Constructor in private section

Yes it is possible to declare constructors as private.

When you declare the constructor as private, you cannot directly instantiate the class from outside. And from only the inside of the class you can create an object of itself. You use static functions for this.

Some examples:

1. When you're creating singleton classes (Where only one object of your class exists in your program).

class Foo
{
private:
   Foo(){}

public:
   static Foo& get_instance()
   {
      static Foo global_instance;
      return global_instance;
   }
};

Create the instance of the above class like this:

Foo& ins = Foo::get_instance();

2. Using static methods to create an instance of a class.

class Foo
{
private:
   Foo(){}
public:
   static Foo* create_instance()
   {
      return new Foo();
   }
};

Create instances of this class like this:

Foo * ins_a = Foo::create_instance();
Foo * ins_b = Foo::create_instance();

Yes, you can: it is often used in the Singleton Pattern to give the class and only the class control over when a class instance can be instantiated.

class MyClass
{
private:
  MyClass();
};

Google or Wiki can help you will more complete examples - look for "Singleton Pattern C++" and you will find plenty of examples.

Yes it is posible only if you do'nt want to create object of that class and you also dont want to make that class abstract. you can make its constructor private so that no one can create its object.

here is sample code

class Shape
{
private Shape();
}