modify an object of a specific class within another class

Hello, I have the Cryptocurrency class having two simple properties and described as follows:

% Cryptocurrency is a class whose objects represent different

% cryptocurrencies.

classdef Cryptocurrency

% Property of the Cryptocurrency class.

% Only the market class can modify the attribute.

properties (GetAccess = public, SetAccess = {?Market})

price (1,1) double {mustBeNonnegative} = 0

end

% The initialSupply property is a constant and therefore can only

% be modified by the constructor.

properties (GetAccess = public, SetAccess = immutable)

initialSupply (1,1) double {mustBePositive} = 1

end

% methods of the Cryptocurrency class.

methods

% Constructor of the Cryptocurrency class. Takes the initial price

% and the inizial supply of the cryptocurrency as input.

function obj = Cryptocurrency(inputPrice, inputSupply)

obj.price = inputPrice;

obj.initialSupply = inputSupply;

end

end

end

Then, I have the Market class which has the task of changing the price of the two cryptocurrencies following a sale. The Market class takes as input two objects of the Cryptocurrency class :

% this class simulates the price dynamics of two cryptocurrencies based on

% the rules of Automated Market Makers.

% The class allows you to buy token A by paying a certain amount of

% token B and vice versa.

classdef Market

% These properties can only be changed by the constructor.

% - assetA and assetB represent the two cryptocurrencies whose price

% dynamics are simulated by this class.

% - supplyA and supplyB indicate the quantity of the two tokens

% in the liquidity pool.

properties (GetAccess = public, SetAccess = private)

assetA (1,1) Cryptocurrency = Cryptocurrency(1,1)

assetB (1,1) Cryptocurrency = Cryptocurrency(1,1)

supplyA (1,1) double {mustBeNonnegative} = 0

supplyB (1,1) double {mustBeNonnegative} = 0

end

% - k is the constant used in AMMs based on the law k = x * y, where x

% is the initial amount of token A and y is the initial amount of

% token B. The value of k must remain constant.

properties (GetAccess = public, SetAccess = immutable)

k (1,1) double {mustBePositive} = 1

end

methods

% Constructor of the Market class. It takes as input two objects

% of Cryptocurrency class

function obj = Market(cryptoA, cryptoB)

arguments (Input)

cryptoA (1,1) Cryptocurrency

cryptoB (1,1) Cryptocurrency

end

obj.assetA = cryptoA;

obj.assetB = cryptoB;

% Formula used in AMMs. k must remain constant.

obj.k = cryptoA.initialSupply * cryptoB.initialSupply;

% the supply in the AMM is not the same as that of the crypto

% in the market.

obj.supplyA = cryptoA.initialSupply;

obj.supplyB = cryptoB.initialSupply;

end

% method that allows you to buy assetA by providing assetB in exchange.

% To buy a quantity of assetA, a quantity of assetB must be

% deposited in the pool.

% In this way the price of assetA increases as there is less

% availability in the pool and the price of assetB decreases as

% there is a greater quantity.

% The method takes as input the quantity of token A to be purchased.

function obj = buyCryptoA(obj, transactionVolume)

arguments (Input)

obj (1,1) Market

transactionVolume (1,1) double {mustBeNonnegative}

end

% An amount of tokenA greater than the amount of tokenA in the

% pool cannot be purchased.

if transactionVolume >= obj.supplyA

disp("The transaction cannot be executed.")

else

% The amount of token A in the pool decreases.

obj.supplyA = obj.supplyA - transactionVolume;

% Amount of tokenB sold to get the amount of tokenA

% specified by transactionVolume

tokenBsold = abs(obj.supplyB - (obj.k / obj.supplyA));

% The amount of token B varies so that k remains constant.

obj.supplyB = obj.k / obj.supplyA;

% The price of token A rises as it becomes scarcer.

obj.assetA.price = (tokenBsold * obj.assetB.price) ...

/ transactionVolume;

% The price of the token B drops as there is more of it

obj.assetB.price = (obj.assetA.price * obj.supplyA) ...

/ obj.supplyB;

end

end

% method that allows you to buy assetB by providing assetA in exchange.

% To buy a quantity of assetB, a quantity of assetA must be

% deposited in the pool (sold).

% In this way the price of assetB increases as there is less

% availability in the pool and the price of assetA decreases as

% there is a greater quantity.

% The method takes as input the quantity of token A to be sold.

function obj = sellCryptoA(obj, transactionVolume)

arguments (Input)

obj (1,1) Market

transactionVolume (1,1) double {mustBeNonnegative}

end

% % The amount of token A in the pool increases.

obj.supplyA = obj.supplyA + transactionVolume;

% Amount of tokenB obtained by selling the amount of tokenA

% specified by transactionVolume

tokenBobtained = abs(obj.supplyB - (obj.k / obj.supplyA));

% The amount of token B varies so that k remains constant.

obj.supplyB = obj.k / obj.supplyA;

% The price of token B rises as it becomes scarcer.

obj.assetB.price = (transactionVolume * obj.assetA.price) / tokenBobtained;

% The price of the token A drops as there is more of it.

obj.assetA.price = (obj.assetB.price * obj.supplyB) / obj.supplyA;

end

end

end

The price of the two properties inside the Market class assetA and assetB is modified correctly.

But how do I change the price property of the two objects that I pass to the Market constructor? For example :

a = Cryptocurrency(1, 1000);

b = Cryptocurrency(0.5, 2000);

m = Market(a, b);

m = m.sellCryptoA(100);

In this example, I'd like the price of item A to change, but it doesn't. So the constructor is passed a copy of a and b? How do I pass the reference of objects a and b so that any modification of them inside the Market class is reflected on the two objects passed to the constructor?

Answer 1

[Harsh Mahalwar]

Hi Francesco,

One way of changing the value of a and b whenever the value of m.assestA and m.assestB is changed can be to create this function inside the class:

function res = func(obj)

res.a = obj.assestA;

res.b = obj.assestB;

end

Then this function can be called to excitly change the values of a and b respectively by:

res = func(obj);

a = res.a;

b = res.b;

Hope this helps! Here are some resources for reference:

https://in.mathworks.com/help/matlab/ref/class.html

Thanks and regards,

Harsh Mahalwar

[Francesco Pio]

Thanks very much! Now I try to implement this solution. However, i solved the problem by putting "< handle" in the Cryptocurrency class definition :

classdef Cryptocurrency< handle
This Is a good solution for you?