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  <title>Undefined Behavior in C++</title>
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<h1 class="title">Undefined Behavior in C++</h1>
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<h2 id="type-punning">Type Punning</h2>
<p>In a systems language like C++ you often want to interpret a value of type A as a value of type B where A and B are completely unrelated types. This is called <em>type punning</em>.</p>
<p>Take for example the ever popular <a href="https://en.wikipedia.org/wiki/Fast_inverse_square_root">Fast Inverse Square Root</a>. The Wikipedia gives us the following code.</p>
<pre><code>float Q_rsqrt( float number )
{
    long i;
    float x2, y;
    const float threehalfs = 1.5F;

    x2 = number * 0.5F;
    y  = number;
    i  = * ( long * ) &amp;y;                       // evil floating point bit level hacking
    i  = 0x5f3759df - ( i &gt;&gt; 1 );               // what the fuck?
    y  = * ( float * ) &amp;i;
    y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
//  y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

    return y;
}</code></pre>
<p>We interpret a float value as integer in the “evil floating point bit level hacking” line. Similar type punning often happens when we want to interpret a stream of bytes as some structure. We try to simply cast the <code>char*</code> input stream to our structure and use the member elements to read values.</p>
<p>Another common way to do this is via a union.</p>
<pre><code>union U {
    long i;
    float f;
};

U u;
u.f = number;
long number_as_int = u.i;</code></pre>
<p>Unfortunately neither are valid C++.</p>
<p>Casting is invalid because of C++’s <a href="http://en.cppreference.com/w/cpp/language/reinterpret_cast">strict aliasing rules</a>. Basically, you mustn’t cast a pointer to a different type and then dereference it (unless you cast to <code>char*</code>).</p>
<p>The union trick is also <em>not valid</em>, because only one member of a union can be “active”. When we set <code>f</code> it becomes active and <code>i</code> is thus inactive. Reading from an inactive member results in undefined behavior. At least that’s how I understand the standard. <a href="https://stackoverflow.com/questions/25664848/unions-and-type-punning">The union trick in valid in modern C99 (but not in C89).</a></p>
<p>Instead of this you should use <code>memcpy</code> and hope that your compiler knows how to optimize it.</p>
<pre><code>memcpy(&amp;i, &amp;y, sizeof(long));</code></pre>
<p>See also:</p>
<ul>
<li><a href="http://blog.regehr.org/archives/959">John Regehr on type punning</a></li>
</ul>
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