design pattern - decorator pattern in c++ and lua
In object-oriented programming, the decorator pattern is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.
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© Hung-Chi's Blog
https://hungchicheng.github.io/2017/11/07/Design-Patterns-Decorator-Pattern-in-lua-and-C++/
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What is Decorator Pattern
Example in C++
#include <iostream>
using namespace std;
class I {
public:
virtual ~I(){}
virtual void show() = 0;
};
class Hero: public I {
public:
Hero(string n):name(n){}
~Hero() {
cout << name << " dtor" << 'n';
}
/virtual/
void show() {
cout << name << " with ";
}
private:
string name;
};
class Equiment: public I {
public:
Equiment(I inner) {
m_wrappee = inner;
}
~Equiment() {
delete m_wrappee;
}
/virtual/
void show() {
m_wrappee->show();
}
private:
I m_wrappee;
};
class Helmet: public Equiment {
public:
Helmet(I *core): Equiment(core){}
~Helmet() {
cout << "Helmet dtor, ";
}
/virtual/
void show() {
Equiment::show();
cout << "Helmet, ";
}
};
class Armour: public Equiment {
public:
Armour(I *core): Equiment(core){}
~Armour() {
cout << "Armour dtor, ";
}
/virtual/
void show() {
Equiment::show();
cout << "Armour, ";
}
};
class Boots: public Equiment {
public:
Boots(I *core): Equiment(core){}
~Boots() {
cout << "Boots dtor, ";
}
/virtual/
void show() {
Equiment::show();
cout << "Boots, ";
}
};
int main() {
I hero1H = new Helmet(new Hero("Hero1"));
I hero2HA = new Armour(new Helmet(new Hero("Hero2")));
I hero3HAB = new Boots(new Armour(new Helmet(new Hero("Hero3"))));
I hero4BH = new Helmet(new Boots(new Hero("Hero4"))); //Hero 4 with no Armour
hero1H->show();
cout << 'n';
hero2HA->show();
cout << 'n';
hero3HAB->show();
cout << 'n';
hero4BH->show();
cout << 'n';
delete hero1H;
delete hero2HA;
delete hero3HAB;
delete hero4BH;
}
Output:
Hero1 with Helmet,
Hero2 with Helmet, Armour,
Hero3 with Helmet, Armour, Boots,
Hero4 with Boots, Helmet,
Helmet dtor, Hero1 dtor
Armour dtor, Helmet dtor, Hero2 dtor
Boots dtor, Armour dtor, Helmet dtor, Hero3 dtor
Helmet dtor, Boots dtor, Hero4 dtor
Program ended with exit code: 0
Example in Lua
function FuncNew( obj ) -- for Inheritance
function obj:new( o )
o = o or {}
setmetatable( o, self )
self.__index = self
return o
end
return obj
end
Hero = {}
function Hero:create( name )
return FuncNew( Hero ):new({ m_name = name })
end
function Hero:show()
print( self.m_name … " with " )
end
Helmet = {}
function Helmet:create( child )
return FuncNew( Helmet ):new({ m_child = child })
end
function Helmet:show()
self.m_child:show()
print( "Helmet, " )
end
Armour = {}
function Armour:create( child )
return FuncNew( Armour ):new({ m_child = child })
end
function Armour:show()
self.m_child:show()
print( "Armour, " )
end
Boots = {}
function Boots:create( child )
return FuncNew( Boots ):new({ m_child = child })
end
function Boots:show()
self.m_child:show()
print( "Boots, " )
end
------------------------------------------------------
local hero1H = Helmet:create(Hero:create("Hero1"))
local hero2HA = Armour:create(Helmet:create(Hero:create("Hero2")))
local hero3HAB = Boots:create(Armour:create(Helmet:create(Hero:create("Hero3"))))
local hero4BH = Helmet:create(Boots:create(Hero:create("Hero4")))
hero1H:show()
hero2HA:show()
hero3HAB:show()
hero4BH:show()
print("nLua automatically deletes objects that become garbagen")
Output:
Hero1 with
Helmet,
Hero2 with
Helmet,
Armour,
Hero3 with
Helmet,
Armour,
Boots,
Hero4 with
Boots,
Helmet,
Lua automatically deletes objects that become garbage
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