counting objects in class

最新推荐文章于 2023-02-09 22:18:50 发布

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本文探讨了C++中几种实现对象计数的方法，包括使用静态成员变量、模板类及私有继承等手段，并分析了各种方法的优缺点。

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#include 
   
    
using namespace std;

//Version 1 : counting objects of a class
class A
{
private:
    static int count;
public:
    A(){++count;}
    A(const A&){++count;}
    ~A(){--count;}
    static int howMany(){return count;}
};
int A::count = 0;   //the static keyword must be ignored when defined

//Version 2: define a class which is only used to count the number of objects of a class
//By the relationship of new expressions and operator delete, we automake the counting of class instantiate

//define a class which used to count the class instantiates for other classes(so it is need to be a template class)
template
    
     
class Counter
{
private:
    static size_t count;
    //void operator delete(void *);
public:
    Counter(){++count;};
    Counter(const Counter &){++count;}
    ~Counter(){--count;}
    static size_t howMany(){return count;}
};
template
     
      
size_t Counter
      
       ::count = 0;

//将Counter直接作为data member 同样是会增加对象的大小，这显然也不是我们期望的.
class Widget
{
private:
    int x;
    Counter
       
         c;    //as a data member,is implemented as...
public:
    static const size_t howMany() 
    {
        return Counter
        
         ::howMany(); } }; //pubic Inherient //but the method require the base dtor is virtual!! //显然这是不合理的，类Widget2只是利用Counter来计数，就导致该类的对象size变大（因为vptr） //可以禁止“通过基类的指针来delete派生类的对象，于是乎将基类的operator delete声明为私有的” //但是很奇怪的是，将operator delete声明为私有的之后，new操作符也不能成功的调用了. //我自己认为是如果new成功了的话，最后却不能调用operator delete这显然会造成内存泄露，于是乎编译器本着早报告错误的原则，直接使得new不通过编译 class Widget2:public Counter
         
           { private: int x; public: static const size_t howMany() { return Counter
          
           ::howMany(); } }; //于是乎，经过尝试之后，我们陷入了困境。 //考虑上述的继承，我们要禁止的是通过基类的指针去释放派生类，但是我们的行为（将opearator delete声明为private）却使得我们连new都不行了（这其实是很合理的）。那么我们干脆就直接禁止基类的指针指向派生类的成员，于是乎 private 继承！！！ //Version 3: private Inherient class Widget3:private Counter
           
             { private: int x; public: // using Counter
            
             ::howMany; static const size_t howMany() { return Counter
             
              ::howMany(); } }; int main() { Widget3 a,b,c; Widget3 d = a; Widget3 e(b); Widget3 *p = new Widget3(); delete p; cout << Widget3::howMany() << endl; return 0; }

参考文献： http://www.drdobbs.com/cpp/counting-objects-in-c/184403484?pgno=1