Design Pattern - Strategy Pattern in C++ and Lua

What is Strategy Pattern
Example in C++
Example in Lua

In computer programming, the strategy pattern (also known as the policy pattern) is a behavioural software design pattern that enables selecting an algorithm at runtime.

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© Hung-Chi's Blog
https://hungchicheng.github.io/2017/09/25/Design-Patterns-Strategy-Pattern-in-lua-and-C++/
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What is Strategy Pattern

The strategy pattern:

defines a family of algorithms,
encapsulates each algorithm, and
makes the algorithms interchangeable within that family.

Example in C++

For example, we create a warrior and archer with different attack strategy.

#include <iostream>
using namespace std;

class AttackStrategy{
public:
    virtual void attack() = 0;
};

class SwordAttackStrategy: public AttackStrategy{
public:
    virtual void attack(){ cout << "use sword to attack" << endl; }  // override 
};

class ArcheryAttackStrategy: public AttackStrategy{
public:
    virtual void attack(){ cout << "use bow to attack" << endl; }  // override 
};

class HeroBase{
private:
    AttackStrategy *m_strategy;
    string m_name;
public:
    HeroBase( string name, AttackStrategy *strategy ):m_name( name ),m_strategy( strategy ){}
    void set_strategy( AttackStrategy *strategy ){ m_strategy = strategy; }
    void attack(){
        cout << m_name << endl;
        m_strategy -> attack();
    }
};

int main( int argc, char *argv[] )
{
    // warrior
    SwordAttackStrategy swordAttackStrategy;
    HeroBase warrior( "warrior1", &swordAttackStrategy );
    warrior.attack();
    // archer
    ArcheryAttackStrategy archeryAttackStrategy;
    HeroBase archer( "archer1", &archeryAttackStrategy );
    archer.attack();
    return 0;
}

Output:

warrior1
use sword to attack
archer1
use bow to attack
Program ended with exit code: 0

Download - Source Code

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

AttackStrategy = {}
function AttackStrategy:create()
    return FuncNew( AttackStrategy ):new()
end
function AttackStrategy:attack() -- virtual
    -- do nothing
end

SwordAttackStrategy = AttackStrategy:create() -- inheritance AttackStrategy
function SwordAttackStrategy:create()
    return FuncNew( SwordAttackStrategy ):new()
end
function SwordAttackStrategy:attack() -- override
    print( "use sword to attack" )
end

ArcheryAttackStrategy = AttackStrategy:create() -- inheritance AttackStrategy
function ArcheryAttackStrategy:create()
    return FuncNew( ArcheryAttackStrategy ):new()
end
function ArcheryAttackStrategy:attack() -- override
    print( "use bow to attack" )
end

HeroBase = {}
function HeroBase:create( n, s )
    return FuncNew( HeroBase ):new({
        m_name = n,
        m_strategy = s or AttackStrategy:create()
    })
end
function HeroBase:attack()
    print( self.m_name )
    print( self.m_strategy:attack() )
end

------------------------------------------------------

-- local warrior0 = HeroBase:create( "warrior0" )
-- warrior
local swordAttackStrategy = SwordAttackStrategy:create()
local warrior1 = HeroBase:create( "warrior1", swordAttackStrategy )
-- archer
local archeryAttackStrategy = ArcheryAttackStrategy:create()
local archer1 = HeroBase:create( "archer1", archeryAttackStrategy )

-- warrior0:attack()
warrior1:attack()
archer1:attack()