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    <p>Direct memory reinterpretation, by definition, means working with lvalues. The most straightforward approach would be to do it though a cast to reference type</p> <pre><code>double d = reinterpret_cast&lt;double &amp;&gt;(voidp); </code></pre> <p>You can also do it through a pointer cast, as other answers suggested, although it "overloads" the procedure with a number of completely unnecessary operator applications. Both approaches are equivalent, since by definition <code>reinterpret_cast</code> to reference type <code>reinterpret_cast&lt;T &amp;&gt;(v)</code> is equivalent to the pointer version <code>*reinterpret_cast&lt;T *&gt;(&amp;v)</code>.</p> <p>However, the above approaches suffer from type-punning issues. Formally, doing this is simply illegal. You are not allowed to read <code>void *</code> objects as <code>double</code> objects in C++. Direct memory reinterpretation exists in C++ for re-interpreting objects as arrays of <code>char</code>s, not for arbitrary type-punning like the above. Even if we ignore the formal issue and stick to purely "practical" considerations, trying to directly reinterpret a <code>void *</code> value as <code>double</code> value might produce completely unexpected and meaningless results in a compiler that follows strict-aliasing semantics when performing optimizations.</p> <p>A better idea might be to <code>memcpy</code> the <code>void *</code> object to the <code>double</code> object</p> <pre><code>double d; assert(sizeof d == sizeof voidp); // &lt;- a static assert would be even better memcpy(&amp;d, &amp;voidp, sizeof d); </code></pre> <p>Alternatively, in C you are now allowed to use unions for that purpose. I'm not sure the formal permission made into C++ yet, but it will typically work in practice.</p>
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    1. COYay for the `assert()` and `memcpy()`. As to the formal permission in C++: it's still explicitly prohibited (UB) to read an inactive union member.
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    2. COI accepted this answer in lieu of the one I accepted first (Avidanborisov's) because you both answered my original question and the suggestion of using a union proved to be more useful.
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    3. COThe union method, in addition to generally resulting in UB, also may not generate the best code: http://blog.regehr.org/archives/959. `memcpy()`, possibly wrapped up in a `bit_cast<>()` template function, should be the preferred method for correctness, safety, and convenience. @Monchoman45
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