Cannot assign to a qstring variable

Question

I am well into programming but new to the world of c++ / qt.

I have been modifying the oscilloscope example from the qwt library to read input from an arduino board using the QtExtSerialPort library. (Yes I know about the QtSerial, but as I discovered that, I was a bit too far into the implementation)

The arduino writes values to the serial port, one number at a line like

1.23
2.33
4.56
2.12
0.32

etc. When the PC reads the data, it comes in in chunks, so in one read, I may get something like

3
2.33
4

and then next time

.56
2.

and so on.

In the header file for the reader thread, I have defined a

QString buffer;

And then when reading I am using this function:

double SamplingThread::value( double timeStamp ) const
{
  double v;
  QByteArray inpt;
  int a = port->bytesAvailable();
  inpt.resize(a);
  port->read(inpt.data(), inpt.size());
  QString strng=buffer+QString::fromAscii(inpt); 
  // This concatenates what is left over since last time to what is read now:
  int j=strng.indexOf("\n");
  if(j>-1){
  // if a newline, ie the first number is complete 
    QString s=strng.left(j-1);
    v=s.toFloat();
    s=strng.mid(j+1,-1); // What is to be saved to next time
    buffer =s; // store it in the global buffer
    return v*d_amplitude/5;
  }
}

(Yes I know I will get into problems as soon as I read a chunk with two \n's in it)

This works fine, except that I cannot store what is left in the global buffer. On that line I get the error:

samplingthread.cpp:89: error: no match for 'operator*=' in
'((const SamplingThread*)this)->SamplingThread::buffer *= s'

I am constantly baffeled what this means. I intended to copy a QString into another QString - but ... ? Have I messed up something with pointers somewhere, but if so, why can I assign to QStrings other places? What is the difference to what I do just one line up? ( s=strng.right(j+1) )

Name · Accepted Answer · 2012-12-18 21:01:22Z

3

Get rid of the "const" on your value function.

"const" functions can only call functions of its classes variables if they are also "const". That is why some functions worked, and some did not.

edited Dec 18, 2012 at 21:01

answered Dec 18, 2012 at 20:43

Name

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7 Comments

MortenSickel Over a year ago

Of cource, not exactly the same, but similiar: error: passing 'const QString' as 'this' argument of 'void QString::clear()' discards qualifiers [-fpermissive]

Name Over a year ago

@MortenSickel That's weird. Give me some time to think about that.

Name Over a year ago

@MortenSickel buffer is certainly not "const", is it?

MortenSickel Over a year ago

nope ;-) The entire block is like: private: virtual double value( double timeStamp ) const; double factor; double d_frequency; double d_amplitude; QextSerialPort* port; QString buffer;

Name Over a year ago

@MortenSickel the code you showed above, is that part of the value function?

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