Re: Type-punning / casting problem

"Victor Bazarov" <>
Sun, 16 Sep 2007 10:35:31 -0400
Greg Herlihy wrote:

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).

A pointer to the argument 'f'? You mean the address of it? The arg
is a reference. The address of a reference is the address of the
referenced object. Nothing is destroyed. Please revise your analysis.

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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