[boost] [Endian] Proposed endian integer types

Subject: [boost] [Endian] Proposed endian integer types
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2011-09-08 13:19:47

Dear All,

I have had a quick look at the proposed endian::big|littleN_t types.  A 
couple of comments:

- I was expecting to find that these types would use the conversion 
functions that I've already looked at at the lowest level, so that 
optimisations made to those functions would be useful here too.  But 
instead, these classes seem to use their own byte-shuffling code.  This 
seems like an odd design.

- I'm not convinced that these types cause the conversion to happen at 
the optimum time.  For example:

struct file_header {
   big32_t n_things;
   ....
};

int main()
{
   file_header h;
   h.n_things = 0;
   while (....) {
     ++h.n_things;
     ....
   }
   write(h);
   ....
}

Here the conversion is happening twice every time that n_things is 
incremented.  It would be better to instead do the conversion once, 
just before the file_header is written.  The same thing applies in 
reverse when reading from a file.  (Surely this is the most common use-case?)

I have been wondering if there is a better design that can avoid this.  
If we had some sort of struct introspection, we could store the fields 
in native byte order and then

template <typename T>
T external_representation(const T& t)
{
   T res(t);
   for each field of T {
     res.field.reorder();
   }
}

++h.n_things;  // cheap
write(external_representation(h));  // conversion happens once here.

Of course we don't have introspection of structs so we can't do that.  
Maybe someone else has another idea.

Regards,  Phil.