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Asked: 2026-06-16T11:58:23+00:00

I have the weird problem that when I include anything from the QtNetwork module

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I have the weird problem that when I include anything from the QtNetwork module into my Qt Desktop application, I get a bunch of errors, before anything of the included is even used.
For example, if I include QtNetwork/QHostAddress, I get the following errors:

..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qabstractsocket.h:66: error: template with C linkage

..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qabstractsocket.h:253: error: declaration of C function 'QDebug operator<<(QDebug, QAbstractSocket::SocketState)' conflicts with

..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qabstractsocket.h:252: error: previous declaration 'QDebug operator<<(QDebug, QAbstractSocket::SocketError)' here


..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qhostaddress.h:141: error: declaration of C function 'QDebug operator<<(QDebug, const QHostAddress&)' conflicts with

..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qabstractsocket.h:253: error: previous declaration 'QDebug operator<<(QDebug, QAbstractSocket::SocketState)' here

..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qhostaddress.h:148: error: declaration of C function 'QDataStream& operator<<(QDataStream&, const QHostAddress&)' conflicts with

..\..\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtNetwork/qhostaddress.h:141: error: previous declaration 'QDebug operator<<(QDebug, const QHostAddress&)' here

Of course, I added the line 

Qt += network

to my .pro file, so this can’t be the issue. Another module (opengl) could be included without problems.

I already ensured that nothing is wrong with my Qt installation itself by creating a dummy project which did nothing but including QtNetwork. It worked just fine.

So, there must be something wrong with my project, but as the error messages only occur in this special case I have no idea what information to provide for you.

All I can say is that the program was pure C-Code before and it is now being changed to C++ with Qt, so there is still a mix in it. But this does not seem to be a problem as long as QtNetwork is not included.

Any ideas on how to solve this or on what information is relevant for this problem?

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1 Answer

  1. Editorial Team

    I finally figured out my problem. The solution is quite easy:

    I included QTNetwork into a header file, which in turn was included into some other .cpp-file by my team member, but he wrongly placed the include-statement in an extern “C” block, so that in the end QTNetwork was included as an extern “C” even though it is C++.

    So, two lessons learned:

    1. Don’t rely on the correctness of your team mates’ code.
    2. If an include leads to errors, follow up the whole include chain to track down the problem.
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