如何使用Qt插件中的宿主应用程序的类？

Question

我通过创建Qt C ++插件来扩展应用程序的功能来链接问题。代码编译和编写工作，但只要我使用Qt库类，例如QString。我给插件的类提供了一个对主机应用程序中定义的类的对象的引用，该项目不再链接。创建插件我按照Qt文档中的步骤进行操作，并考虑给定的示例 - echo和plug＆paint。但是，这种情况并不包括在内。

更新

这是错误：

myPlugin.obj：-1：错误：LNK2019：未解析的外部符号“public：class QString __cdecl myClass :: answer（void）const”（？answer @ myClass @@ QEBA？AVQString @@ XZ）在函数“public：virtual”中引用class QString __cdecl myPlugin :: echo（class myClass *）“（？echo @ myPlugin @@ UEAA？AVQString @@ PEAVmyClass @@@ Z）

以下是导致它的项目：

plug test.pro

TEMPLATE = subdirs

SUBDIRS += 
    host 
    plugin

host.pro

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = host
TEMPLATE = app


SOURCES += main.cpp
        MainWindow.cpp 
    myClass.cpp

HEADERS  += MainWindow.h 
    myClass.h 
    plugInterface.h

FORMS    += MainWindow.ui

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QMessageBox>
#include <QPluginLoader>
#include <QDir>
#include "myClass.h"
#include "plugInterface.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void on_button_clicked();

private:
    bool loadPlugin();

    Ui::MainWindow *ui;

    plugInterface *m_interface;

    myClass *m_class;
};

#endif // MAINWINDOW_H

myClass.h

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QObject>

class myClass : public QObject
{
    Q_OBJECT
public:
    explicit myClass(QObject *parent = 0);

    QString answer() const;
    void setAnswer(const QString &str);

private:
    QString m_answer;
};

#endif // MYCLASS_H

plugInterface.h

#ifndef PLUGINTERFACE_H
#define PLUGINTERFACE_H

#include <QString>
#include "myClass.h"

class plugInterface
{
public:
    virtual ~plugInterface() {}
    virtual QString echo(myClass *value) = 0;
};

#define PlugInterface_iid "example.suite.app.PluginInterface"

Q_DECLARE_INTERFACE(plugInterface, PlugInterface_iid)

#endif // PLUGINTERFACE_H

MainWindow.cpp

#include "MainWindow.h"
#include "ui_MainWindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    if (!loadPlugin())
    {
        QMessageBox::information(this, "Error", "Could not load the plugin");
    }

    m_class = new myClass(this);
    m_class->setAnswer("Good!");
}

MainWindow::~MainWindow()
{
    delete ui;
}

bool MainWindow::loadPlugin()
  {
      QDir pluginsDir(qApp->applicationDirPath());

      if (pluginsDir.dirName().toLower() == "debug" || pluginsDir.dirName().toLower() == "release")
      pluginsDir.cd("plugins");

      foreach (QString fileName, pluginsDir.entryList(QDir::Files))
      {
          QPluginLoader pluginLoader(pluginsDir.absoluteFilePath(fileName));
          QObject *plugin = pluginLoader.instance();
          if (plugin) {
              m_interface = qobject_cast<plugInterface *>(plugin);
              if (m_interface)
                  return true;
          }
      }

      return false;
  }

void MainWindow::on_button_clicked()
{
    ui->lineResult->setText(m_interface->echo(m_class));
}

myClass.cpp

#include "myClass.h"

myClass::myClass(QObject *parent) : QObject(parent)
{

}

QString myClass::answer() const
{
    return m_answer;
}

void myClass::setAnswer(const QString &str)
{
    m_answer = str;
}

plugin.pro

  TEMPLATE        = lib
  CONFIG         += plugin
  QT             += widgets
  INCLUDEPATH    += ../host
  HEADERS         = myPlugin.h
  SOURCES         = myPlugin.cpp
  TARGET          = $$qtLibraryTarget(myPlugin)
  DESTDIR         = ../plugins

myPlugin.h

#ifndef MYPLUGIN_H
#define MYPLUGIN_H

#include <QObject>
#include "plugInterface.h"

class myPlugin : public QObject, plugInterface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "example.suite.app.PluginInterface")
    Q_INTERFACES(plugInterface)

public:
    QString echo(myClass *value) Q_DECL_OVERRIDE;
};

#endif // MYPLUGIN_H

myPlugin.cpp

#include "myPlugin.h"

QString myPlugin::echo(myClass *value)
{
    return value->answer();
}