Ahhh - so try

ar >> static_cast<A* &>(a2p);

Robert Ramey

troy d. straszheim wrote:

...

Ben wrote:

...

{ std::ifstream ifs(fname); boost::archive::binary_iarchive ar(ifs); ar >> static_cast<A*>(a2p); }

You're trying to write to an rvalue. Note that

int *ip = 0; static_cast<int*>(ip) = 0; // lvalue required as // left operand of assignment

doesn't compile either...

Wow, that was silly of me. Thanks for the quick response. Any clue to the main question I had (repeated below) about serializing classes with references? I don't think it's an rvalue thing, as the problem occurs even in the save_construct_data bit. Generally, I'm trying to copy what the documentation says to do. Best, Ben Date: Fri, 23 Oct 2009 15:27:48 -0700 From: Ben <midfield@gmail.com> To: boost-users@lists.boost.org Subject: [Boost-users] [serialization] Serializing a reference Message-ID: <9157df230910231527x686ff3b4n16ec1e9eabc9515@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Hello Boost Experts -- And now for my real question. Again, g++ 4.2.1, OS X 10.6.1, boost 1.40. I'm trying to follow the instructions in the document for serializing classes with reference members, which suggests I serialize them as pointers. First, a program which works, that uses pointers and not references: #include <iostream> #include <fstream> #include <boost/archive/binary_oarchive.hpp> #include <boost/archive/binary_iarchive.hpp> struct A { int v; template<class Ar> void serialize(Ar & ar, const unsigned int ver) { ar & v; } }; struct B { A *ap; B(A *ap_) : ap(ap_) {} template<class Ar> void serialize(Ar & ar, const unsigned int ver) {} }; template<class Ar> void save_construct_data(Ar & ar, const B * b, const unsigned int ver) { ar << b->ap; } template<class Ar> void load_construct_data(Ar & ar, B * b, const unsigned int ver) { A * ap_; ar >> ap_; ::new(b)B(ap_); } int main(int ac, char ** argv) { A a; a.v = 10; B * b1p; b1p = new B(&a); const char * fname = "test.bin"; { std::ofstream ofs(fname); boost::archive::binary_oarchive ar(ofs); ar << b1p; } B * b2p; { std::ifstream ifs(fname); boost::archive::binary_iarchive ar(ifs); ar >> b2p; } std::cout << b2p->ap->v << std::endl; return 0; } What I would really like to do is to modify this program so that class B has a reference to class A instead of a pointer. The documentation suggests I do the following: "This raises the question of how and where the objects being referred to are stored and how are they created. Also there is the question about references to polymorphic base classes. Basically, these are the same questions that arise regarding pointers. This is no surprise as references are really a special kind of pointer. We address these questions by serializing references as though they were pointers." The example given after this serializes the address of the referenced object, similar to the above. My translation of that code is: #include <iostream> #include <fstream> #include <boost/archive/binary_oarchive.hpp> #include <boost/archive/binary_iarchive.hpp> struct A { int v; template<class Ar> void serialize(Ar & ar, const unsigned int ver) { ar & v; } }; struct B { A &a; B(A &a_) : a(a_) {} template<class Ar> void serialize(Ar & ar, const unsigned int ver) {} }; template<class Ar> void save_construct_data(Ar & ar, const B * b, const unsigned int ver) { ar << (&(b->a)); } template<class Ar> void load_construct_data(Ar & ar, B * b, const unsigned int ver) { A * ap_; ar >> ap_; ::new(b)B(*ap_); } int main(int ac, char ** argv) { A a; a.v = 10; B * b1p; b1p = new B(a); const char * fname = "test.bin"; { std::ofstream ofs(fname); boost::archive::binary_oarchive ar(ofs); ar << b1p; } B * b2p; { std::ifstream ifs(fname); boost::archive::binary_iarchive ar(ifs); ar >> b2p; } std::cout << b2p->a.v << std::endl; return 0; } This however fails to compile, with the message: Compilation started at Fri Oct 23 15:25:29 make -k debug make all "CXX_FLAGS = -g -fno-inline -m64 -I. -I.. -I/opt/local/include -Wall -Wextra -Wpointer-arith -Wcast-align -Wwrite-strings -Wconversion -Woverloaded-virtual -Wno-sign-compare -Wno-unused -fno-nonansi-builtins" \ "LD_FLAGS = -g -m64" g++ -g -fno-inline -m64 -I. -I.. -I/opt/local/include -Wall -Wextra -Wpointer-arith -Wcast-align -Wwrite-strings -Wconversion -Woverloaded-virtual -Wno-sign-compare -Wno-unused -fno-nonansi-builtins -c test.cc -o test.o test.cc: In function 'void save_construct_data(Ar&, const B*, unsigned int) [with Ar = boost::archive::binary_oarchive]': /opt/local/include/boost/serialization/serialization.hpp:148: instantiated from 'void boost::serialization::save_construct_data_adl(Archive&, const T*, unsigned int) [with Archive = boost::archive::binary_oarchive, T = B]' /opt/local/include/boost/archive/detail/oserializer.hpp:179: instantiated from 'void boost::archive::detail::pointer_oserializer<Archive, T>::save_object_ptr(boost::archive::detail::basic_oarchive&, const void*) const [with Archive = boost::archive::binary_oarchive, T = B]' test.cc:60: instantiated from here test.cc:27: error: no match for 'operator<<' in 'ar << (A*)b->B::a' /opt/local/include/boost/archive/detail/interface_oarchive.hpp:63: note: candidates are: Archive& boost::archive::detail::interface_oarchive<Archive>::operator<<(T&) [with T = A*, Archive = boost::archive::binary_oarchive] make[1]: *** [test.o] Error 1 make[1]: Target `all' not remade because of errors. make: *** [debug] Error 2 Compilation exited abnormally with code 2 at Fri Oct 23 15:25:30 What am I doing wrong? Best regards, Ben