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  1. Qt
  2. QTBUG-75899

QDataStream setDevice() doesn't work on a default contructed QDataStream Object

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Not Evaluated
    • Resolution: Invalid
    • Affects Version/s: 5.12.3
    • Fix Version/s: None
    • Component/s: Core: I/O
    • Labels:
      None
    • Environment:
      MinGW 64 bit (GCC 8.1.0)
      Windows 10 64 bit pro
    • Platform/s:
      Windows

      Description

      Here is a simple program I wrote, that, in theory, should compile without issues. I create an object using the QDataStream Default constructor.

       

      #include <QtCore/QDataStream>
      #include <QtCore/QIODevice>
      #include <QtCore/QFile>
      int main( int argc, char **argv )
      {
       QDataStream out_stream(); // we will serialize the data into the file
       QFile file("file.dat");
       file.open(QIODevice::WriteOnly);
       out_stream.setDevice(&file);
      }
       
      

      When attempting to compile this program, I get the following error:

      datastream_test.cpp: In function 'int main(int, char**)':
      datastream_test.cpp:12:16: error: request for member 'setDevice' in 'out_stream', which is of non-class type 'QDataStream()'
           out_stream.setDevice(&file);
                      ^~~~~~~~~

       

      This error makes no sense, as out is initialized as a QDataStream object. From what I can tell, calling the QDataStream constructor without passing an argument results in  an object ("out_stream") that is not initialized as a QDataStream object.

       

      The only way I can use setDevice successfully is by passing a QIODevice into the QDataStream constructor. This is demonstrated in the below code, which compiles without issues:

       

      #include <QtCore/QDataStream>
      #include <QtCore/QIODevice>
      #include <QtCore/QFile>
      int main( int argc, char **argv )
      {
       QFile dummy_file("asdf.dat");    //dummy that is used to initialize the datastream object
       dummy_file.open(QIODevice::WriteOnly);
       QDataStream out_stream(); // we will serialize the data into the file
       QFile file("file.dat");
       file.open(QIODevice::WriteOnly);
       out_stream.setDevice(&file);
      }
      

       

       

      This workaround gets Qt to (correctly) recognize "out_stream" as a QDataStream object. However, calling the QDataStream Constructor without passing arguments does not result in an object that is recognized as a QDataStream object.

       

       

      When I create a default constructed QDatastream object and try to call setDevice() on it, I get

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            • Assignee:
              thiago Thiago Macieira
              Reporter:
              jrj99 jrj99
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              • Created:
                Updated:
                Resolved:

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