在 C++ 中“重新创建”对象不会重置属性值。 (Qt)

Question

我认为只写下代码会更容易，但我想要的是“重新创建”一个对象。 所以在这个简单的例子中，我有一个带有 int 作为属性的类。当我使用重新创建课程时 object = new Object();，int保持不变，不重置。

这是简单问题的代码（看起来过于复杂，但这是有原因的）：

main.cpp

#include <iostream>
#include "main.h"

using namespace std;

int main(int argc, const char * argv[]) {
    Object *object = new Object();
    object->setNumber(4);
    cout << endl << object->getNumber()<<endl;
    restart(object);
    cout << endl << object->getNumber()<<endl;
    return 0;
}

void restart(Object *object)
{
    object = new Object();
}

main.h

#ifndef test_main_h
#define test_main_h
#include "object.h"

void restart(Object *object);

#endif

object.cpp

using namespace std;

Object::Object()
{
    m_number = new int(2);
}

void Object::setNumber(int number)
{
    m_number = new int(number);
}

int Object::getNumber()
{
    return *m_number;
}

object.h

#ifndef __test__object__
#define __test__object__

#include <stdio.h>

class Object
{
public:
    Object();
    void setNumber(int number);
    int getNumber();

private:
    int *m_number;
};

#endif /* defined(__test__object__) */

所以输出将是：

4

4
Program ended with exit code: 0

我们希望最后一个输出为 2。

现在如果你想知道我为什么要做这一切，那是因为我正在用 Qt 做一个程序，我也遇到了同样的问题，但有点复杂：

main.cpp

#include "mainwindow.h"
#include <QApplication>

 int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Main main;
    main.show();
    return a.exec();
}

主窗口.h

#ifndef LESSON_H
#define LESSON_H

#include <QAudioProbe>
#include <QAudioRecorder>
#include <QDir>
#include <QFileDialog>
#include <QMediaRecorder>
#include <fstream>
#include <string>
#include <QVariant>
#include <QDir>
#include <QtCore>
#include <QtWidgets>
#include <QMediaPlayer>
#include <QKeyEvent>

 class MainWindow : public QWidget
{
    Q_OBJECT

 public:
    MainWindow();

 private slots:
    void togglePause();
    void toggleRecord();
    void setEnabled();

 signals:
    void end();

 private:
    QPushButton *m_record;
    QPushButton *m_recordPause;
    QPushButton *m_enableB;
    bool m_enable;
    QLabel *m_recordState;
    QMediaPlayer *m_audio;
    QAudioRecorder *m_audioRecorder;
    QString m_dossier;
};

 class Main : public QWidget
{
    Q_OBJECT

 public:
    Main();
 private slots:
    void restart();

 private:
    MainWindow *main;
};

#endif // LESSON_H

主窗口.cpp

#include "mainwindow.h"

 using namespace std;

 MainWindow::MainWindow()
{

    m_record = new QPushButton("Record");
    m_recordPause = new QPushButton("Pause");
    m_recordPause->setEnabled(false);
    m_enableB = new QPushButton("Paste enabled");
    m_recordState = new QLabel("Recording paused.");
    m_recordState->setAlignment(Qt::AlignCenter);

    m_enable = true;

    QGridLayout *layout = new QGridLayout;
    layout->addWidget(m_record,0,0,1,1);
    layout->addWidget(m_recordPause,1,0,1,1);
    layout->addWidget(m_enableB,2,0,1,1);
    layout->addWidget(m_recordState,3,0,1,1);
    setLayout(layout);

     m_dossier = QFileDialog::getSaveFileName(this, "Save Audio File To:", QString(), "Audio (*.wav)");

     m_audioRecorder = new QAudioRecorder(this);
     m_audioRecorder->setOutputLocation(QUrl::fromLocalFile(m_dossier));
     m_audioRecorder->setAudioInput(m_audioRecorder->defaultAudioInput());

     QAudioEncoderSettings settings;
     settings.setCodec("audio/amr");
     settings.setSampleRate(16000);
     settings.setBitRate(32);
     settings.setQuality(QMultimedia::HighQuality);
     settings.setEncodingMode(QMultimedia::ConstantQualityEncoding);

     m_audioRecorder->setEncodingSettings(settings);

     m_audio = new QMediaPlayer;
     m_audio->setMedia(QUrl::fromLocalFile(m_dossier));

     connect(m_recordPause, SIGNAL(clicked()), this, SLOT(togglePause()));
     connect(m_record, SIGNAL(clicked()), this, SLOT(toggleRecord()));
     connect(m_enableB, SIGNAL(clicked()), this, SLOT(setEnabled()));
}

void MainWindow::toggleRecord()
{
     if (m_audioRecorder->state() == QMediaRecorder::StoppedState) {
        m_record->setText("Stop");
        m_recordPause->setEnabled(true);
        m_audioRecorder->record();
        m_recordState->setText("Recording.");
    }
     else
    {
        m_audioRecorder->stop();
        m_audio->setMedia(QUrl::fromLocalFile(m_dossier));
        emit end();
    }
}

      void MainWindow::togglePause()
{
     if (m_audioRecorder->state() != QMediaRecorder::PausedState)
    {
        m_audioRecorder->pause();
        m_recordPause->setText("Resume");
        m_recordState->setText("Recording paused.");
    }
    else
    {
        m_audioRecorder->record();
        m_recordPause->setText("Pause");
        m_recordState->setText("Recording.");
    }
}

     void MainWindow::setEnabled()
{
    if(m_enable)
    {
        m_enableB->setText("Paste disabled");
        m_enable = false;
    }
    else
    {
        m_enableB->setText("Paste enable");
        m_enable = true;
    }
}

 Main::Main()
{
    main = new MainWindow();
    QGridLayout *layout = new QGridLayout;
    layout->addWidget(main,0,0,1,1);
    setLayout(layout);
    this->setFixedSize(200,200);
    this->setWindowTitle("Notes");
    connect(main, SIGNAL(end()), this, SLOT(restart()));
}

 void Main::restart()
{
    main = new MainWindow();
}

非常感谢您的帮助。

Answer 1

嗯，您需要了解指针在 C 和 C++ 中的工作原理。当你写这个时：

void restart(Object *object)
{
    object = new Object();
}

您正在复制指针，指向新创建对象的指针将被分配给函数内部的“内部”复制变量。我想你打算写的是改变外面的指针。

此外，如果在外部更改指针，则需要确保旧对象已被破坏。

话虽如此，这是一种非常糟糕的重置对象的方法。在我看来，你应该忘记这个概念有几个原因。首先，它的性能代价可能很高。你基本上破坏然后再次构建。例如，如果您有足够的可用内存，我会建议缓存默认值，但毕竟这都是关于权衡的，不是吗...

如果你仍然坚持，这里是你需要在实践中应用的代码：

您需要为此应用以下代码：

void restart(Object **object)
{
    delete *object;
    *object = new Object();
}

然后你的调用应该是这样的：

restart(&object);