt;T&gt; swap(T &amp;left, T &amp;right) { T temp = left; left = right; right = temp; } </code></pre> <p>The problem is that <code>left</code> and <code>right</code> are both references to a <code>bitset::reference</code>. They have the same underlying data (because they are proxies; same meaning the both point to the same <code>bitset</code>!) they just encapsulate different positions in that <code>bitset</code>. Thus, think of it like this <code>left</code> is position 0 in some <code>bitset</code> and <code>right</code> is position 1 in some <code>bitset</code> and that <code>bitset</code> is the same <code>bitset</code> as <code>left</code>! Let's forever refer to this <code>bitset</code> as <code>BS</code> (chosen intentionally).</p> <p>So,</p> <pre><code>T temp = left; </code></pre> <p>says that <code>temp</code> is position 0 in <code>BS</code>.</p> <pre><code>left = right; </code></pre> <p>sets position 0 in left to position 1 in <code>BS</code> (which simultaneously changes position 0 in <code>temp</code>!)</p> <pre><code>right = temp; </code></pre> <p>sets position 1 in right to position 0 in <code>BS</code> (which was just set to position 1 in <code>BS</code>!). So at the end of this mess have that position 0 is whatever position 1 was and position 1 is unchanged! Now, because position 0 is the LSB and position 1 is the MSB we have that "10" becomes "11". Ugly.</p> <p>You can get around this with a <a href="http://en.wikipedia.org/wiki/Template_specialization" rel="noreferrer">template specialization</a>:</p> <pre><code>namespace std { template&lt;&gt; void swap&lt;bitset&lt;2&gt;::reference&gt;( bitset&lt;2&gt;::reference &amp;left, bitset&lt;2&gt;::reference &amp;right ) { bool temp = (bool)left; left = (bool)right; right = (bool)temp; } } </code></pre> <p>Then:</p> <pre><code>int main() { bitset&lt;2&gt; test(string("10")); cout &lt;&lt; test; // Prints "10" swap(test[0], test[1]); cout &lt;&lt; test; // Prints "01", hallelujah! } </code></pre>
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    1. POWhy the std::swap of Bits in a std::bitset instance doesn't work?
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    1. USCassio Neri
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    1. POWhy the std::swap of Bits in a std::bitset instance doesn't work?
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    1. CONice work! Incredibly not even C++0x mentions `swap` at all in relation to bitsets.
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