Simple example code does not link under Windows
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Key: QPID-2926
URL: https://issues.apache.org/jira/browse/QPID-2926
Project: Qpid
Issue Type: Bug
Components: C++ Client
Affects Versions: 0.7
Environment: Windows platform, native C++ client example
Reporter: Chuck Rolke
Assignee: Chuck Rolke
Fix For: 0.7
Using one of the existing examples that has a line such as
"connection.close();"
change to
"if (connection) connection.close();"
causes error:
Error 2 error LNK2019: unresolved external symbol "__declspec(dllimport)public:
__thiscall qpid::messaging::Handle<class
qpid::messaging::ConnectionImpl>::operator bool(void)const
"(__imp_??b?$han...@vconnectionimpl@messag...@qpid@@@messag...@qpid@@QBE_NXZ)
referenced in function __catch$_main$0 client.obj messaging_clien
The missing routine undecorates to:
"public: __thiscall qpid::messaging::Handle<class
qpid::messaging::ConnectionImpl>::operator bool(void)const "
I think the problem is that the messaging Connection class has a template
Handle class in its interface, and then this interface is exported by a DLL. At
DLL-compile time the DLL source code never instantiates the Handle class
template and thus has no actual instance of Handle to export in the DLL. On a
Linux build the instances of Handle are created on the client host system and
are not imported from a library - but this is purely a guess. See also
http://www.codeproject.com/kb/cpp/templatesourceorg.aspx
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Possible fixes
1. Add a separate compilation unit to Windows builds that creates actual
instances of Handle<>. Then export these in the DLL to satisfy the link.
This may work but it does not sound easy to support in the long run.
2. Define the function definitions in Connection proper and not let the work
fall into class Handle. In Connection.cpp adding
QPID_MESSAGING_EXTERN operator bool() const {
return qpid::messaging::Handle<ConnectionImpl>::operator bool();
}
solves the issue. This same pattern applies to several functions in each of
Connection, Receiver, Sender, and Session.
This would probably work in Linux, too, but could be conditionaled to only be
active in Windows.
If anyone has some other ideas I'd love to hear them. Also, is having a
template class in a Messaging library interface a good idea?
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