对象树 & 所有权

概述

QObjects 在对象树中组织自身。当创建 QObject 采用另一对象作为父级，它被添加到父级的 children() 列表，和被删除当父级被删除时。事实证明，这种方式非常适合 GUI 对象的需要。例如， QShortcut (键盘快捷方式) 是相关窗口的子级，因此当用户关闭该窗口时，快捷方式也会被删除。

QQuickItem ，Qt Quick 模块的基本视觉元素，继承自 QObject , but has a concept of the visual parent which differs from that of the QObject parent . An item's visual parent may not necessarily be the same as its object parent. See 概念 - Qt Quick 中的可视父级了解更多细节。

QWidget , the fundamental class of the Qt Widgets module, extends the parent-child relationship. A child normally also becomes a child widget, i.e. it is displayed in its parent's coordinate system and is graphically clipped by its parent's boundaries. For example, when the application deletes a message box after it has been closed, the message box's buttons and label are also deleted, just as we'd want, because the buttons and label are children of the message box.

You can also delete child objects yourself, and they will remove themselves from their parents. For example, when the user removes a toolbar it may lead to the application deleting one of its QToolBar objects, in which case the tool bar's QMainWindow parent would detect the change and reconfigure its screen space accordingly.

The debugging functions QObject::dumpObjectTree () 和 QObject::dumpObjectInfo () are often useful when an application looks or acts strangely.

QObject 的构造/销毁次序

当 QObjects are created on the heap (i.e., created with new ), a tree can be constructed from them in any order, and later, the objects in the tree can be destroyed in any order. When any QObject in the tree is deleted, if the object has a parent, the destructor automatically removes the object from its parent. If the object has children, the destructor automatically deletes each child. No QObject is deleted twice, regardless of the order of destruction.

当 QObjects are created on the stack, the same behavior applies. Normally, the order of destruction still doesn't present a problem. Consider the following snippet:

int main()
{
    QWidget window;
    QPushButton quit("Quit", &window);
    ...
}

父级， window ，和子级， quit ，两者是 QObjects 因为 QPushButton 继承 QWidget ，和 QWidget 继承 QObject . This code is correct: the destructor of quit is not called twice because the C++ language standard (ISO/IEC 14882:2003) specifies that destructors of local objects are called in the reverse order of their constructors. Therefore, the destructor of the child, quit , is called first, and it removes itself from its parent, window , before the destructor of window 被调用。

But now consider what happens if we swap the order of construction, as shown in this second snippet:

int main()
{
    QPushButton quit("Quit");
    QWidget window;
    quit.setParent(&window);
    ...
}

In this case, the order of destruction causes a problem. The parent's destructor is called first because it was created last. It then calls the destructor of its child, quit , which is incorrect because quit is a local variable. When quit subsequently goes out of scope, its destructor is called again, this time correctly, but the damage has already been done.