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Qt文档阅读笔记-QScopedPointer解析及实例

发布日期：2021-06-30 10:43:11 浏览次数：3 分类：技术文章

本文共 4251 字，大约阅读时间需要 14 分钟。

当指针超出范围后就会删除被引用的对象。

与QPointer不同，他可以在任意类型中使用（QPointer只能在identity type中使用）

4个不同的清除类

1. QScopedPointerDeleter

2. QScopedPointerArrayDeleter

3. QScopedPointerArrayDeleter

4. QScopedPointerDeleteLater

如下例子：QScopedPointer<int, QScopedPointerArrayDeleter<int>> intArrayPointer(new int(100));

在如下几个方面用得比较多：

异常

使代码简洁

通常用于删除动态分配的变量。

QPointerQScopedPointer基本上用用动态分配的对象，在范围外自动进行释放，这个范围是通过{}包起来的，也就是函数的生存周期。

如下面这个原始代码：

void myFunction(bool useSubClass)  {      MyClass *p = useSubClass ? new MyClass() : new MySubClass;      QIODevice *device = handsOverOwnership();      if (m_value > 3) {          delete p;          delete device;          return;      }      try {          process(device);      }      catch (...) {          delete p;          delete device;          throw;      }      delete p;      delete device;  }

使用了QScopedPointer后：

void myFunction(bool useSubClass)  {      // assuming that MyClass has a virtual destructor      QScopedPointer
   
     p(useSubClass ? new MyClass() : new MySubClass);      QScopedPointer
    
      device(handsOverOwnership());      if (m_value > 3)          return;      process(device);  }

带限定符的C++指针使用QScopedPointer：

const QWidget *const p = new QWidget();      // is equivalent to:      const QScopedPointer
   
     p(new QWidget());      QWidget *const p = new QWidget();      // is equivalent to:      const QScopedPointer
    
      p(new QWidget());      const QWidget *p = new QWidget();      // is equivalent to:      QScopedPointer
     
       p(new QWidget());

自定义清空

相当于delete操作QScopedPointerDeleter

相当于delete []操作QScopedPointerArrayDeleter

相当于free操作QScopedPointerPodDeleter

相当于deleteLater操作QScopedPointerDeleteLater

代码如下：

// this QScopedPointer deletes its data using the delete[] operator:  QScopedPointer

> arrayPointer(new int[42]); // this QScopedPointer frees its data using free(): QScopedPointer

podPointer(reinterpret_cast

(malloc(42))); // this struct calls "myCustomDeallocator" to delete the pointer struct ScopedPointerCustomDeleter { static inline void cleanup(MyCustomClass *pointer) { myCustomDeallocator(pointer); } }; // QScopedPointer using a custom deleter: QScopedPointer

customPointer(new MyCustomClass);

下面是在类声明的时候要注意的问题：

class MyPrivateClass; // forward declare MyPrivateClass  class MyClass  {  private:      QScopedPointer
   
     privatePtr; // QScopedPointer to forward declared class  public:      MyClass(); // OK      inline ~MyClass() {} // VIOLATION - Destructor must not be inline  private:      Q_DISABLE_COPY(MyClass) // OK - copy constructor and assignment operators                               // are now disabled, so the compiler won't implicitely                               // generate them.  };

运行截图如下：

代码如下：

#ifndef CLASSA_H#define CLASSA_H#include 
   
    #include 
    
     class ClassA;struct ClassACustomDeleter{    static inline void cleanup(ClassA *p){        qDebug() << "static inline void cleanup(ClassA *p) called";        delete p;    }};class ClassA{public:    ClassA();    ~ClassA();    const QString &getStr();    const int &getValue();private:    QString m_str;    int m_value;};#endif // CLASSA_H

ClassA.cpp

#include "ClassA.h"#include 
   
    ClassA::ClassA() : m_str("字符串"), m_value(10){}ClassA::~ClassA(){    qDebug() << "ClassA::~ClassA() called";}const QString &ClassA::getStr(){    return m_str;}const int &ClassA::getValue(){    return m_value;}

main.cpp

#include 
   
    #include 
    
     #include 
     
      #include "ClassA.h"void normalFunction(){    ClassA *a = new ClassA;    try {        if(0 == 0){            throw "error";        }    }    catch(char const *err) {        qDebug() << err;        delete a;        return;    }    //want to do sth    //...    //well    delete a;}void usingQScopedFunction(){    QScopedPointer
      
        p(new ClassA);}void usingQScopedCustomFunction(){    QScopedPointer
       
         p(new ClassA);}int main(int argc, char *argv[]){    QCoreApplication a(argc, argv);    normalFunction();    qDebug() << "------------华丽的分割线------------";    usingQScopedFunction();    qDebug() << "------------华丽的分割线------------";    usingQScopedCustomFunction();    return a.exec();}

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