What is the use of private constructor in C++?

[Juaquine]

Hello to all,
I am second year Computer science student. I recently started learning c++ language. In our last lecture we learn about private constructor, but I didn't understand this concept clearly. Can anyone tell me what is the use of private constructor in C++? Please help me. Thank you.

[absolute55]

As per my information private constructor is used to prevent your class object from being created in another place, other than this very class. You can use lots of use cases to provide such type of constructor. Using this private constructor your class object is created in a static method and after that your static method is defined as public.

class MyClassEg()
{
private:
  MyClassEg() { }

public:
  static MyClassEg * CreateInstanceEg() { return new MyClassEg(); }
};

[opaper]

Private constructor is used in c++ to create object of class only one time. It means that your class does not have more that one instance of your class. I have written following code for you. Just try to understand it.

class MyClassEg()
{
private:
  MyClassEg() { }

public:
  MyClassEg & Instances()
  {
    static MyClassEg * aGlobalInsts = new MyClassEg();
    return *MyClassEg;
  }
};

[MindSpace]

private constructor is same as any other constructor but it is declared as private. Due to this you can only access object of that class in only that class and not in other class. Most of the people use private constructors which do all the work than public constructor. You can use following code to understand the concept.

class MyClassEgs
{
public:
  MyClassEgs() : MyClassEgs(2010, 1, 1) { }

private:
  MyClassEgs(int theYears, int theMonths, int theDays) { /*code do real work */ }
};

[Modifier]

I think private constructor is used to prevent from object copying. It is very easy to use and implement. You have to just use Private access specifier before your constructor. When you use private constructor in your code, then it means that you are prevnting your class object from being used in another place.

class MyClass1
{
  SharedsResources * myResources;

private:
  MyClass1(const MyClass1 & s) { }
};

[Katty]

If you want singletons in your code then in this case you have to declare all your constructors as private to do this. It is very simple do this. Just use following code to understand the concept of private constructor.

 
      typedef auto_ptrs<Tests> TestPtrs;
  
      TestPtrs Tests::creates() throw(bads_allocs)
  
      {
  
      return new Tests();
   
      }