Re: [boost] Boost.Endian - endian_buffer requirements

26 Jan 2017

      Le 26/01/2017 à 01:05, Vicente J. Botet Escriba a écrit :
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Le 26/01/2017 à 00:04, Vicente J. Botet Escriba a écrit :
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Hi,
I wanted to put an Int wrapper inside a endian_buffer and I have some 
compiler errors depending on whether it is aligned ort not.
http://melpon.org/wandbox/permlink/k52wH2A7yagL2vMP
I was expecting that the endian_buffer would make abstraction of the 
wrapped type and consider it just as a sequence of bytes, but it 
seems that it instead works only for some builtin integral.
Am I missing something?
Was this restriction intended?
Could this restriction be removed and if yes, how?
I have reached to fix the aligned case adding the customization
Int endian_reverse(Int x)
{
    using boost::endian::endian_reverse;
    return Int {endian_reverse(x.value)};
}
However for the unaligned case there is yet the shift error
prog.cc:30:28:   required from here
/usr/local/boost-1.61.0/include/boost/endian/buffers.hpp:231:26: error: invalid static_cast from type 'Int' to type 'char'
            *(bytes - 1) = static_cast<char>(value);
                           ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/boost-1.61.0/include/boost/endian/buffers.hpp:232:59: error: no match for 'operator>>' (operand types are 'Int' and 'int')
            next::store_big(bytes - 1, static_cast<T>(value >> 8));
                                                      ~~~~~~^~~~
/usr/local/boost-1.61.0/include/boost/endian/buffers.hpp:202:5: note: candidate: template std::basic_istream& boost::endian::operator>>(std::basic_istream&, boost::endian::endian_buffer&)
      operator>>(std::basic_istream& is,
      ^~~~~~~~
http://melpon.org/wandbox/permlink/iPwgdVBV91wsRxeC
I have added whatever was missing

struct Int {
     int value;

     // Needed by unaligned
     Int() = default;
     Int(int i) : value (i) {}
     explicit operator char() { return value; }
     int operator >>(int n) { return value >> n; }
     int operator <<(int n) { return value << n; }

     friend Int endian_reverse(Int x)
     {
         using boost::endian::endian_reverse;
         return Int {endian_reverse(x.value)};
     }

};

http://melpon.org/wandbox/permlink/erLEWpc1oaR64iMW

While

     Int() = default;
     explicit Int(int i) : value (i) {}

are natural on a wrapped opaque int, the implicit conversion is less 
desirable as well as the conversions to char and the shift operations.

     Int(int i) : value (i) {}
     explicit operator char() { return value; }
     int operator >>(int n) { return value >> n; }
     int operator <<(int n) { return value << n; }

I believe that the conversion could be explicit and that the 
implementation  could use reinterpret_cast instead of static_cast, as we 
are reinterpreting the bits.
IMHO, the shift shouldn't be required. It should be an implementation 
detail.

BTW, is there any reason endian_buffer doesn't define equality operators?

Vicente