[Boost-users] Serialization--a peculiar observation regarding operator<<

From: Drumheller, Michael (michael.drumheller_at_[hidden])
Date: 2006-04-03 04:21:44

The test program below (under boost 1_33, vc71) compiles fine, unless I
change the "#if 1" to "#if 0", in which case it produces this:
error C2679: binary '<<': no operator found which takes a right-hand
operand of type 'double' (or there is no acceptable conversion)

Can anyone explain why that should happen?  A double is a double--why
should '<<' care whether the rhs comes from funcall a variable?

Note:  The test program is clearly a nonsense program (in real life the
role of foo() is played by some public (get-accessor) on klass--a very
common pattern when doing a non-intrusive serialization).  But I did not
want to cloud the issue--I wanted to emphasize that the bug, if it is a
bug, seems to have nothing to with klass.

Thank you.

Michael Drumheller
-----------------------------------------------------------------begin--
-------------------------------------------------------------
        #include <fstream>
        #include <boost/archive/text_oarchive.hpp>
        #include <boost/serialization/split_free.hpp>
        class klass {};
        double foo() { return 3.1416; }
        namespace boost { namespace serialization {
          template<class Archive>
          void save(Archive &ar, const klass & b, const unsigned int
ver)
          {
        #if 1
            double x=foo();
            ar << x;
        #else
            ar << foo();
        #endif
          }
          template<class Archive>
          void load(Archive &ar, klass & b, const unsigned int
ver){}//N/A for this demo
        }}
        BOOST_SERIALIZATION_SPLIT_FREE(klass);
        void out(const char *testfile, const klass & c)
        {
          std::ofstream ofs(testfile);
          boost::archive::text_oarchive oa(ofs);
          oa << c;
        }
----------------------------------------------------------------end-----
-----------------------------------------------------------------
Michael Drumheller
Boeing Phantom Works
Mathematics and Engineering Analysis
425.865.3520 michael.drumheller_at_[hidden]