interface_segregation_principle.cpp

/*** 
Acknowledagement: This is a lecture note for course https://www.udemy.com/course/patterns-cplusplus/ by 
@Dmitri Nesteruk
Title: Interface segregation principle
Task: Clients should not be forced to depend upon interfaces that they do not use. 
Avoid stuffing too much into a single interface.

Shiyu Mou
***/
#include <vector>
struct Document;

/*** bad interface, all subclass will has these 3 functions but usually only 
      one is needed. You have to leave the rest as empty or throw exceptions ***/

//struct IMachine
//{
//  virtual void print(Document& doc) = 0;
//  virtual void fax(Document& doc) = 0;
//  virtual void scan(Document& doc) = 0;
//};
//
//struct MFP : IMachine
//{
//  void print(Document& doc) override;
//  void fax(Document& doc) override;
//  void scan(Document& doc) override;
//};

// 1. Recompile
// 2. Client does not need this
// 3. Forcing implementors to implement too much

/*** better interface, seperate them ***/

struct IPrinter
{
  virtual void print(Document& doc) = 0;
};

struct IScanner
{
  virtual void scan(Document& doc) = 0;
};

struct Printer : IPrinter
{
  void print(Document& doc) override;
};

struct Scanner : IScanner
{
  void scan(Document& doc) override;
};

struct IMachine: IPrinter, IScanner
{
};

struct Machine : IMachine
{
  IPrinter& printer;
  IScanner& scanner;

  Machine(IPrinter& printer, IScanner& scanner)
    : printer{printer},
      scanner{scanner}
  {
  }

  void print(Document& doc) override {
    printer.print(doc);
  }
  void scan(Document& doc) override;
};

// IPrinter --> Printer
// everything --> Machine