private constructor

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Possible Duplicate    What is the use of making constructor private in a class        Where do we need private constructor   How can we instantiate a class having private constructor

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One common use is in the singleton pattern where you want only one instance of the class to exist  In that case  you can provide a static method which does the instantiation of the object  This way the number of objects instantiated of a particular class can be controlled

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One common use is for template-typedef workaround classes like following   template  lt class TObj gt  class MyLibrariesSmartPointer     MyLibrariesSmartPointer      public      typedef smart ptr lt TObj gt  type       Obviously a public non-implemented constructor would work aswell  but a private construtor raises a compile time error instead of a link time error  if anyone tries to instatiate MyLibrariesSmartPointer lt SomeType gt  instead of MyLibrariesSmartPointer lt SomeType gt   type  which is desireable

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Private constructor means a user cannot directly instantiate a class  Instead  you can create objects using something like the Named Constructor Idiom  where you have static class functions that can create and return instances of a class   The Named Constructor Idiom is for more intuitive usage of a class  The example provided at the C   FAQ is for a class that can be used to represent multiple coordinate systems   This is pulled directly from the link  It is a class representing points in different coordinate systems  but it can used to represent both Rectangular and Polar coordinate points  so to make it more intuitive for the user  different functions are used to represent what coordinate system the returned Point represents     include  lt cmath gt                   To get std  sin   and std  cos     class Point    public     static Point rectangular float x  float y           Rectangular coord s    static Point polar float radius  float angle        Polar coordinates       These static methods are the so-called  named constructors          private     Point float x  float y          Rectangular coordinates    float x   y         inline Point  Point float x  float y       x  x   y  y        inline Point Point  rectangular float x  float y     return Point x  y       inline Point Point  polar float radius  float angle     return Point radius std  cos angle   radius std  sin angle        There have been a lot of other responses that also fit the spirit of why private constructors are ever used in C    Singleton pattern among them    Another thing you can do with it is to prevent inheritance of your class  since derived classes won t be able to access your class  constructor  Of course  in this situation  you still need a function that creates instances of the class

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For example  you can invoke a private constructor inside a friend class or a friend function   Singleton pattern usually uses it to make sure that nobody creates more instances of the intended type

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It s common when you want to implement a singleton  The class can have a static  factory method  that checks if the class has already been instantiated  and calls the constructor if it hasn t

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private constructor are useful when you don t want your class to be instantiated by user  To instantiate such classes  you need to declare a static method  which does the  new  and returns the pointer   A class with private ctors can not be put in the STL containers  as they require a copy ctor

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It is reasonable to make constructor private if there are other methods that can produce instances  Obvious examples are patterns Singleton  every call return the same instance  and Factory  every call usually create new instance

[c++] private constructor

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Examples related to private-constructor