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POCorrect way to serialize binary data in C++
 

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  1. POCorrect way to serialize binary data in C++
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    <p>After having read the following <a href="https://stackoverflow.com/questions/12612488/aliasing-t-with-char-is-allowed-is-it-also-allowed-the-other-way-around">1</a> and <a href="https://stackoverflow.com/questions/13280771/is-this-use-of-stdarray-undefined-behavior?">2</a> Q/As and having used the technique discussed below for many years on x86 architectures with GCC and MSVC and not seeing a problems, I'm now very confused as to what is supposed to be the correct but also as important "most efficient" way to serialize then deserialize binary data using C++.</p> <p>Given the following "wrong" code:</p> <pre><code>int main() { std::ifstream strm("file.bin"); char buffer[sizeof(int)] = {0}; strm.read(buffer,sizeof(int)); int i = 0; // Experts seem to think doing the following is bad and // could crash entirely when run on ARM processors: i = reinterpret_cast&lt;int*&gt;(buffer); return 0; } </code></pre> <p>Now as I understand things, the reinterpret cast indicates to the compiler that it can treat the memory at buffer as an integer and subsequently is free to issue integer compatible instructions which require/assume certain alignments for the data in question - with the only overhead being the extra reads and shifts when the CPU detects the address it is trying to execute alignment oriented instructions is actually not aligned. </p> <p>That said the answers provided above seem to indicate as far as C++ is concerned that this is all undefined behavior.</p> <p>Assuming that the alignment of the location in buffer from which cast will occur is not conforming, then is it true that the only solution to this problem is to copy the bytes 1 by 1? Is there perhaps a more efficient technique?</p> <p>Furthermore I've seen over the years many situations where a struct made up entirely of pods (using compiler specific pragmas to remove padding) is cast to a char* and subsequently written to a file or socket, then later on read back into a buffer and the buffer cast back to a pointer of the original struct, (ignoring potential endian and float/double format issues between machines), is this kind of code also considered undefined behaviour?</p> <p>The following is more complex example:</p> <pre><code>int main() { std::ifstream strm("file.bin"); char buffer[1000] = {0}; const std::size_t size = sizeof(int) + sizeof(short) + sizeof(float) + sizeof(double); const std::size_t weird_offset = 3; buffer += weird_offset; strm.read(buffer,size); int i = 0; short s = 0; float f = 0.0f; double d = 0.0; // Experts seem to think doing the following is bad and // could crash entirely when run on ARM processors: i = reinterpret_cast&lt;int*&gt;(buffer); buffer += sizeof(int); s = reinterpret_cast&lt;short*&gt;(buffer); buffer += sizeof(short); f = reinterpret_cast&lt;float*&gt;(buffer); buffer += sizeof(float); d = reinterpret_cast&lt;double*&gt;(buffer); buffer += sizeof(double); return 0; } </code></pre>
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    1. COYou can solve the alignment problem using, e.g., `std::aligned_storage<sizeof(int), std::alignment_of<int>::value>::type` instead of `char[sizeof(int)]` (or, if you don't have C++11, there may be similar compiler-specific functionality).
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    2. CO@abarnert the example above is simple, in general one would serialize many different types sequentially into one buffer and expect to read them all back, furthermore the "buffer" may be request from a pool which may not have any alignas done etc...
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    3. COFrom the tone of your question, it sounds like you're not so much asking "can I do this" (you know you can't) or "why won't it work?" (it's already been explained to you) so much as "why didn't they design the language differently so it _would_ work?" (Or maybe even, "Is there a language close to C++ that was designed so it would work?")
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