#include <QApplication>
#include <QFont>
#include <QPushButton>
#include <QWidget>
class MyWidget : public QWidget
{
public:
MyWidget(QWidget *parent = 0);
};
MyWidget::MyWidget(QWidget *parent)
: QWidget(parent)
{
setFixedSize(200, 120);
QPushButton *quit = new QPushButton(tr("Quit"), this);
quit->setGeometry(62, 40, 75, 30);
quit->setFont(QFont("Times", 18, QFont::Bold));
connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));
}
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MyWidget widget;
widget.show();
return app.exec();
}
For this line : MyWidget(QWidget *parent = 0);
why we need to put = 0 here??
It is called an Default parameter
Basically you are saying unless you pass another value, the function (or constructor in this case) will be called with parent as 0.
When you'd had MyWidget(QWidget *parent); as constructor, you'd had to call it like MyWidget widget(0);
You do not have to put zero there. C++ allows you to put default value for a parameter. In this case, parameter parent will default to 0 if the constructor is invoked without specifying an argument.
Don't need to put it there, but it's a default value. If you don't pass any value to the constructor, it will take the '0' as value. It's making things a bit easier in some cases for the programmer.
Related
I'm trying to create a class for signal/slot connection (old syntax, Qt 4.8) and I am doing something wrong as I keep receiving a template error: invalid declaration of member template in local class... That has obviously something to do with the Q_OBJECT macro... What should I do? Here is a modeled program:
#include <QtGui>
#include <QtCore>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QWidget mw;
mw.setWindowTitle("Main Window");
mw.resize(400, 400);
mw.show();
QLabel label ("Enter something:", &mw);
label.setAlignment(Qt::AlignHCenter);
label.show();
QLineEdit line (&mw);
line.show();
QString a = line.text();
QTextEdit text (&mw);
text.show();
class MyObject : public QObject
{
Q_OBJECT /* the problem is somewhere here... */
public:
QTextEdit text;
QString a;
public slots:
void onClicked() {
text.setText(a);
}
};
QPushButton btn ("Convert", &mw);
QObject::connect(
&btn,
SIGNAL(clicked()),
this,
SLOT(onClicked()));
btn.show();
QVBoxLayout layout_mw;
layout_mw.addWidget(&label);
layout_mw.addWidget(&line);
layout_mw.addWidget(&btn);
layout_mw.addWidget(&text);
mw.setLayout(&layout_mw);
return app.exec();
}
Qt's MOC can process neither nested classes nor local classes. You will have to move the class definition outside main. The documentation only mentions nested classes, but the limitation does apply to local classes too.
I made class inherited from QLabel. This class also have public slot, that should change label caption. I "call" this SLOT with clicked() SIGNAL of button.
So nothing happened when I press the button.
#include <QApplication>
#include <QLabel>
#include <QPushButton>
class Label : public QLabel
{
public:
Label(QString a) : QLabel(a){}
public slots:
void change()
{
this->setNum(2);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QPushButton* button = new QPushButton("Button");
Label* lbl = new Label("Label");
button->show();
lbl->show();
QObject::connect(button, SIGNAL(clicked(bool)), lbl, SLOT(change()));
return a.exec();
}
What should I do to change caption from slot?
In order for the signals and slots to be recognized, the classes must use the Q_OBJECT macro in the private part.
Another thing to do is to include "main.moc", for more information on this point read this.
#include <QApplication>
#include <QLabel>
#include <QPushButton>
class Label : public QLabel
{
Q_OBJECT
public:
Label(const QString &text, QWidget *parent = Q_NULLPTR, Qt::WindowFlags f = Qt::WindowFlags()) :
QLabel(text, parent, f){}
public slots:
void change()
{
setNum(2);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QPushButton* button = new QPushButton("Button");
Label* lbl = new Label("Label");
button->show();
lbl->show();
QObject::connect(button, SIGNAL(clicked()), lbl, SLOT(change()));
return a.exec();
}
#include "main.moc"
At the end of making these changes you must execute the following:
Press clean all in the Build menu.
then run qmake in the same menu.
And you just compose your project.
Add Q_OBJECT after
class Label : public QLabel
{
and then you should
either place your Label class declaration to a .h file or write #include "main.moc" after main function declaration.
try to get the return value from your connect call an check it for true or false.
Add Q_OBJECT Macro to the beginning of your derived class.
Add some debug output to your slot like
qDebug()<<"This is my slot.";
Maybe this would help to get a little further.
Best regards
//disconnect.h
#include <QWidget>
#include <QPushButton>
class Disconnect : public QWidget {
Q_OBJECT
public:
Disconnect(QWidget *parent = 0);
private slots:
void onClick();
void onCheck(int);
private:
QPushButton *clickBtn;
};
//disconnect.cpp
#include <QTextStream>
#include <QCheckBox>
#include <QHBoxLayout>
#include "disconnect.h"
Disconnect::Disconnect(QWidget *parent)
: QWidget(parent) {
QHBoxLayout *hbox = new QHBoxLayout(this);
hbox->setSpacing(5);
clickBtn = new QPushButton("Click", this);
hbox->addWidget(clickBtn, 0, Qt::AlignLeft | Qt::AlignTop);
QCheckBox *cb = new QCheckBox("Connect", this);
cb->setCheckState(Qt::Checked);
hbox->addWidget(cb, 0, Qt::AlignLeft | Qt::AlignTop);
connect(clickBtn, &QPushButton::clicked, this, &Disconnect::onClick);
connect(cb, &QCheckBox::stateChanged, this, &Disconnect::onCheck);
}
void Disconnect::onClick() {
QTextStream out(stdout);
out << "Button clicked" << endl;
}
void Disconnect::onCheck(int state) {
if (state == Qt::Checked) {
connect(clickBtn, &QPushButton::clicked, this, &Disconnect::onClick);
} else {
disconnect(clickBtn, &QPushButton::clicked, this,
&Disconnect::onClick);
}
}
//main.cpp
#include <QApplication>
#include "disconnect.h"
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
Disconnect window;
window.resize(250, 150);
window.setWindowTitle("Disconnect");
window.show();
return app.exec();
}
In the above code，A Disconnect object has two child object——a QPushButton and a QcheckBox, but class Disconnect only have a QPushButton pointer, not a QcheckBox pointer. Can anyone tell me when should the child object be declared as a member variable of its parent class in qt?
