在Singleton类的情况下我应该如何编写复制构造函数？如何重载=运算符？

Question

How should I write a copy constructor for my singleton class to prevent the creation of a new object as I already have one . And what is the best practice to overload = operator for same

 #include <iostream>
 #include <stdio.h>
 #include <conio.h>

 using namespace std;

 class Rect
 { 
  int length;
  int breadth;
  static int count;
  static int maxcount;
  Rect()
  {};
  Rect(const Rect& abc){};
  public :

      ~Rect();
      int area_rect()          
      {return length*breadth;}
      void set_value(int a,int b);

      static Rect* instance()
      {     
            Rect* ptr=NULL;
            if(count < maxcount)
            {
              ptr=new Rect ;
              count++;
            }
             return ptr;
       }
     };
    int Rect::count = 0;
    int Rect::maxcount = 1;
    void Rect::set_value(int a,int b)
   {
    length=a;
    breadth=b;
   }
  Rect::~Rect()
  {
   count --;          
  }  
 int main()
 {
  Rect* a= Rect::instance();  //creates first object
  // Rect* c= Rect::instance();  //fails to create second object 
  //as maxcount=1 and returns NULL
  a->set_value(10,3);
 cout << "area realted to object a : "  << a->area_rect() <<"\n"; 
 Rect* b=a;//allows creation of second object which I dont want
 b->set_value(10,4);
 cout << "area realted to object b : "  << b->area_rect() <<"\n"; 
 delete a;
 delete b;
 getch();
 return 0;
}       

How to write copy constructor code and overloading equal operator for prevention of creation of further object?

Answer 1

Either you make it non-copyable as explained here

How do I make this C++ object non-copyable?

or you define the copy constructor and assignment operators such that you get the same singleton. Depending on the functionality you actually want.

Assignment should typically also be forbidden (as on the link above):

class Rect{
     Rect( const Rect& ) = delete;
     Rect& operator=( const Rect& ) = delete;
     . . .
}

This also forbids move operations.

You may also want to know this: Are Singletons really that bad?

Answer 2

Singletons are ridiculous, just use free functions.

Still, to answer your question...

C++11:

class Foo {
public:
    Foo (const Foo &) = delete;
    Foo & operator = (const Foo &) = delete;
};

C++03;

class Foo {
private:
    // Don't write bodies.
    Foo (const Foo &);
    Foo & operator = (const Foo &);
};

Answer 3

Here is related answer : if you don't want someone to create an object, don't give them public constructor!

Seriously, you can have a factory object, make it a friend to class which you want to restrict construction of, and make constructor of that class private. This way the only way to access an object (without even necessarily creating it at this time) would be through your factory.

eg

#include <boost/noncopyable.hpp>
#include <string>

class ConnectionFactory;

class DatabaseConnection : boost::noncopyable {
  // can only be created by ConnectionFactory which happens to 
  // know how to connect to our database server!
  friend class ConnectionFactory;
  DatabaseConnection(std::string username, std::string password /*etc*/);
  // don't want any random user to reset the connection!
  ~DatabaseConnection();

public:
  // public interface bits
  void Execute(const Select&);
  // ...
};

ConnectionFactory {
public:
  DatabaseConnection& conn();
};

class MayNeedDbConnection {
  ConnectionFactory factory;
public:
  explicit MayNeedDbConnection(const ConnectionFactory& f) : factory(f)
  {}

  void SelectStuff() {
    DatabaseConnection& conn = factory.conn();
    conn.Execute(....);
  }
};

int main()
{
  ConnectionFactory factory;
  MayNeedDbConnection md(factory);
  md.SelectStuff();
}