为什么在这个程序中不调用析构函数？

Question

我做了一个实例化这个类的函数，并做了一些操作。在函数返回之后，这意味着作用域已经结束。一旦作用域结束，它就应该调用类的析构函数。但是，我没有看到析构函数被调用。背后的原因是什么？我是不是漏掉了什么？

/***

The static member in the class is just a declaration, whose namespace scope is limited to the class.
One need to define it in the corresponding source of the class header. This way, the static member is
allocated a space on the Data segment. In case the static member is not defined, then the linker will
throw an error.

**/

class category_to_subcategory
{
     /** static members have just declaration not definition **/
     static QMultiMap<QString,QString>m_cat_to_subcat;

 public:
     /** The const can give a definition to it within the class, thus no need to define it **/
     /** C++ allows only "integer const static" types to be defined within a class */
     /** both are same const static or static const **/
     const static  int categ_const = 10;
     static void insert_subcat();
     /** template **/
     QList<QString> get_subcat(QString &cat);
     QList<QString> get_subcat(const QString &cat);
     ~category_to_subcategory();
};

/** When you want to map a string to string, you can use QMultimap **/
/** It is quite easy to get the values using QMultimap **/
/** definition of m_cat_to_subcat, the namescope is bind to the class name - category_to_subcategory  **/
QMultiMap<QString,QString> category_to_subcategory::m_cat_to_subcat;

 void category_to_subcategory::insert_subcat()
 {
     qDebug()<<__PRETTY_FUNCTION__;
     m_cat_to_subcat.clear();
     m_cat_to_subcat.insert("OS", "Linux");
     m_cat_to_subcat.insert("OS", "Windows");
     m_cat_to_subcat.insert("OS", "MAC");
     m_cat_to_subcat.insert("SHELL", "BASH");
     m_cat_to_subcat.insert("SHELL", "KSH");
     m_cat_to_subcat.insert("SHELL", "CSH");
     m_cat_to_subcat.insert("MOUSE", "WIRED");
     m_cat_to_subcat.insert("MOUSE", "WIRELESS");

 }

 QList<QString> category_to_subcategory::get_subcat(QString &cat)
 {
     qDebug()<<__PRETTY_FUNCTION__;
     if(category_to_subcategory::m_cat_to_subcat.empty()) {
          category_to_subcategory::insert_subcat();
      }
      QList<QString> subcat_list = m_cat_to_subcat.values(cat);
      return subcat_list;
 }

 QList<QString> category_to_subcategory::get_subcat(const QString &cat)
 {
     qDebug()<<__PRETTY_FUNCTION__;
     if(category_to_subcategory::m_cat_to_subcat.empty()) {
          category_to_subcategory::insert_subcat();
     }
     QList<QString> subcat_list = m_cat_to_subcat.values(cat);
     return subcat_list;
 }

category_to_subcategory::~category_to_subcategory()
{
   qDebug()<<__PRETTY_FUNCTION__;
   delete this;
}

void function_to_QMAP()
{
    qDebug()<<__PRETTY_FUNCTION__;
    category_to_subcategory *cat_to_subcat_Instance = new category_to_subcategory;
    QString cat = "OS";
    qDebug()<<cat_to_subcat_Instance->get_subcat(cat);
    cat = "SHELL";
    qDebug()<<cat_to_subcat_Instance->get_subcat(cat);
    cat = "MOUSE";
    qDebug()<<cat_to_subcat_Instance->get_subcat(cat);
    /** Passing just the string will throw an error **/
    //qDebug()<<cat_to_subcat_Instance->get_subcat("MOUSE");
    /** no matching function for call to 'category_to_subcategory::get_subcat(const char[6]); */

    qDebug()<<"The const category is"<<cat_to_subcat_Instance->get_subcat("OS");
    qDebug()<<"The static const integer defined in class value is "<<category_to_subcategory::categ_const;
}

void function_op_finish()
{
    qDebug()<<__PRETTY_FUNCTION__;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    function_to_QMAP();
    function_op_finish();
    return a.exec();
}

Answer 1

看起来你混淆了基于堆栈的内存分配(局部变量在堆栈上分配，并在函数退出时自动销毁)和基于堆的内存分配(使用‘C++’分配，程序员负责确保在某个时刻调用'delete‘)。

下面是关于不同之处的friendly tutorial，欢迎使用C++手动管理内存(这可能是该语言中错误和问题的最大来源)。请注意，该链接特别指向C，而不是C++，但两者的概念是相同的。

Answer 2

因为你不会delete cat_to_subcat_Instance。这是一个内存泄漏:对象被创建，但从未被销毁。

在您的特定代码片段中，我看到了动态分配cat_to_subcat_Instance的零需求。为什么不用简单的方法呢？

void function_to_QMAP()
{
    qDebug()<<__PRETTY_FUNCTION__;
    category_to_subcategory cat_to_subcat_Instance;
    QString cat = "OS";
    qDebug()<<cat_to_subcat_Instance.get_subcat(cat);
    cat = "SHELL";
    qDebug()<<cat_to_subcat_Instance.get_subcat(cat);
    cat = "MOUSE";
    qDebug()<<cat_to_subcat_Instance.get_subcat(cat);
    /** Passing just the string will throw an error **/
    //qDebug()<<cat_to_subcat_Instance.get_subcat("MOUSE");
    /** no matching function for call to 'category_to_subcategory::get_subcat(const char[6]); */

    qDebug()<<"The const category is"<<cat_to_subcat_Instance.get_subcat("OS");

    qDebug()<<"The static const integer defined in class value is "<<category_to_subcategory::categ_const;


}

Answer 3

您使用运算符new在堆中分配了一个category_to_subcategory类型的对象。

category_to_subcategory *cat_to_subcat_Instance = new category_to_subcategory;

要删除此对象，需要显式调用operator delete。

还要考虑到类category_to_subcategory的析构函数是无效的。

category_to_subcategory::~category_to_subcategory()
{
   qDebug()<<__PRETTY_FUNCTION__;
   delete this;
}

它的调用将导致析构函数的递归调用，直到堆栈内存耗尽。使用delete删除该语句。

category_to_subcategory::~category_to_subcategory()
{
   qDebug()<<__PRETTY_FUNCTION__;
}