1、设置父对象的作用

众所周知，Qt中，有为对象设置父对象的方法——setParent。

而设置父对象的作用主要有，在父对象析构的时候，会自动去析构其子对象。如果是一个窗口对象，如果其父对象设置了样式表(Style Sheet)，子对象也会继承父对象的样式。

所以，这篇文章，咱们主要看一下setParent的源码以及QObject是怎么进行对象管理的。

本文福利，莬费领取Qt开发学习资料包、技术视频，内容包括（C++语言基础，C++设计模式，Qt编程入门，QT信号与槽机制，QT界面开发-图像绘制，QT网络，QT数据库编程，QT项目实战，QSS，OpenCV，Quick模块，面试题等等）↓↓↓↓↓↓见下面↓↓文章底部点击莬费领取↓↓

2、设置父对象(setParent)

我们可以看到，setParent这个函数就是调用了QObjectPrivate类的setParent_helper这个函数。

void QObject::setParent(QObject *parent)
{
    Q_D(QObject);
    Q_ASSERT(!d->isWidget);
    d->setParent_helper(parent);
}

所以，我们进一步分析setParent_helper这个函数

3、完整源码

void QObjectPrivate::setParent_helper(QObject *o)
{
    Q_Q(QObject);
	// 不能把自己设为父对象
    Q_ASSERT_X(q != o, Q_FUNC_INFO, "Cannot parent a QObject to itself");
#ifdef QT_DEBUG
	// 检查对象树的循环
    const auto checkForParentChildLoops = qScopeGuard([&](){
        int depth = 0;
        auto p = parent;
        while (p) {
            if (++depth == CheckForParentChildLoopsWarnDepth) {
                qWarning("QObject %p (class: '%s', object name: '%s') may have a loop in its parent-child chain; "
                         "this is undefined behavior",
                         q, q->metaObject()->className(), qPrintable(q->objectName()));
            }
            p = p->parent();
        }
    });
#endif

	// 如果要设置的父对象就是当前的父对象，直接返回
    if (o == parent)
        return;

    if (parent) {
        QObjectPrivate *parentD = parent->d_func();
        if (parentD->isDeletingChildren && wasDeleted
            && parentD->currentChildBeingDeleted == q) {
            // don't do anything since QObjectPrivate::deleteChildren() already
            // cleared our entry in parentD->children.
        } else {
            const int index = parentD->children.indexOf(q);
            if (index < 0) {
                // we're probably recursing into setParent() from a ChildRemoved event, don't do anything
            } else if (parentD->isDeletingChildren) {
                parentD->children[index] = 0;
            } else {
				// 如果对象已经存在父对象的列表中，将原先存在的对象删除，并发送事件
                parentD->children.removeAt(index);
                if (sendChildEvents && parentD->receiveChildEvents) {
                    QChildEvent e(QEvent::ChildRemoved, q);
                    QCoreApplication::sendEvent(parent, &e);
                }
            }
        }
    }
	// 设置父对象
    parent = o;
    if (parent) {
        // object hierarchies are constrained to a single thread
        if (threadData != parent->d_func()->threadData) {
            qWarning("QObject::setParent: Cannot set parent, new parent is in a different thread");
            parent = nullptr;
            return;
        }
		// 父对象添加子对象，并发送事件
        parent->d_func()->children.append(q);
        if(sendChildEvents && parent->d_func()->receiveChildEvents) {
            if (!isWidget) {
                QChildEvent e(QEvent::ChildAdded, q);
                QCoreApplication::sendEvent(parent, &e);
            }
        }
    }
    if (!wasDeleted && !isDeletingChildren && declarativeData && QAbstractDeclarativeData::parentChanged)
        QAbstractDeclarativeData::parentChanged(declarativeData, q, o);
}

4、片段分析

一些先决条件的判断

判断设置的父对象是否是自己

	// 不能把自己设为父对象 
	Q_ASSERT_X(q != o, Q_FUNC_INFO, "Cannot parent a QObject to itself"); 
	/*...*/ 
	// 如果要设置的父对象就是当前的父对象，直接返回 
	if (o == parent)    return;

判断原来的父对象是否处于正在删除子对象的过程中，并且当前对象已经被删除了，如果是，则什么都不做（有点迷惑）

	if (parentD->isDeletingChildren && wasDeleted            
		&& parentD->currentChildBeingDeleted == q) {
	            // don't do anything since QObjectPrivate::deleteChildren() 
	            //already  cleared our entry in parentD->children.
	 }

判断是不是通过从ChildRemoved事件递归到setParent()

if (index < 0) {                
	// we're probably recursing into setParent() from a ChildRemoved event,
	// don't do anything 
} else if (parentD->isDeletingChildren) {    
	parentD->children[index] = 0; 
}

判断对象是不是已存在父对象的列表中，如果存在，就将对象删除，并发送事件

else { 			
// 如果对象已经存在父对象的列表中，将原先存在的对象删除，并发送事件                
	parentD->children.removeAt(index);                
	if (sendChildEvents && parentD->receiveChildEvents) {                    
		QChildEvent e(QEvent::ChildRemoved, q);
		QCoreApplication::sendEvent(parent, &e);                
	}            
}

设置的父对象，必须和当前对象在同一个线程，否则不能设置。

// 设置父对象
    parent = o;
    if (parent) {
        // object hierarchies are constrained to a single thread
        if (threadData != parent->d_func()->threadData) {
            qWarning("QObject::setParent: Cannot set parent, \
             new parent is in a different thread");
            parent = nullptr;
            return;
        }
		// 父对象添加子对象，并发送事件
        parent->d_func()->children.append(q);
        if(sendChildEvents && parent->d_func()->receiveChildEvents) {
            if (!isWidget) {
                QChildEvent e(QEvent::ChildAdded, q);
                QCoreApplication::sendEvent(parent, &e);
            }
        }
    }

对象的删除

然后就是对象的管理，也就是在父对象析构的时候，自动析构掉所有的子对象。这一个在我们使用窗口部件的时候很有用，因为一个界面可能有很多个子控件，比如按钮、label等，这时候，如果一个小窗口被关闭，我们也不需要一个一个的去析构，由Qt的对象树去进行析构就好了。

QObject::~QObject()
{
   /*...*/

	// 删除子对象
    if (!d->children.isEmpty())
        d->deleteChildren();

#if QT_VERSION < 0x60000
    qt_removeObject(this);
#endif
    if (Q_UNLIKELY(qtHookData[QHooks::RemoveQObject]))
        reinterpret_cast<QHooks::RemoveQObjectCallback>(qtHookData[QHooks::RemoveQObject])(this);

    Q_TRACE(QObject_dtor, this);

    if (d->parent)        // remove it from parent object
        d->setParent_helper(nullptr);
}

将所有的子对象进行删除，遍历容器，按照子对象所加入进来的顺序进行析构。

void QObjectPrivate::deleteChildren()
{
	// 清空子对象
    Q_ASSERT_X(!isDeletingChildren, "QObjectPrivate::deleteChildren()", "isDeletingChildren already set, did this function recurse?");
    isDeletingChildren = true;
    // delete children objects
    // don't use qDeleteAll as the destructor of the child might
    // delete siblings
    for (int i = 0; i < children.count(); ++i) {
        currentChildBeingDeleted = children.at(i);
        children[i] = 0;
        delete currentChildBeingDeleted;
    }
    children.clear();
    currentChildBeingDeleted = nullptr;
    isDeletingChildren = false;
}