The following is a dead simple example of using Boost.Serialization using an
XML output archive which follows the patterns laid down in examples:
/*
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
*/
#include <iostream>
#include
#include
// Warning level 4.
#pragma warning(push, 4)
#pragma message("********")
struct MyData
{
MyData(int val) : m_Value(val) {}
template<typename Archive>
void serialize(Archive & ar, const unsigned int)
{
ar & BOOST_SERIALIZATION_NVP(m_Value); // <-- Warning here.
}
int m_Value;
};
void main()
{
using namespace std;
MyData d(3);
boost::archive::xml_oarchive oa(cout);
oa << BOOST_SERIALIZATION_NVP(d); <-- Warning here.
}
// Warning level back to normal.
#pragma warning(pop)
/*
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
*/
When compiling this code I get the following error:
"warning C4239: nonstandard extension used : 'argument' : conversion from
'struct boost::serialization::nvp<struct MyData>' to 'struct
boost::serialization::nvp<struct MyData> &'
A reference that is not to 'const' cannot be bound to a non-lvalue"
I tried MSVC 6 and "Visual C++ 2005" which can be downloaded from here:
http://msdn.microsoft.com/vstudio/express/visualC/default.aspx
Interestingly I could not get it to happen with "Microsoft Visual C++
Toolkit 2003".
The warning seems valid to me. The "BOOST_SERIALIZATION_NVP" looks like
this:
--------------------------------------------------------------------------------------------------------------------------
#define BOOST_SERIALIZATION_NVP(name) \
boost::serialization::make_nvp(BOOST_PP_STRINGIZE(name), name)
--------------------------------------------------------------------------------------------------------------------------
The "make_nvp" function looks like as follows:
-------------------------------------------------------
nvp<T> make_nvp(const char * name, T & t){
return nvp<T>(name, t);
}
-------------------------------------------------------
"Operator<<", which is used in "main" is reproduced below:
--------------------------------------------------------------------------------------
template<class Archive>
class interface_oarchive
{
// ... Code removed ... //
template<class T>
Archive & operator<<(T & t){
this->This()->save_override(t, 0);
return * this->This();
}
// ... Code removed ... //
};
--------------------------------------------------------------------------------------
So, to cut to the chase, "BOOST_SERIALIZATION_NVP" expands to a call to
make_nvp" which returns a temporary which is bound to a non-const reference
in "interface_oarchive::operator<<": this is not legal in standard C++
(unless I'm mistaken).
It seems that if you follow to documentation and examples the resulting code
isn't standard C++!?!
