举个例子来陈述这个问题:

1、写一个C++方法

class A: public B

{
    Q_OBJECT
public:
    A(QQuickItem *parent = 0);
    ~A();
    Q_INVOKABLE void init();     //注意这里我们用到了Q_INVOKABLE这个宏

}

2、注册这个类

qmlRegisterType<A>("A", 1, 0, "A");

3、QML中调用

import QtQuick 2.5
import A1.0

Rectangle {
    color: "transparent"
    property alias painter: painter

    Component.onCompleted: {
        painter.penColor = "red"
        painter.init()
    }

    A{
        id: painter;
        onWidthChanged: {
            painter.init()
        }
    }
}