Encapsulation & Information Hiding Rules

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

• Don't expose data items
• Don't expose the difference between stored data and derived data
• Don't expose implementation details of a class
• Don't expose a class's internal structure

class Good

class Good

• Actually, is a "good class"
• Follows encapsulation and information-hiding rules, or does it?
• The client code for Good Class:
• Must #include "Good.hpp"
• Knows data member types
• Good.hpp may depend on a large number of direct and indirect include files for the data members

Client recompiled whenever a change in class Good:

• Add a data member
• Delete a data member
• Change the type of a data member
• Update the include files for a data member
• Even if there is no change in the interface

PIMPL

Pointer to IMPLementation

• AKA, opaque pointer, d-pointer, compiler firewall, Cheshire Cat
• Use a pointer to a class/struct declaration for an implementation class in the target class declaration file (i.e., .hpp)
• Define the implementation class in the target class definition file (i.e., .cpp)

class Good PIMPL

PIMPL Advantages

• The class implementation is hidden
• Add a new data member to the (private) structure without violating binary compatibility
• Header file containing the class declaration only needs to include those files required for the class interface rather than for its implementation
• Not a new technique archive_entry declaration, archive_entry definition

class Good PIMPL Implementation

PIMPL: Include File

• Implement using a class or struct. Typically, it is a struct since there is no need to keep it private.
• A struct/class declaration forms an incomplete type
• std::unique_ptr<> can work with incomplete types (with one issue)
• std::unique_ptr<> used instead of raw pointer for RAII. I.e., don't depend on the destructor of the class to run

PIMPL: Implementation File

• Define the PIMPL struct in the definition file
• Target class data member initialization is now done in the PIMPL struct
• Access to "members" is through a pointer
• If you have a default destructor, use the keyword default with the destructor definition in the .cpp file so that std::unique_ptr<> knows how to delete the incomplete type.

class Stack PIMPL

A Step Further

A Step Further

• We write most of the client code based on the abstract class Good
• Often, the abstract class is a type we use as a parameter, constructor parameter, or field
• Somewhere else in the code, we choose a particular implementation
• A Framework or Toolkit may be written using the abstract class Good
• Then, we provide the implementation class that the Framework or Toolkit uses

Choices

EndNotes

• Abstractions can have a time efficiency cost
• Abstractions can have a comprehension cost
• Requirements for abstraction can vary over time on a project
• Have to develop the skill to know when is enough