Re: Type-punning / casting problem

 Greg Herlihy <>
Sun, 16 Sep 2007 06:18:00 -0700
On Sep 14, 12:02 pm, Phil Endecott
<> wrote:

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;


And it shouldn't. The encode_arg() function is effectively returning a
pointer to a local variable (the parameter f). So the caller of
encode_arg() receives a pointer to an object that no longer exists;
and therefore the value of the bytes obtained by dereferencing the
returned pointer - could be anything.


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