How to convert ’QString 1’ to ’unsigned char’ (solved)

  • i thought using toLocal8bit can convert from QString to unsigned char but i am wrong.

    error: cannot convert 'QString' to 'unsigned char' in assignment


    Brightness0[6] = value.toLatin1();
    'QByteArray::operator QNoImplicitBoolCast() const' is private

    Brightness0[6] =;
    Invalid conversion from 'QChar*' to 'unsigned char'

    Brightness0[6] = value.toLocal8bit().data();
    Invalid conversion from 'QChar*' to 'unsigned char'

    Brightness0[6] = (unsigned char *)(value);
    Invalid cast from type 'QString' to type 'unsigned char'

    void MainWindow::SendBrightness()
    //total 8 bytes,first 6 bytes fixed,7 bytes-value,8 bytes-CRC
    unsigned char Brightness0[]= {0x08,0x11,0x01,0x00,0x00,0x00,0x00,0x00};
    unsigned char Brightness1[]={0x04,0x12,0x83,'\0'};

            //append GUI value
            QString value=ui->BrightnessCombo->currentText();
            Brightness0[6] = (value);                         //!!!!!error
            //calculate CRC and appending
            char crc0 = ModBusCRC(Brightness0,sizeof(Brightness0));
            //send value out
            serial->write((char *)Brightness0);             



  • Moderators

    Are you trying to assign the whole string, or just one character? (Remember that a string is an array of characters)

    Asked another way: Do you want to use "unsigned char" or "unsigned char *"?

  • Trying to assigned one character
    Trying to replace Array[6] with QString value & Array[7] with char value.

    Initially, I tried append to variable Brightness0[] index6 & index7, but not successful.

  • Moderators

    [quote author="houmingc" date="1420706407"]Trying to assigned one character[/quote]Think of a QString as an array of QChar.

    Brightness[6] = value[6].toLatin1();

  • Value of ui->BrightnessCombo is from 0 to 100
    QString value=ui->BrightnessCombo->currentText();

    I need to convert QString to one QChar and insert into index 6, but this is not successful
    Brightness[6] = value.toLatin1();

  • If value is a string that represents a number, then you'll first want to convert the string to an actual number: use one of the functions that create an unsigned number for you, like toUShort or toUInt. Then, it's a simple cast to put that value into a char (which is now used as a one-byte number, not as a character), which can then be assigned to your array.

    So, you end up with something like this:
    bool ok;
    ushort sValue = value.toUShort(&ok);
    if (!ok) {
    //handle error
    char cValue = static_cast<char>(sValue);

    Brightness[6] = cValue;

    However, I think it would be simpler if you replace your combo box with something that actually returns a numeric value to begin with, like a QSpinBox and/or a QSlider.

  • QString value=ui->BrightnessCombo->currentText();

    is change to

    QString value=ui->BrightnessCombo->currentindex();

    to solve the problem

  • [quote author="houmingc" date="1420710723"]QString value=ui->BrightnessCombo->currentText();

    is change to

    QString value=ui->BrightnessCombo->currentindex();

    to solve the problem[/quote]
    That's just wrong too. The last variant returns an integer, and then you put it into a string again. Why?

  • i need to put it into a unsigned char array bit 6; my code is in the first post :>

  • But... A char is a number, not a string. Why convert a number to a string, and then jump through hoops to get a number again?

  • Need to add hex value in an array[] and minus the total with 0x100.
    Below is the code i scribbled.
    Is there an easier way to parse it. Hope to get some feedback before bedtime.

    unsigned char Brightness0[]= {0x08,0x11,0x01,0x00,0x00,0x00,0x00,0x64};
    unsigned char EndHexValue =0x100;

       //convert Hex value to int
       int EndIntValue=EndHexValue.toUInt(0,16);
       //Array below is integer Equal of Brightness0
       int EqualArray[];
       //IntSum to contain total value of integer  
       int IntSum=0;
       unsigned char HexSum=0;
       for (int a=0; a<sizeof(Brightness0); a++)
             //Convert hex to int and set it into Integer array              
             Equalarray[a]= Brightness0[a].toUInt(0,16); 
             //Integer total value
             IntSum += EqualArray[a];
            //Integer total value
            EndIntValue -= intSum; 


    Edit: merged with your previous thread on the same issue. Please don't start multiple threads on the same problem. Andre

  • Hi,

    You can replace the for loop with std::accumulate().

    By the way I don't get why there are so many conversions there. The computer doesn't know number bases.

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