Type-punning / casting problem

Phil Endecott <spam_from_usenet_0606@chezphil.org>
Fri, 14 Sep 2007 19:02:21 GMT
Dear Experts,

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:

template <typename T>
inline const char* encode_arg(T& t); // linker error if you try to
                                       // encode a type for which there
                                       // is no implementation

// This one works:
template <>
inline const char* encode_pq_arg<int>(int& i) {
   i = htonl(i);
   return reinterpret_cast<const char*>(&i);

// This one doesn't:
template <>
inline const char* encode_arg<float>(float& f) {
   uint32_t* ptr = reinterpret_cast<uint32_t*>(&f);
   *ptr = htonl(*ptr);
   const char* cptr = reinterpret_cast<const char*>(ptr);
   return cptr;

When I dump cptr[0] to cptr[3] before the return, it works. Without the
  debug, it fails; it's obviously hard to see what ends up in the result
in that case, but it looks undefined.

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?

Many thanks,


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