QT button click to change plaintextedit

I'm totally new to QT. I've written a little form with 4 radio buttons, click button and plain text edit field (in QT Creator). My goal is to press push button and get a text change in the plaintextedit ("Some text here"). Unfortunately I can't find the right formula for connecting the button and the text field, so I get an error, I can't figure out yet. Here's the code (I omit the GUI XML part):

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QRadioButton>
#include <QPushButton>
#include <QPlainTextEdit>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow *ui;
    QRadioButton radioButton,radioButton_2,radioButton_3,radioButton_4;
    QPushButton pushButton;
    QPlainTextEdit plainTextEdit;
};

#endif // MAINWINDOW_H

main.cpp:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
//#include <QPushButton>
//#include <QPlainTextEdit>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QObject::connect(pushButton,SIGNAL(clicked()),plainTextEdit,SLOT(on_pushButton_clicked()));
}

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

void MainWindow::on_pushButton_clicked()
{
    plainTextEdit.setPlainText("Some text here");
}

Here's the error output:

/home/user/workspace/ButtonRadio/mainwindow.cpp:-1: In constructor 'MainWindow::MainWindow(QWidget*)':
/home/user/workspace/ButtonRadio/mainwindow.cpp:11: error: no matching function for call to 'MainWindow::connect(QPushButton&, const char*, QPlainTextEdit&, const char*)'
/home/user/workspace/ButtonRadio/mainwindow.cpp:11: candidates are:
/usr/include/qt4/QtCore/qobject.h:204: static bool QObject::connect(const QObject*, const char*, const QObject*, const char*, Qt::ConnectionType)
/usr/include/qt4/QtCore/qobject.h:204: note:   no known conversion for argument 1 from 'QPushButton' to 'const QObject*'
/usr/include/qt4/QtCore/qobject.h:217: static bool QObject::connect(const QObject*, const QMetaMethod&, const QObject*, const QMetaMethod&, Qt::ConnectionType)
/usr/include/qt4/QtCore/qobject.h:217: note:   no known conversion for argument 1 from 'QPushButton' to 'const QObject*'
/usr/include/qt4/QtCore/qobject.h:337: bool QObject::connect(const QObject*, const char*, const char*, Qt::ConnectionType) const
/usr/include/qt4/QtCore/qobject.h:337: note:   no known conversion for argument 1 from 'QPushButton' to 'const QObject*'

If I write just

QObject::connect(pushButton,SIGNAL(clicked()),SLOT(on_pushButton_clicked()));

then the error output slightly changes, but the essence is still the same. How can I solve this problem?

--EDIT--15.11.14--

mainwindow.h: ...

private:
    Ui::MainWindow *ui;
    QRadioButton *radioButton,*radioButton_2,*radioButton_3,*radioButton_4;
    QPushButton *pushButton;
    QPlainTextEdit *plainTextEdit;
};

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    pushButton(new QPushButton(this)),
    plainTextEdit(new QPlainTextEdit(this))
{
    ui->setupUi(this);
    //pushButton = new QPushButton(this);
    //plainTextEdit = new QPlainTextEdit(this);
    QObject::connect(pushButton,SIGNAL(clicked()),plainTextEdit,SLOT(on_pushButton_clicked()));
}

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

void MainWindow::on_pushButton_clicked()
{
    plainTextEdit->setPlainText("Some text here");
}

As a result, I get SIGSEGV with the following output:

amember "1on_pushButton_clicked()" char *
asender @0x2 QObject
asignal "2clicked()" char *
atype Qt::AutoConnection(0) Qt::ConnectionType
this @0x7fffffffe640 QObject

It also points to the following lines in qobject.h:

339 { return connect(asender, asignal, this, amember, atype); }
349 T qFindChild(const QObject *o, const QString &name = QString());
350 QList<T> qFindChildren(const QObject *oobj, const QString &name = QString());
351 QList<T> qFindChildren(const QObject *o, const QRegExp &re);

You get this error because connect use pointers. You should use pointers:

QRadioButton *radioButton,*radioButton_2,*radioButton_3,*radioButton_4;
QPushButton *pushButton;
QPlainTextEdit *plainTextEdit;

When you use pointer every . replace with ->. Compiler tells you about mistake with next error:

...(maybe you meant to use '->' ?) radioButton_2.show();

And connect should be:

connect(pushButton,SIGNAL(clicked()),this,SLOT(on_pushButton_clicked()));

or

connect(pushButton,SIGNAL(clicked()),SLOT(on_pushButton_clicked()));

On future: you can also write this:

QObject obj1;
QObject obj2;
connect(&obj1,SIGNAL(someSigna()),&obj2,SLOT(slot()));

As you can see, you need &

You need to write QObject::connect in not QObject subclasses (for example in main.cpp) In your MainWindow you can write just connect

Also when you use pointers you should allocate memory with new. For example:

pushButton = new QPushButton(this);//in constructor
//and so on

where this shows us that this (our MainWindow) is parent, so pushButton will appear in MainWindow, not in the separate window.

I think I mentioned all big mistakes in this code.

on_pushButton_clicked() is slot of mainwindow, not a plaintexteditTry next:

pushButton = new QPushButton(this);
plainTextEdit = new QPlainTextEdit(this);
connect(pushButton,SIGNAL(clicked()),this,SLOT(on_pushButton_clicked()));