Can anyone tell me when should the child object be declared as a
member variable of its parent class in qt?
If you think you will need to call methods on (or otherwise access) the child object from other methods of parent class (as your example code does with clickBtn, referencing it from the onCheck method), then you'll want to create a member variable that is a pointer to that child object, to allow you to do so.
If, on the other hand, you will not need to access the child object outside of the method where it was created, then you can keep your parent class's definition shorter and simpler by declaring the pointer-to-the-child-object as a local variable inside the creating method only.
Issue Resolved: Q_OBJECT macro was necessary and proper signal slot declarations are also important for any other handles.
I am unable to focus on any input type widgets like QTextEdit,QListWidget etc.
Note: There are no compile time or runtime errors.
Update: QSplitter is working properly! I have a QListWidget, whose items I click but they are highlighted only when I make the next move with the splitter.
I have a MainWindow class derived from QMainWindow as declared in main_window.h:
class MainWindow : public QMainWindow{
Q_OBJECT
public:
MainWindow(QWidget *parent = 0);
//some other members like menu and statusbar here
}
I have another class called Stack derived from QWidget defined in stack.h:
class Stack: public QWidget{
public:
Stack(QWidget *parent=0);
//some other members
}
Constructor of Stack as in stack.cpp :
Stack::Stack(QWidget *parent):QWidget(parent){
main = new QHBoxLayout;
handle = new QSplitter;
setupList();
setupScreens();
//above functions add the widgets to the handle splitter
main->addWidget(handle);
setLayout(main);
}
If i open up this widget in a separate window from the MainWindow using test->show(), the things work as expected/as i want.
But doing this in the MainWindow constructor, renders it unclickable.
MainWindow::MainWindow(QWidget *parent):QMainWindow(parent){
Stack *test = new Stack(this);
//test->show();
setCentralWidget(test);
}
This is strange. Why am i not able to focus any widget that can take input e.g. QTextEdit,QListWidget or click any QPushButton widget?
Please compile following code, it was working..you are getting focus and edit on QTextEdit...
stack.h
#include <QWidget>
class Stack: public QWidget
{
Q_OBJECT
public:
Stack(QWidget *parent = 0);
~Stack(void);
};
stack.cpp
#include "Stack.h"
#include<QTextEdit>
#include<QHBoxLayout>
Stack::Stack(QWidget *parent):QWidget(parent){
QHBoxLayout* main = new QHBoxLayout;
QTextEdit *test = new QTextEdit;
main->addWidget(test);
//other things added to main layout
setLayout(main);
}
Stack::~Stack(void)
{
}
mainwindow1.h
#ifndef MAINWINDOW1_H
#define MAINWINDOW1_H
#include <QtGui/QMainWindow>
//#include "ui_mainwindow1.h"
class Mainwindow1 : public QMainWindow
{
Q_OBJECT
public:
Mainwindow1(QWidget *parent = 0, Qt::WFlags flags = 0);
~Mainwindow1();
private:
//Ui::Mainwindow1Class ui;
};
#endif // MAINWINDOW1_H
mainwindow1.cpp
#include "mainwindow1.h"
#include "Stack.h"
#include <QTextEdit>
Mainwindow1::Mainwindow1(QWidget *parent, Qt::WFlags flags)
: QMainWindow(parent, flags)
{
Stack *test = new Stack;
setCentralWidget(test);
}
Mainwindow1::~Mainwindow1()
{
}
main.cpp
#include "mainwindow1.h"
#include <QtGui/QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Mainwindow1 w;
w.show();
return a.exec();
}
If some1 would find this looking for answer on how to set focus on input widget from UI in QT5 you can just use:
ui->plainTextEdit->setFocus();
I was trying to figure out how this code will loke like in plain C++ without any dependency so I was using the moc compiler but apparently I'm wrong.
moc always returns
main.cpp:0: Note: No relevant classes found. No output generated.
the code is
#include <QApplication>
#include <QWidget>
#include <QPushButton>
class MyButton : public QWidget
{
public:
MyButton(QWidget *parent = 0);
};
MyButton::MyButton(QWidget *parent)
: QWidget(parent)
{
QPushButton *quit = new QPushButton("Quit", this);
quit->setGeometry(50, 40, 75, 30);
connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));
}
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MyButton window;
window.resize(250, 150);
window.move(300, 300);
window.setWindowTitle("button");
window.show();
return app.exec();
}
from http://www.zetcode.com/gui/qt4/firstprograms/
In general terms I'm interested in creating my own signal slot system using only the C++ standard library ( no boost signal, no QT, no nothing else ) so I'm doing this for research purpose and I'm only interested on the infrastructure about signals and slots.
Thanks.
Add the Q_OBJECT macro to the private section of your class, so moc converts it.
class MyButton : public QWidget
{
Q_OBJECT
public:
MyButton(QWidget *parent = 0);
};
Here is what the documentation says.