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Asked: 2026-06-01T20:36:22+00:00

I am a newbie at Qt programming and I have some issues with the

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I am a newbie at Qt programming and I have some issues with the code below. I wanted to create a simple two button application with one label. One button is for positive increment one, and the other button is for decrement one. The label should be updated once I click the plus or minus button. But it doesn’t work. Any ideas why the code doesn’t work? I get a compiler error for an element function void. But the error message is rather unspecific.

I checked the internet (intensive google search and also here on stackoverflow) for a solution but I could not find one. I very much appreciate any direct help or linkage to other websites / links. Please let me know in case I should rephrase the question or add some more information. Thanks for your time. Stefan 

#include "widget.h"

int counter = 0;

Widget::Widget(QWidget *parent) :
QWidget(parent)
{
    setFixedSize(200, 120);

    QPushButton *Plus = new QPushButton(tr("+"), this);
    Plus -> setGeometry(62, 40, 75, 30);
    Plus -> setFont(QFont("Times", 10, QFont::Bold));

    QPushButton *Minus = new QPushButton(tr("-"), this);
    Minus -> setGeometry(62, 40, 75, 30);
    Minus -> setFont(QFont("Times", 10, QFont::Bold));

    QLabel *MyLabel = new QLabel();
    MyLabel ->setAlignment(Qt::AlignCenter);
    MyLabel ->setGeometry(62, 40, 75, 30);
    MyLabel ->setNum(counter);

    QVBoxLayout *layout = new QVBoxLayout;
    layout ->addWidget(MyLabel);
    layout ->addWidget(Plus);
    layout ->addWidget(Minus);
    setLayout(layout);


    connect(Plus, SIGNAL(clicked()), this, SLOT(myClickPlus()));
    connect(Minus, SIGNAL(clicked()), this, SLOT(myClickMinus()));
}

void Widget::myClickPlus(){
    counter ++;
    qDebug("Das ist der Plus-Button");
    MyLabel -> setNum(counter);
}

void Widget::myClickMinus()
{
    counter --;
    qDebug("Das ist der Minus-Button");
    MyLabel ->setNum(counter);
}
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1 Answer

  1. Editorial Team

    The MyLabel that you are creating in the Widget’s constructor is a local variable. The MyLabel that you are using in the myClickPlus and myClickMinus member functions is a member variable. So they are completely different instances.

    If your code compiles, then this change in the Widget’s constructor will probably fix the problem:

    QLabel *MyLabel = new QLabel(); // create local QLabel

    –> 

    MyLabel = new QLabel();  // create member QLabel
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