《More Effective C++》技术篇——限制某个class所能产生的对象数量

• 如下例，只要继承Counted template就可以限制class所能产生的对象数量，超过了设置的maxObjects就会抛出异常。

  #include <stdlib.h>
  #include <iostream>
  
  template<class BeingCounted>
  class Counted {
  public:
    class TooManyObjects{}; //这是可能被抛出的exceptions。
    static int objectCount() { return numObjects; }
  
  protected:
    Counted();
    Counted(const Counted& rhs);
    ~Counted() { --numObjects; }
  
  private:
    static int numObjects;
    static const size_t maxObjects;
  
    void init(); //用以避免ctor码重复出现。
  };
  
  template<class BeingCounted>
  Counted<BeingCounted>::Counted()
  { init(); }
  
  template<class BeingCounted>
  Counted<BeingCounted>::Counted(const Counted<BeingCounted>& rhs)
  { init(); }
  
  template<class BeingCounted>
  void Counted<BeingCounted>::init()
  {
    if (numObjects >= maxObjects)
    {
        throw TooManyObjects();
    }
  
    ++numObjects;
  }
  
  template<class BeingCounted>
  int Counted<BeingCounted>::numObjects = 0;
  
  class Printer: private Counted<Printer> {
  public:
    // pseudo-constructors
    static Printer* makePrinter() { return new Printer(); }
    static Printer* makePrinter(const Printer& rhs) { return new Printer(rhs); }
    ~Printer() {}
    
    using Counted<Printer>::objectCount;
    using Counted<Printer>::TooManyObjects;
  
  private:
    Printer() {}
    Printer(const Printer& rhs) {}
  };
  
  template<class Printer>
  const size_t Counted<Printer>::maxObjects = 5;
  
  int main()
  {
    auto p1 = Printer::makePrinter();
    auto p2 = Printer::makePrinter();
    auto p3 = Printer::makePrinter();
    std::cout << "now Printer num = " << p1->objectCount() << std::endl;
    auto p4 = Printer::makePrinter(*p1);
    auto p5 = Printer::makePrinter(*p2);
    std::cout << "now Printer num = " << p1->objectCount() << std::endl;
    //auto p6 = Printer::makePrinter(*p3); //抛出TooManyObjects的异常。
  }