QPushButton not executing connected function on click - c++

Question

I have a class called UserInterface. In this class there are different functions. build_start_screen() adds all the widgets (labels, buttons,...) for the initial startscreen. build_option_a_screen() removes everything from the startscreen and adds all the widgets needed for the screen when the users click on the button for option A, and so on. The class is stripped down for the sake of this question.
Now I have declared a button in build_start_screen() and connected it to a simple MessageBox.exec() so it should pop-up on click.
However, nothing happens when the button gets clicked.
What am I doing wrong? Has it something to do with the lifetime of variables expiring after the function finishes?
#include <QApplication>
#include <QPushButton>
#include <QAbstractButton>
#include <QLabel>
#include <QFont>
#include <QVBoxLayout>
#include <QMessageBox>
//Class handling all the UI in this Application
class UserInterface {
public:
//Build the initial UI the user sees
void build_start_screen(QWidget& window) {
//Make new QVBoxLayout for this startscreen UI
this->layout = new QVBoxLayout(&window);
//Test messagebox
QMessageBox msgBox;
msgBox.setText("Button test.");
//Button to go to Option A-screen
QPushButton* showMsgBox = new QPushButton("Show pop-up");
QAbstractButton::connect(showMsgBox, SIGNAL (clicked()), &window, SLOT (msgBox.exec()));
//Add labels and button to QVBoxLayout
layout->addWidget(showMsgBox);
}
private:
//Properties
QVBoxLayout* layout;
};
int main(int argc, char **argv) {
QApplication app (argc, argv);
//Initialize Window
QWidget Window;
Window.resize(400, 250);
//Create new UserInterface object
//This will allow us to create different user-interfaces
//depending on the function we call
UserInterface* ui = new UserInterface();
ui->build_start_screen(Window);
Window.show();
return app.exec();
}
And what if I'd like to do the same, but instead of calling a messageBox I'd like to call another function?
#include <QApplication>
#include <QPushButton>
#include <QAbstractButton>
#include <QLabel>
#include <QFont>
#include <QVBoxLayout>
#include <QMessageBox>
//Class handling all the UI in this Application
class UserInterface {
public:
//Build the initial UI the user sees
void build_start_screen(QWidget& window) {
//Make new QVBoxLayout for this startscreen UI
this->layout = new QVBoxLayout(&window);
//Test messagebox
QMessageBox msgBox;
msgBox.setText("Button test.");
//Button to go to Option A-screen
QPushButton* showMsgBox = new QPushButton("Show pop-up");
QAbstractButton::connect(showMsgBox, SIGNAL (clicked()), &window, SLOT (build_option_a_screen()));
//Add labels and button to QVBoxLayout
layout->addWidget(showMsgBox);
}
void build_option_a_screen(QWidget& window) {
//Do stuff here with window
//e.g
window.resize(500, 500);
}
private:
//Properties
QVBoxLayout* layout;
};
int main(int argc, char **argv) {
QApplication app (argc, argv);
//Initialize Window
QWidget Window;
Window.resize(400, 250);
//Create new UserInterface object
//This will allow us to create different user-interfaces
//depending on the function we call
UserInterface* ui = new UserInterface();
ui->build_start_screen(Window);
Window.show();
return app.exec();
}

Answer 1

Your code has 2 problems:
The window "object" does not have slot "msgBox.exec()" as pointed out by the error:
QObject::connect: No such slot QWidget::msgBox.exec() in ../main.cpp:23
Correcting the above, the solution would be:
QObject::connect(showMsgBox, &QPushButton::clicked, &msgBox, &QMessageBox::exec);
but now the problem is that "msgBox" is a local variable that will be destroyed and cannot be displayed.
So the solution is to make msgBox a member of the class or a pointer (in the case of the pointer you must manage the dynamic memory to avoid memory leaks):
//Class handling all the UI in this Application
class UserInterface {
public:
//Build the initial UI the user sees
void build_start_screen(QWidget& window) {
//Make new QVBoxLayout for this startscreen UI
this->layout = new QVBoxLayout(&window);
msgBox.setText("Button test.");
//Button to go to Option A-screen
QPushButton* showMsgBox = new QPushButton("Show pop-up");
QObject::connect(showMsgBox, &QPushButton::clicked, &msgBox, &QMessageBox::exec);
//Add labels and button to QVBoxLayout
layout->addWidget(showMsgBox);
}
private:
//Properties
QVBoxLayout* layout;
QMessageBox msgBox;
};
Plus:
It is recommended not to use the old connection syntax as it has limitations and hides the problems.
It is recommended not to use the old connection syntax as it has limitations and hides the problems.
If you want to connect to a method of some kind that is not a QObject (for example X as you want the OP) then the solution is to use a lambda method:
//Class handling all the UI in this Application
class UserInterface {
public:
//Build the initial UI the user sees
void build_start_screen(QWidget& window) {
//Make new QVBoxLayout for this startscreen UI
this->layout = new QVBoxLayout(&window);
//Button to go to Option A-screen
QPushButton* showMsgBox = new QPushButton("Show pop-up");
QObject::connect(showMsgBox, &QPushButton::clicked, [this, &window](){
build_option_a_screen(window);
});
//Add labels and button to QVBoxLayout
layout->addWidget(showMsgBox);
}
void build_option_a_screen(QWidget& window) {
//Do stuff here with window
//e.g
window.resize(500, 500);
}
private:
//Properties
QVBoxLayout* layout;
};