【Qt】: QPointer详解

考古：《Qt智能指针之QScopedPointer》

QPointer是Qt框架中的一个智能指针类，用于安全地管理QObject派生对象的指针。它的主要功能是自动将指针置为nullptr，当它所指向的QObject对象被销毁时。这在Qt应用程序中非常有用，因为QObject对象通常会在其父对象被销毁时自动销毁。

一、`QPointer`的特性和用法

自动置空：
- QPointer会在其指向的QObject对象被销毁时自动将自身置为nullptr。这可以防止悬空指针（dangling pointer）问题。
使用场景：
- 适用于需要在多个地方引用同一个QObject对象的场景，尤其是当对象的生命周期不由你直接控制时。
- 常用于Qt信号和槽机制中，确保槽函数中使用的对象在信号发出时仍然有效。
与QSharedPointer和QScopedPointer的区别：
- QPointer不负责对象的内存管理，它只是一个观察者。
- QSharedPointer和QScopedPointer负责对象的生命周期管理，前者通过引用计数，后者通过作用域。

二、示例代码

以下是一个简单的示例，展示如何使用QPointer：

#include <QCoreApplication>
#include <QObject>
#include <QPointer>
#include <QDebug>

class MyObject : public QObject {
    Q_OBJECT
public:
    MyObject() { qDebug() << "MyObject created"; }
    ~MyObject() { qDebug() << "MyObject destroyed"; }
};

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);

    MyObject* obj = new MyObject();
    QPointer<MyObject> ptr(obj);

    qDebug() << "Pointer is valid:" << !ptr.isNull();

    delete obj;  // Manually delete the object

    qDebug() << "Pointer is valid after deletion:" << !ptr.isNull();

    return a.exec();
}

1. 创建对象：
  MyObject* obj = new MyObject();：动态分配一个MyObject实例。
1. 使用QPointer：
  -QPointer<MyObject> ptr(obj);：创建一个QPointer，指向obj。
1. 检查指针有效性：
  ptr.isNull()：检查QPointer是否为空。
1. 删除对象：
  delete obj;：手动删除obj，此时ptr会自动变为nullptr。
1. 输出结果：
  在对象被删除后，ptr.isNull()返回true，表明指针已被置空。

关键点

安全性：QPointer提供了一种安全的方式来引用QObject对象，避免悬空指针。
非所有权：QPointer不负责对象的内存管理，只是一个观察者。
适用性：适用于需要在多个地方引用同一个QObject对象的场景，尤其是在信号和槽机制中。

三、`QPointer` 源码赏析

/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or (at your option) the GNU General
** Public license version 3 or any later version approved by the KDE Free
** Qt Foundation. The licenses are as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
** included in the packaging of this file. Please review the following
** information to ensure the GNU General Public License requirements will
** be met: https://www.gnu.org/licenses/gpl-2.0.html and
** https://www.gnu.org/licenses/gpl-3.0.html.
**
** $QT_END_LICENSE$
**
****************************************************************************/

#ifndef QPOINTER_H
#define QPOINTER_H

#include <QtCore/qsharedpointer.h>
#include <QtCore/qtypeinfo.h>

#ifndef QT_NO_QOBJECT

QT_BEGIN_NAMESPACE

class QVariant;

template <class T>
class QPointer
{
    Q_STATIC_ASSERT_X(!std::is_pointer<T>::value, "QPointer's template type must not be a pointer type");

    template<typename U>
    struct TypeSelector
    {
        typedef QObject Type;
    };
    template<typename U>
    struct TypeSelector<const U>
    {
        typedef const QObject Type;
    };
    typedef typename TypeSelector<T>::Type QObjectType;
    QWeakPointer<QObjectType> wp;
public:
    inline QPointer() { }
    inline QPointer(T *p) : wp(p, true) { }
    // compiler-generated copy/move ctor/assignment operators are fine!
    // compiler-generated dtor is fine!

