Re: Type-punning / casting problem

"Victor Bazarov" <>
Fri, 14 Sep 2007 15:15:47 -0400
Phil Endecott wrote:

I need a function that takes a float, swaps its endianness (htonl) in
place, and returns a char* pointer to its first byte. This is one of
a family of functions that prepare different data types for passing to
another process.

I have got confused by the rules about what won't work, what will
work, and what might work, when casting. Specifically, I have an
implementation that works until I remove my debugging, at which point
the compiler seems to decide that it can optimise away the writes to
the bytes other than the first, or something like that. Here it is:
So, is there some tweak that will make this work, and be certain to
work? Am I better off using a union, or is that even less defined?
Does anyone know what the rules actually are?

You need to make it platform-specific. When you need to reorder, do:

    template<> char* encode<float>(float& f)
        char* pc = reinterpret_cast<char*>(&f);
        for (int i = 0, s = sizeof(float); i < s/2; ++i)
            std::swap(pc[i], pc[s - i - 1]);
        return pc;

When you don't need to reorder, don't.

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