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The Facade pattern

Facade design pattern is a structural design pattern and it's used widely. The aim of this design pattern is to provide a simple interface to the client for a complex underlying system. So, facade means face of the building. This design pattern hides all complexities of the system and just displays a simple face. Very common example could be URL interface of a browser, which hides all complexities behind and only accepts a URL which user intends to browse. Another common example could be withdrawal / deposit of money from banking system via ATM.

To withdraw money we need following steps to achieve successfully:

1. Validate card / account number

2. Validate pin

3. In case of withdrawal check account balance and allow / disallow withdrawal

4. Follow steps 1 & 2 for account deposit (Steps 3 not needed).

5. Finally show the balance.


Code Example: 

<Code>

File: WelcomeBank.h

#pragma once
#include <fmt/format.h>

class WelcomeBank{
    public:
        // ctor...
        WelcomeBank(){
            fmt::print("{}\n", "Welcome to ABC Bank");
        }

};

File: AccountNumberCheck.h

class AccountNumberCheck {
    private:
        unsigned int _acctNumber = 12345678;
    public:
         unsigned int getAccountNumber() { return _acctNumber; }
         bool isAccountActive(unsigned int accountNumber)
         {
             if(getAccountNumber() == accountNumber) return true;
             else return false;
         }
};

File: SecurityCodeCheck.h

#pragma once
class SecurityCodeCheck{
    private:
        unsigned int _securityCode = 1234;
    public:
         unsigned int getSecurityCode() { return _securityCode; }
         bool isCodeCorrect(unsigned int securityCodeToCheck)
         {
             if(getSecurityCode() == securityCodeToCheck)
                return true;
             else
                return false;
         }
};

File: FundsCheck.h

#pragma once
#include <fmt/format.h>

class FundsCheck {
    double cashInAccount = 1000.00;
    void decreaseCashInAccount(double cashWithDrawn) {
            cashInAccount -= cashWithDrawn;
        }

        void increaseCashInAccount(double cashDeposited) {
            cashInAccount += cashDeposited;
        }
    public:
        double getCashInAccount() {
            return cashInAccount;
        }

        bool haveEnoughMoney(double cashWithDrawal) {
            if(cashWithDrawal > getCashInAccount()) {
                fmt::print("Error: Don't have enough balanace!\n");
                fmt::print("Current Balance: {}\n", getCashInAccount());
                return false;
            }
            else {
                decreaseCashInAccount(cashWithDrawal);
                fmt::print("Withdrawal complete, New Balance is: {}\n", getCashInAccount());
                return true;
            }
        }

        void makeDeposits(double cashToDeposit){
            increaseCashInAccount(cashToDeposit);
            fmt::print("Deposit complete, new balance is: {}\n", getCashInAccount());
        }
};

File: BankAccountFacade.h

/* This is our facade implementation */
#include "AccountNumberCheck.h"
#include "FundsCheck.h"
#include "SecurityCodeCheck.h"
#include "WelcomeBank.h"

class BankAccountFacade{
    private:
        unsigned int _accountNumber;
        unsigned int _securityCode;

        AccountNumberCheck accCheker;
        SecurityCodeCheck codeChecker;
        FundsCheck fundCheker;

    public:
        BankAccountFacade(int accNum, int secCode) :
            _accountNumber(accNum),
            _securityCode(secCode) {}
        unsigned int getAccountNumber() { return _accountNumber; }
        unsigned int getSecurityCode() { return _securityCode; }

        void withdrawCash(double cashToGet) {
           
            WelcomeBank bankGreet;
            if(accCheker.isAccountActive(getAccountNumber()) &&
                codeChecker.isCodeCorrect(getSecurityCode()) &&
                fundCheker.haveEnoughMoney(cashToGet)) {
                    fmt::print("Transaction complete\n");
                }
                else
                    fmt::print("Transaction failed\n");
        }

        void depositCash(double cashToDeposit) {
            WelcomeBank bankGreet;

            if(accCheker.isAccountActive(getAccountNumber()) &&
                codeChecker.isCodeCorrect(getSecurityCode())) {
                    fundCheker.makeDeposits(cashToDeposit);
                    fmt::print("Transaction complete\n");
                }
                else
                    fmt::print("Transaction failed\n");
        }
};

// This part contains our client code
File: example.cpp

#include "BankAccountFacade.h"
int main()
{
    BankAccountFacade accessingBank(12345678, 1234);
    accessingBank.withdrawCash(50.00);
    accessingBank.withdrawCash(900.00);
    accessingBank.depositCash(50.00);
    return 0;
}

</Code> 

Demo (Compiler Explorer code)

Credit: Design Patterns: Elements of Reusable Object-Oriented Software (by GoF)

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