# SOLID cairo - OOP-like pattern

**Published by:** [Only Dust](https://paragraph.com/@only-dust/)
**Published on:** 2022-05-16
**URL:** https://paragraph.com/@only-dust/solid-cairo-oop-like-pattern

## Content

When I learn a new language, my first focus is to build SOLID components while leveraging the feature of this new language. In this article, I would like to present a new convention we at OnlyDust are adopting with regards to cairo development. This new kind of architecture feels like object-oriented programming in a way and leverage several cairo features like:implicit parametersstructsnamespacesFollowing OpenZeppelin’s recommendations about extensibility, we aims at building nice modules that can be re-usable and well tested. A module is composed of:structs to represent the current state of an objectexternal functions that are aimed at being imported (in smart contracts and unit tests)internal functions that are meant to be kept privateAs the memory is immutable in cairo, when interacting with an object, this object is usually being copied into a new version to be used. Using explicit parameters, this would result in providing the object both as input parameter and output parameter. Implicit parameters are better suited for this need as this is automatically done by the language. This convention is especially useful when dealing with in-memory object when usage of storage is to be limited. Let’s see how it looks:# person.cairo %lang starknet from starkware.cairo.common.math import assert_nn, assert_not_zero struct Wallet: member token : felt member balance : felt end # External functions namespace wallet_access: func create(token : felt) -> (wallet : Wallet): let wallet = Wallet(token, 0) internal.check_wallet{wallet=wallet}() return (wallet=wallet) end func increase_balance{wallet : Wallet}(amount : felt): let wallet = Wallet(wallet.token, wallet.balance + amount) return () end end # Internal functions namespace internal: func check_wallet{wallet : Wallet}(): with_attr error_message("Token cannot be 0"): assert_not_zero(wallet.token) end return () end end And here is the contract file that interact with it:# dex.cairo %lang starknet from src.wallet import Wallet, wallet_access @external func do_something(wallet : Wallet) -> (wallet : Wallet): with wallet: wallet_access.increase_balance(42) # Here wallet has been updated end return (wallet=wallet) end Using this convention, it is actually possible to build SOLID modules and interact with them. It allows to build some design patterns on top of it like the iterator pattern presented here.

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