QT槽函数获取信号发送对象

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Qt 在槽函数中获取信号发送对象

Qt中提供了一个函数 qobject_cast(QObject *object),可以通过这个函数判断信号发出对象

Qt 帮助文档的解释：

Returns the given object cast to type T if the object is of type T (or of a subclass); otherwise returns nullptr. If object is nullptr then it will also return nullptr.

The class T must inherit (directly or indirectly) QObject and be declared with the Q_OBJECT macro.

A class is considered to inherit itself.

The qobject_cast() function behaves similarly to the standard C++ dynamic_cast(), with the advantages that it doesn't require RTTI support and it works across dynamic library boundaries.

• 简单来说 就是使用这个函数会得到你转型的的一个实例，但是这个类必须继承自QObject 或者其子类并且声明Q_OBJECT 这个宏
• QT 帮助文档中一个Example

  QObject *obj = new QTimer;
QTimer *timer = qobject_cast<QTimer *>(obj);


这里顺便记录一下RTTI 与RAII，RAII 通常应用于对象资源管理，RTTI 可以动态判断对象类型，但使用RTTI会增加程序运行时间，这里简单记录区分一下 。

1. RTTI : Run-time type information

   #include <iostream>
#include <typeinfo>class Base {
public:
virtual ~Base() = default;
};class Derived : public Base {};int main() {
Base base;
Derived derived;
Base* ptr = &derived;
Base& ref = derived;
std::cout << typeid(base).name()<< std::endl; // class Base
std::cout << typeid(derived).name()<< std::endl; // class Derived
std::cout << typeid(ptr).name()<< std::endl; // class Base *
std::cout << typeid(*ptr).name() << std::endl; //class Derived
std::cout << typeid(ref).name() << std::endl; //class Derived}


2. RAII : Resource Acquisition Is Initialization

   代码来源 https://en.cppreference.com/w/cpp/language/raii
std::mutex m;void bad()
{
m.lock(); // acquire the mutex
f(); // if f() throws an exception, the mutex is never released
if(!everything_ok()) return; // early return, the mutex is never released
m.unlock(); // if bad() reaches this statement, the mutex is released
}void good()
{
std::lock_guard<std::mutex> lk(m); // RAII class: mutex acquisition is initialization
f(); // if f() throws an exception, the mutex is released
if(!everything_ok()) return; // early return, the mutex is released
}


下面是QT通过qobject_cast获取信号发送对象的一个Demo,通过Qt Desinger 绘制两个按钮和一个文本框，将两个按钮的点击事件连接到同一个槽函数，在槽函数里面判断信号的发送者并作出不同的响应

主要的代码如下：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H#include <QMainWindow>QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACEclass MainWindow : public QMainWindow
{
Q_OBJECTpublic:
MainWindow(QWidget *parent = nullptr);
~MainWindow();public slots:
void onButtonClicked();private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H#include "mainwindow.h"
#include "ui_mainwindow.h"
#include<QDebug>
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
connect(ui->leftButton,&QPushButton::clicked,this,&MainWindow::onButtonClicked);
connect(ui->rightButton,&QPushButton::clicked,this,&MainWindow::onButtonClicked);}MainWindow::~MainWindow()
{
delete ui;
}void MainWindow::onButtonClicked()
{
QPushButton *button = qobject_cast<QPushButton *>(sender()); ui->textLabel->setText(button->text()); if(ui->leftButton == button)
{
qDebug()<<"left Button clicked";
ui->textLabel->setStyleSheet("background-color:yellow");
button->setStyleSheet("background-color:yellow"); }
else
{
ui->textLabel->setStyleSheet("background-color:green");
button->setStyleSheet("background-color:green");
}}

完整的代码已上传Github