main.cpp

#include "Counter.h"
#include <QtCore/QDebug>

void Counter::setValue(int value)
{
    if (value != m_value)
    {
        m_value = value;
        emit valueChanged(value);
    }
}

int main(void)
{
    Counter a, b;
    QObject::connect(&a, SIGNAL(valueChanged(int)), &b, SLOT(setValue(int)));

    a.setValue(12); // a.value() == 12, b.value() == 12
    b.setValue(48); // a.value() == 12, b.value() == 48

    qDebug() << "a value " << a.value();
    qDebug() << "b value " << b.value();

    return 0;
}

Counter.h

#pragma once

#include <QtCore/QObject>

class Counter : public QObject
{
    Q_OBJECT

public:
    Counter() { m_value = 0; }

    int value() const { return m_value; }

public slots:
    void setValue(int value);

signals:
    void valueChanged(int newValue);

private:
    int m_value;
};

demo.pro

QT += core

TARGET = qtdemo

SOURCES += main.cpp

CONFIG += console 

HEADERS  += Counter.h

編譯運行

mkdir build && cd build
qmake .. && make