Credits

Adapted from: Encapsulation is not information hiding

Definitions

• Encapsulation

  the bundling of data with the methods that operate on that data

• Information Hiding

  hide the internal representation, or state, of an object from the outside

• Note: Some definitions of encapsulation include the definition of information hiding

Language Features

• C language support:
  • struct with fields only and no access specifiers
  • free functions
• C++ language support:
  • class Integrate data with methods
  • Access specifiers: public, private, protected

Access & Visibility for Class A

First Rule: Limit the Interface

• Limit methods available
• Limit the access and visibility
• … I.e., private whenever possible
• Remove any unneeded parameters
• … E.g., code where a local variable could be used instead of a parameter
• Use const whenever possible for reference and pointer parameters and methods

Minimum Essential Interface

• Unless required, more is not better
• Developers tend to overdesign and provide more than is needed (sometimes missing what is needed)
• … E.g., A study at Microsoft showed that 30% of methods identified in the design of software were never implemented
• Start with the minimum functionality and configurability, and add as needed

Position v0

Position v0 Analysis

• Data, latitude and longitude, in the Position class
• Operations, distance() and heading(), are free functions in a separate file
• Client program has to include two files
• Easy to get the order of arguments backward heading() and calculate the opposite of what is needed

Other Position v0 Issues

• Fields of struct Position are public and directly accessible. It is possible to store invalid latitude and longitude values, producing garbage distance and headings (GIGO)
• Changes to the field/data member names, types, or even storage in Position require changes in the implementation of distance() and heading() (∆ Position.hpp ⟶ ∆ PathCalculations.cpp)

Encapsulation Rule

Place data and the operations that perform on that data in the same class

• Don't make the client tie data and operations together
• Provide it in one class (or the fewest number of classes needed)
• Improves the class cohesion

Position v1

Position v1 Analysis

• Only need to include a single include file, Position.hpp
• Clear direction of operations heading()

Information Hiding Rules

1. Don't expose data items
2. Don't expose the difference between stored data and derived data
3. Don't expose implementation details of a class
4. Don't expose a class's internal structure

Information Hiding Rule 1

Don't expose data items

• Make all data members private
• If access is needed externally to the class, use get and set methods for access
• Isolates client from changes to data members (if the same external functionality is needed)

Position v2

What Not To Do

• Non-const reference takes const off of the method
• Pretty much the same as making the field public
• Very few applications where this makes sense
• Specially a problem in Java, as Objects are not passed by value but by Object Reference, and there isn't a const
• Instead: Always return data by value or const reference

As Far As A Mutator

Information Hiding Rule 2

Don't expose the difference between stored data and derived data

• Derived data is data calculated from stored data
• Don't reveal whether an attribute is stored or derived
• Use get method names for property methods
• get method names: speed(), getSpeed()
• property method names: calculateSspeed(), determineSspeed()

Position v2 Position::distance()

• Changing to radians internally would make everything much simpler
• Do not want to change the current interface (i.e., in degrees), as clients expect it
• However, we could add radians to the interface while preserving what we have

Position v2 → v3

Position v2 & v3 Interface

Position v2 & v3 Client

Position::distance() v2 → v3

Position:getLatitude() v2 → v3

New Requirement

• Need to handle a multi-position route

Route v0

Information Hiding Rules

1. Don't expose data items
2. Don't expose the difference between stored data and derived data
3. Don't expose implementation details of a class
4. Don't expose a class's internal structure

Route v0 Issues

• Can directly access elements of an internal container
• Can we change the container?
• Can we change the container values?

Route v0 & v1 Interface

Information Hiding Rules

1. Don't expose data items
2. Don't expose the difference between stored data and derived data
3. Don't expose implementation details of a class
4. Don't expose a class's internal structure

Route v1 Issues

• Exposes direct design detail that we are using a std::vector
• If we change to a different container, the type returned by the getPositions() has to change, and the client code has to change
• Use of auto in the client code, and perhaps a typedef can help, but not entirely prevent this
• Which container we use is an internal structure and implementation detail that should be hidden in the class
• Do we need this type of access?

Route v1 & v2 Interface

Route v2 Issues

• The setPosition() exposes direct design detail that we are using an indexable container
• If we change to a container that does not allow indexing, the client code would have to change
• The characteristics of which container we use is an internal structure and implementation detail that should be hidden in the class
• Do we need this type of access?

Route v2 & v3 Interface

Route v3 Issues

• The getPosition() is expected to be a O(1) or constant-time algorithm
• However, with a non-indexable container, e.g., std::list, this is not true
• If we change a container that does not allow indexing, the client code would have to change
• The characteristics of which container we use is an internal structure and implementation detail that should be hidden in the class
• Do we need this type of access?

Route v3 & v4 Interface

Route v4 Improvements

• Use standard C++ iterators

Route v4 & v5 Interface

Route v5 Additional Improvements

• A separate iterator object would allow multiple, simultaneous access
• A begin() and end() would allow use in range-for statement
• The segment is problematic. Perhaps allow iteration through segments and not positions