Q:

reinterpret casting to and from unsigned char* and char*

I'm wondering if it is necessary to reinterpret_cast in the function below.  ITER_T might be a char*, unsigned char*, std::vector<unsigned char> iterator, or something else like that.  It doesn't seem to hurt so far, but does the casting ever affect how the bytes are copied at all?
template<class ITER_T>
char *copy_binary(
  unsigned char length,
  const ITER_T& begin)
{
  // alloc_storage() returns a char*
  unsigned char* stg = reinterpret_cast<unsigned char*>(alloc_storage(length));
  std::copy(begin, begin + length, stg);
  return reinterpret_cast<char*>(stg);
}

A:

reinterpret_casts are used for low-level implementation defined casts. According to the standard, reinterpret_casts can be used for the following conversions (C++03 5.2.10):

Pointer to an integral type
Integral type to Pointer
A pointer to a function can be converted to a pointer to a function of a different type
A pointer to an object can be converted to a pointer to an object of different type
Pointer to member functions or pointer to data members can be converted to functions or objects of a different type. The result of such a pointer conversion is unspecified, except the pointer a converted back to its original type.
An expression of type A can be converted to a reference to type B if a pointer to type A can be explicitly converted to type B using a reinterpret_cast.

That said, using the reinterpret_cast is not a good solution in your case, since casting to different types are unspecified by the standard, though casting from char * to unsigned char * and back should work on most machines.
In your case I would think about using a static_cast or not casting at all by defining stg as type char *:
template<class ITER_T>
char *copy_binary(
  unsigned char length,
  const ITER_T& begin)
{
  // alloc_storage() returns a char*
  char* stg = alloc_storage(length);
  std::copy(begin, begin + length, stg);
  return stg;
}

