[Boost-users] [endian] Endian types in union

From: Dave Dribin (dave-ml_at_[hidden])
Date: 2006-07-19 22:03:27

Hello,

This may be more appropriate for the devel list, but someone just  
pointed out on this list that an endian library has been submitted  
for approval.  I've *just* needed classes like this (actually I wrote  
my own similar classes), and decided to see how the Boost library  
stacked up.  It looks quite nice, and I'd be excited to see this  
included.  One suggestion I would make is that the endian classes  
should not have constructors.  As of version 0.5, trying to put them  
into a union causes a compiler error.  Take the struct from the  
example, and add a union:

   struct header
   {
     big4_t     file_code;
     big4_t     file_length;
     little4_t  version;
     little4_t  shape_type;
     union
     {
         big4_t foo;
         little4_t bar;
     } baz;
   };

I get the following errors:

endian_example.cpp:37: error: member 'boost::integer::big4_t  
<unnamed>::header::<anonymous union>::foo' with constructor not  
allowed in union
endian_example.cpp:38: error: member 'boost::integer::little4_t  
<unnamed>::header::<anonymous union>::bar' with constructor not  
allowed in union

By removing all constructors, the errors go away.  I cam across this  
very issue when developing my classes, and found the answer in this  
comp.lang.c++.moderated post:

  <http://groups.google.com/group/comp.lang.c++.moderated/msg/ 
103835a7b493723e?hl=en&>

As an experiment, I modified boost/integer/endian.hpp to remove the  
constructors, and add an operator=() to see if this worked, and it did:

     template <typename T, std::size_t n_bytes>
     class endian< big, T, n_bytes, unaligned >
       : cover_operators< endian< big, T, n_bytes >, T >
     {
       public:
         typedef T value_type;
         operator T() const
           { return detail::load_big_endian<T, n_bytes>(bytes); }
         void operator=(T i) { detail::store_big_endian<T, n_bytes> 
(bytes, i); }
       private:
           char bytes[n_bytes];
     };

I'd suggest making this change to all classes so that they can be  
used in unions.

-Dave