#ifdef Q_QDOC
    // Stop qdoc from complaining about missing function
    ~QPointer();
#endif

    inline void swap(QPointer &other) { wp.swap(other.wp); }

    inline QPointer<T> &operator=(T* p)
    { wp.assign(static_cast<QObjectType*>(p)); return *this; }

    inline T* data() const
    { return static_cast<T*>( wp.data()); }
    inline T* operator->() const
    { return data(); }
    inline T& operator*() const
    { return *data(); }
    inline operator T*() const
    { return data(); }

    inline bool isNull() const
    { return wp.isNull(); }

    inline void clear()
    { wp.clear(); }
};
template <class T> Q_DECLARE_TYPEINFO_BODY(QPointer<T>, Q_MOVABLE_TYPE);

template <class T>
inline bool operator==(const T *o, const QPointer<T> &p)
{ return o == p.operator->(); }

template<class T>
inline bool operator==(const QPointer<T> &p, const T *o)
{ return p.operator->() == o; }

template <class T>
inline bool operator==(T *o, const QPointer<T> &p)
{ return o == p.operator->(); }

template<class T>
inline bool operator==(const QPointer<T> &p, T *o)
{ return p.operator->() == o; }

template<class T>
inline bool operator==(const QPointer<T> &p1, const QPointer<T> &p2)
{ return p1.operator->() == p2.operator->(); }

template <class T>
inline bool operator!=(const T *o, const QPointer<T> &p)
{ return o != p.operator->(); }

template<class T>
inline bool operator!= (const QPointer<T> &p, const T *o)
{ return p.operator->() != o; }

template <class T>
inline bool operator!=(T *o, const QPointer<T> &p)
{ return o != p.operator->(); }

template<class T>
inline bool operator!= (const QPointer<T> &p, T *o)
{ return p.operator->() != o; }

template<class T>
inline bool operator!= (const QPointer<T> &p1, const QPointer<T> &p2)
{ return p1.operator->() != p2.operator->() ; }

template<typename T>
QPointer<T>
qPointerFromVariant(const QVariant &variant)
{
    return QPointer<T>(qobject_cast<T*>(QtSharedPointer::weakPointerFromVariant_internal(variant).data()));
}

QT_END_NAMESPACE

#endif // QT_NO_QOBJECT

#endif // QPOINTER_H

QPointer 只有一个成员变量 QWeakPointer<QObjectType> wp; 几乎所有成员函数的操作都是对wp成员变量的操作。其他，比较简单，不过多赘述。

四、`QWeakPointer`

/****************************************************************************

template <class T>
class QWeakPointer
{
    typedef T *QWeakPointer:: *RestrictedBool;
    typedef QtSharedPointer::ExternalRefCountData Data;

public:
    typedef T element_type;
    typedef T value_type;
    typedef value_type *pointer;
    typedef const value_type *const_pointer;
    typedef value_type &reference;
    typedef const value_type &const_reference;
    typedef qptrdiff difference_type;

    bool isNull() const Q_DECL_NOTHROW { return d == nullptr || d->strongref.load() == 0 || value == nullptr; }
    operator RestrictedBool() const Q_DECL_NOTHROW { return isNull() ? nullptr : &QWeakPointer::value; }
    bool operator !() const Q_DECL_NOTHROW { return isNull(); }
    T *data() const Q_DECL_NOTHROW { return d == nullptr || d->strongref.load() == 0 ? nullptr : value; }

    inline QWeakPointer() Q_DECL_NOTHROW : d(nullptr), value(nullptr) { }
    inline ~QWeakPointer() { if (d && !d->weakref.deref()) delete d; }

#ifndef QT_NO_QOBJECT
    // special constructor that is enabled only if X derives from QObject
#if QT_DEPRECATED_SINCE(5, 0)
    template <class X>
    QT_DEPRECATED inline QWeakPointer(X *ptr) : d(ptr ? Data::getAndRef(ptr) : nullptr), value(ptr)
    { }
#endif
#endif

#if QT_DEPRECATED_SINCE(5, 0)
    template <class X>
    QT_DEPRECATED inline QWeakPointer &operator=(X *ptr)
    { return *this = QWeakPointer(ptr); }
#endif

