[jira] Commented: (QPID-2926) Simple example code does not link under Windows

["Chuck Rolke (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/QPID-2926?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12928691#action_12928691 ] 

Chuck Rolke commented on QPID-2926:
-----------------------------------

Here's my second suggestion. It is actually Possible Fix #1 from the last post:

In a brand new windows-only qpidmessaging source file

void HandleInstantiatorDoNotCall(void)
{
    // This function exists to instantiate various template Handle
    // bool functions. The instances are then available to
    // the qpidmessaging DLL and subsequently exported.
    // This function must not be exported nor called called.
    // For further information refer to
    // https://issues.apache.org/jira/browse/QPID-2926

    Connection connection;
    if (connection.isValid()) connection.close();
    if (connection.isNull() ) connection.close();
    if (connection ) connection.close();
    if (!connection ) connection.close();

    Receiver receiver;
    if (receiver.isValid()) receiver.close();
    if (receiver.isNull() ) receiver.close();
    if (receiver ) receiver.close();
    if (!receiver ) receiver.close();

    Sender sender;
    if (sender.isValid()) sender.close();
    if (sender.isNull() ) sender.close();
    if (sender ) sender.close();
    if (!sender ) sender.close();

    Session session;
    if (session.isValid()) session.close();
    if (session.isNull() ) session.close();
    if (session ) session.close();
    if (!session ) session.close();
}

Points:
1. The code is in a new, windows-only qpidmessaging source file.
2. The fix does not change any source module for linux and it changes no interface definitions.


> Simple example code does not link under Windows
> -----------------------------------------------
>
>                 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?$Handle@VConnectionImpl@messaging@qpid@@@messaging@qpid@@QBE_NXZ)
> referenced in function __catch$_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
> --------------
> 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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