How to send message from child widget to parent window in Qt? - c++

Question

How do I send a message from a child widget to a parent window in qt?
I tried sending a signal from the child widget to the parent window in qt. When I call the function test in subwidget.cpp, the signal is sent but the mainwindow slot does not execute. How can I send the message?
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QStackedWidget>
#include <QPushButton>
#include <QProcess>
#include <QDebug>
#include <subwidget.h>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
void check_adb_exists();
private:
Ui::MainWindow *ui;
QStackedWidget *stack;
QPushButton *start_btn;
SubWidget *f2;
private slots:
void StartApplication();
void ReceiveCustomMessage(const QString &msg);
};
#endif // MAINWINDOW_H
subwidget.h
#define SUBWIDGET_H
#include <QWidget>
namespace Ui {
class SubWidget;
}
class SubWidget : public QWidget
{
Q_OBJECT
public:
explicit SubWidget(QWidget *parent);
~SubWidget();
void test();
private:
Ui::SubWidget *ui;
signals:
void SendCustomMessage(const QString& msg);
};
#endif // SUBWIDGET_H
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
// #include <formcustomnew.h>
#include <subwidget.h>
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
stack = MainWindow::findChild<QStackedWidget *>("stackedWidget");
start_btn = MainWindow::findChild<QPushButton *>("start_btn");
stack->setCurrentIndex(0);
connect(start_btn,SIGNAL(released()),this,SLOT(StartApplication()));
f2 = new SubWidget(this);
//connect(f2,SIGNAL(SendCustomMessage(QString)),this,SLOT(ReceiveCustomMessage(QString)));
//f2->test();
//auto f1 = new FormCustomNew(this);
//connect(f1,SIGNAL(sendMessageNewMessage(QString)),this,SLOT(receiveMessage(QString)));
//f1->test();
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::StartApplication(){
//check_adb_exists();
f2->test();
}
void MainWindow::ReceiveCustomMessage(const QString &msg){
qDebug("Recieved message from child");
qDebug("Message: " + msg.toLatin1());
}
void MainWindow::check_adb_exists(){
QProcess *p = new QProcess();
connect(p,&QProcess::readyReadStandardOutput,[&](){
auto data = p->readAllStandardOutput();
qDebug("Stdout: " + data);
});
connect(p,&QProcess::readyReadStandardError,[&](){
auto data = p->readAllStandardError();
qDebug("Error: " + data);
if(data.toStdString().compare("File Not Found")){
qDebug("File Not Found is the error");
}
});
QStringList args;
args << "/c dir C:\\Users\\%USERNAME%\\AppData\\Local\\Android\\Sdk";
p->setArguments(args);
p->setProgram("C:\\Windows\\System32\\cmd.exe");
p->start();
}
subwidget.cpp
#include "subwidget.h"
#include "ui_subwidget.h"
SubWidget::SubWidget(QWidget *parent) :
QWidget(parent),
ui(new Ui::SubWidget)
{
ui->setupUi(this);
qDebug("parent: " + parent->objectName().toLatin1());
connect(this,SIGNAL(SendCustomMessage(QString)),parent,SLOT(ReceiveCustomMessage(QString)));
}
SubWidget::~SubWidget()
{
delete ui;
}
void SubWidget::test(){
emit SendCustomMessage("trial message");
}
void SubWidget::SendCustomMessage(const QString &msg){
qDebug("Sending Message: " + msg.toLatin1());
}

Answer 1

Signals must not be defined in Qt.
From the Qt wiki on Signal & Slots:
Signals are automatically generated by the moc and must not be implemented in the .cpp file
Remove your implementation and this should work. However you should not be binding the signal within subclass as that reduces encapsulation and reusability (parent must have a ReceiveCustomMessage(QString) slot). Instead bind it outside, as you have in your commented out code.