Answer by retinotop for How do I display a counter function in qt

#include "form.h"
#include "ui_form.h"
#include <QString>
#include <QLabel>

Form::Form(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Form)
{
    counter = 0;
    ui->setupUi(this);
}

Form::~Form()
{
    delete ui;
}

void Form::on_pushButton_clicked()
{
    counter++;
    ui->label->setText(QString::number(counter));
}

Initialize your member in the constructor not in the header. And put the definition of your counter member to the private section not to the private slot section.