IPlugin with Q_DECLARE_INTERFACE?

Hu Junhao

I‘m reading Qt Creator's source code.
Documents says that definite of an interface class as below:

//header
class FilterInterface
{
public:
    virtual ~FilterInterface() {}

    virtual QStringList filters() const = 0;
    virtual QImage filterImage(const QString &filter, const QImage &image,
                               QWidget *parent) = 0;
};

Q_DECLARE_INTERFACE(FilterInterface, "org.qt-project.Qt.Examples.PlugAndPaint.FilterInterface")

//source
#include <QObject>
#include <QtPlugin>
#include <QStringList>
#include <QImage>

#include <plugandpaint/interfaces.h>

class ExtraFiltersPlugin : public QObject, public FilterInterface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.Examples.PlugAndPaint.FilterInterface" FILE "extrafilters.json")
    Q_INTERFACES(FilterInterface)

public:
    QStringList filters() const;
    QImage filterImage(const QString &filter, const QImage &image,
                       QWidget *parent);
};

I found many of writings like this in code.
However, other usage like IPlugin, IContext, IMode is also used.
Why IPlugin don't use Q_DECLARE_INTERFACE like above?

jsulm

@Hu-Junhao Because the plugin implements the interface, it does not declare an interface.

Hu Junhao

@jsulm IPlugin is the interface.

class EXTENSIONSYSTEM_EXPORT IPlugin : public QObject
{
    Q_OBJECT
    //...
}

class HelloWorldPlugin
  : public ExtensionSystem::IPlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QtCreatorPlugin" FILE "HelloWorld.json")
    //...
}

why not

class IPlugin
{
public:
    virtual ~IPlugin() {}
    //...
};

Q_DECLARE_INTERFACE(FilterInterface, "org.qt-project.Qt.QtCreatorPlugin")
class HelloWorldPlugin
  : public QObject, public ExtensionSystem::IPlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QtCreatorPlugin" FILE "HelloWorld.json")
    Q_INTERFACES(IPlugin)
    //...
}

maybe IPlugin has a signal?