    QWeakPointer(const QWeakPointer &other) Q_DECL_NOTHROW : d(other.d), value(other.value)
    { if (d) d->weakref.ref(); }
#ifdef Q_COMPILER_RVALUE_REFS
    QWeakPointer(QWeakPointer &&other) Q_DECL_NOTHROW
        : d(other.d), value(other.value)
    {
        other.d = nullptr;
        other.value = nullptr;
    }
    QWeakPointer &operator=(QWeakPointer &&other) Q_DECL_NOTHROW
    { QWeakPointer moved(std::move(other)); swap(moved); return *this; }
#endif
    QWeakPointer &operator=(const QWeakPointer &other) Q_DECL_NOTHROW
    {
        QWeakPointer copy(other);
        swap(copy);
        return *this;
    }

    void swap(QWeakPointer &other) Q_DECL_NOTHROW
    {
        qSwap(this->d, other.d);
        qSwap(this->value, other.value);
    }

    inline QWeakPointer(const QSharedPointer<T> &o) : d(o.d), value(o.data())
    { if (d) d->weakref.ref();}
    inline QWeakPointer &operator=(const QSharedPointer<T> &o)
    {
        internalSet(o.d, o.value);
        return *this;
    }

    template <class X>
    inline QWeakPointer(const QWeakPointer<X> &o) : d(nullptr), value(nullptr)
    { *this = o; }

    template <class X>
    inline QWeakPointer &operator=(const QWeakPointer<X> &o)
    {
        // conversion between X and T could require access to the virtual table
        // so force the operation to go through QSharedPointer
        *this = o.toStrongRef();
        return *this;
    }

    template <class X>
    bool operator==(const QWeakPointer<X> &o) const Q_DECL_NOTHROW
    { return d == o.d && value == static_cast<const T *>(o.value); }

    template <class X>
    bool operator!=(const QWeakPointer<X> &o) const Q_DECL_NOTHROW
    { return !(*this == o); }

    template <class X>
    inline QWeakPointer(const QSharedPointer<X> &o) : d(nullptr), value(nullptr)
    { *this = o; }

    template <class X>
    inline QWeakPointer &operator=(const QSharedPointer<X> &o)
    {
        QSHAREDPOINTER_VERIFY_AUTO_CAST(T, X); // if you get an error in this line, the cast is invalid
        internalSet(o.d, o.data());
        return *this;
    }

    template <class X>
    bool operator==(const QSharedPointer<X> &o) const Q_DECL_NOTHROW
    { return d == o.d; }

    template <class X>
    bool operator!=(const QSharedPointer<X> &o) const Q_DECL_NOTHROW
    { return !(*this == o); }

    inline void clear() { *this = QWeakPointer(); }

    inline QSharedPointer<T> toStrongRef() const { return QSharedPointer<T>(*this); }
    // std::weak_ptr compatibility:
    inline QSharedPointer<T> lock() const { return toStrongRef(); }

#if defined(QWEAKPOINTER_ENABLE_ARROW)
    inline T *operator->() const { return data(); }
#endif

private:

#if defined(Q_NO_TEMPLATE_FRIENDS)
public:
#else
    template <class X> friend class QSharedPointer;
    template <class X> friend class QPointer;
#endif

    template <class X>
    inline QWeakPointer &assign(X *ptr)
    { return *this = QWeakPointer<X>(ptr, true); }

#ifndef QT_NO_QOBJECT
    template <class X>
    inline QWeakPointer(X *ptr, bool) : d(ptr ? Data::getAndRef(ptr) : nullptr), value(ptr)
    { }
#endif

    inline void internalSet(Data *o, T *actual)
    {
        if (d == o) return;
        if (o)
            o->weakref.ref();
        if (d && !d->weakref.deref())
            delete d;
        d = o;
        value = actual;
    }

    Data *d;
    T *value;
};

QWeakPointer有两个成员变量，
Data *d; T *value;
它自身在构造函数中只是接收指针变量，并赋值給成员函数，因此他也是也是观察者。其他比较简单，不过多赘述。关于Qt中的引用计数原理和实现，后续另外进行介绍。