Scope: Composition between classes in C++
Audience: Those who have a basic understanding of classes, and pass by value functions.
Specs: Written with the C++ Standard Library. Compiled in VS2008 C++ Express Edition.
Implementation: When creating classes, we aim to have each class perform one function. For more complex problems, one solution is to use composition. I use the ol' computer example. A computer performs many operations. It directs the flow of data, saves and retrieves data, provides a UI for input/output. Instead of writing one giant class for handling all of these operations, we could break this into multiple classes that handle objects between eachother. Below are three files that represent a simple example of such. There is main.cpp which provides the UI, CPU.h which directs objects to they're proper class, and Storage.h which accepts the data in this scenario. This is a simple example, but it gives you an idea of how Composition can make your complex class more simple to maintain through sub-classes.
main.cpp
//Composition
#include<iostream>
#include<string>
#include<stdlib.h>
#include"CPU.h"
using namespace std;
int main()
{
int val = 0;
string choice;
for(int j = 0; j < 5; j++)
{
cout<<"Booting..."<<endl;
}
cout<<"Boot Success!"<<endl;
cout<<endl;
cout<<"\nPress any key to continue..."<<endl;
cin.get();
//system("CLS"); //Uncomment if running Windows
cout<<endl;
cout<<"Welcome User, you may now store numerical data."<<endl;
cout<<endl;
do
{
cout<<endl;
cout<<"Please enter a number to save to disk."<<endl;
cout<<endl;
if(!(cin>>val))
{
cout<<endl;
cout<<"Invalid Input"<<endl;
return 0;
}
else
cout<<endl;
cout<<"Saving..."<<endl;
cout<<endl;
CPU beginProc((Storage(val))); //explicitly initialize CPU
cout<<endl; //to pass val to class Storage
cout<<"Would you like to save another number? (y or n)"<<endl;
cout<<endl;
cin>>choice;
//system("CLS"); //Uncomment if running Windows
}while(choice=="y");
cout<<endl;
cout<<"Goodbye"<<endl;
cout<<endl;
cin.get();
return 0;
}
cpu.h
#ifndef CPU_H
#define CPU_H
#include<iostream>
#include"Storage.h" //Must include Storage.h to allow
//passing of object from CPU
class CPU
{
Storage objPass;
public:
CPU(){};
CPU(const Storage &obj) : objPass(obj) //creating a reference to
{ //storage class, allow initialization
} //of passed object to storage
~CPU()
{
}
};
#endif
storage.h
#ifndef STORAGE_H
#define STORAGE_H
#include<iostream>
class Storage
{
int store;
public:
Storage() : store(0)
{
}
Storage(int passIn) : store(passIn) //Since an int was passed out of
{ //main(), we accept an int in storage
std::cout<<store<<" has been saved to disk.\n";
}
~Storage(){};
};
#endif
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