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POOpinions on type-punning in C++?
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copied!<p>I'm curious about conventions for type-punning pointers/arrays in C++. Here's the use case I have at the moment:</p> <blockquote> Compute a simple 32-bit checksum over a binary blob of data by treating it as an array of 32-bit integers (we know its total length is a multiple of 4), and then summing up all values and ignoring overflow. </blockquote> <p>I would expect such an function to look like this:</p> <pre><code>uint32_t compute_checksum(const char *data, size_t size) { const uint32_t *udata = /* ??? */; uint32_t checksum = 0; for (size_t i = 0; i != size / 4; ++i) checksum += udata[i]; return udata; } </code></pre> <p>Now the question I have is, what do you consider the "best" way to convert <code>data</code> to <code>udata</code>?</p> <p>C-style cast?</p> <pre><code>udata = (const uint32_t *)data </code></pre> <p>C++ cast that assumes all pointers are convertible?</p> <pre><code>udata = reinterpret_cast<const uint32_t *>(data) </code></pre> <p>C++ cast that between arbitrary pointer types using intermediate <code>void*</code>?</p> <pre><code>udata = static_cast<const uint32_t *>(static_cast<const void *>(data)) </code></pre> <p>Cast through a union?</p> <pre><code>union { const uint32_t *udata; const char *cdata; }; cdata = data; // now use udata </code></pre> <p>I fully realize that this will not be a 100% portable solution, but I am only expecting to use it on a small set of platforms where I know it works (namely unaligned memory accesses and compiler assumptions on pointer aliasing). What would you recommend?</p>

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