Private class data pattern

Private class data is a design pattern in computer programming used to encapsulate class attributes and their manipulation.

tandard documentation

The following documentation categories for the "private class data" design pattern follows the design pattern documentation style precedent set by the Gang of Four.

Name and classification

; Pattern Name : This pattern is known as the "private class data" design pattern.; Pattern Classification : This pattern is a structural pattern.

Intent

The "private class data" design pattern seeks to reduce exposure of attributes by limiting their visibility. It reduces the number of class attributes by encapsulating them in single "Data" object. It allows the class designer to remove write privilege of attributes that are intended to be set only during construction, even from methods of the target class.

Also known as

PIMPL (Private IMPLementation) or Opaque pointer

Motivation

A class may expose its attributes (class variables) to manipulation when manipulation is no longer desirable, e.g. after construction. Using the private class data design pattern prevents that undesirable manipulation.

A class may have one-time mutable attributes that cannot be declared "final". Using this design pattern allows one-time setting of those class attributes.

The motivation for this design pattern comes from the design goal of protecting class state by minimizing the visibility of its attributes (data).

Applicability

This design pattern applies to any class in any object oriented language.

tructure

Participants

Collaboration

Consequences

The consequences of using this design pattern include the following:
* Controlling write access to class attributes;
* Separating of data from methods that use it;
* Encapsulating class attribute (data) initialization; and
* Providing new type of "final": "final after constructor".

Implementation

The private class data design pattern solves the problems above by extracting a "data class" for the targed class and giving the target class instance an instance of the extracted "data class".

*The "data class" exposes each attribute (variable or property) through a "getter".
*The "data class" exposes each attribute that must change after construction through a "setter".

ample code

The following C# code illustrates an opportunity to use the private class data design pattern: public class Circle { private double radius; private Color color; private Point origin; public Circle(double radius, Color color, Point origin) { this.radius = radius; this.color = color; this.origin = origin; } public double Circumference { get { return 2 * Math.PI * this.radius; } } public double Diameter { get { return 2 * this.radius; } } public void Draw(Graphics graphics) { //... } }The attributes "radius", "color", and "origin" above should not change after the "Circle()" constructor. Note that the visibility is already limited by scoping them as "private", but doing methods of class "Circle" can still modify them.

Although marking attributes of classes as "const" (or "final" or "ReadOnly" in other programming languages) restricts their manipulation, the attributes above are set in the constructor and hence cannot be marked as such.

The excess exposure of the attributes creates a type of (undesirable) coupling between methods that access those attributes. To reduce the visibility of the attributes and thus reduce the coupling, implement the private class data design pattern, as follows: public class CircleData { private double radius; private Color color; private Point origin; public CircleData(double radius, Color color, Point origin) { this.radius = radius; this.color = color; this.origin = origin; } public double Radius { get { return this.radius; } } public Color Color { get { return this.color; } } public Point Origin { get { return this.origin; } } } public class Circle { private CircleData circleData; public Circle(double radius, Color color, Point origin) { this.circleData = new CircleData(radius, color, origin); } public double Circumference { get { return this.circleData.Radius * Math.PI; } } public double Diameter { get { return this.circleData.Radius * 2; } } public void Draw(Graphics graphics) { //... } }

The "Circle" class in the resulting code has an attribute of type "CircleData" to encapsulate the attributes previously exposed to all of the methods of the class "Circle". That encapsulation prevents methods from changing the attributes after the "Circle()" constructor. Note, however, that any method of "Circle" can still retrieve the values of the encapsulated attributes.

Known uses

Related patterns

See Structural pattern for related patterns.

References