Lesson 3: Query about SimpleStorage Array and SimpleStorage Object.

[p0lpixel]

Hi I'm a little confused about what this code is doing. What is happening in the line "SimpleStorage[] public simpleStorageArray?" What does the line "SimpleStorage simpleStorage = new SimpleStorage();" doing?

//
contract StorageFactory {
SimpleStorage[] public simpleStorageArray;

function createSimpleStorageContract() public {
simpleStorage simpleStorage = new SimpleStorage();
simpleStorageArray.push(simpleStorage);

}

[alymurtazamemon]

@p0lpixel

SimpleStorage[] public simpleStorageArray;

This is called declaring an Array in solidity. In that case, this array type is SimpleStorage.

- simpleStorage simpleStorage = new SimpleStorage();
+ SimpleStorage simpleStorage = new SimpleStorage();

There is a typo mistake, correct it.

SimpleStorage simpleStorage = new SimpleStorage();

This line is creating an object of the SimpleStorage contract and we are pushing to the array after it.

[p0lpixel]

Thanks. What does it mean if array type is SimpleStorage?

What does it mean to create an object of SimpleStorage contract and to push to the array after it?

[Muchunguzi]

Hi,
Qn. What does it mean if array type is SimpleStorage?

 In Solidity, you can create a datatype from just a given word you want. And In this case, we choose to use "SimpleStorage", so the array is of the type SimpleStorage.

Qn. What does it mean to create an object of the SimpleStorage contract and to push to the array after it?

     Well  in any programming language that deals with objects , objects can mean anything in the real world like a car maybe and this car can have properties which are its characteristics, such as color and engine type.

     I hope you got me there , so to create this object in that specific programming language we use the new keyword and the word on the right hand side of the (  =  ) equals sign after the **new** is the object created.

    So in Solidity **SimpleStorage()** is our object ( Which is seen as a contract by Solidity) with one of its properties being its address.  And Everytime **SimpleStorage simplestorage = new SimpleStorage()** is executed a new object is created.  So we use one of the inbuilt methods of Arrays ( .push() method ) to add these contracts to the array we created which is referred to as pushing.

ie

SimpleStorage simpleStorage = new SimpleStorage() // Telling Solidity about the contract SimpleStorage()

SimpleStorage[] public simpleStorageArray //Array where we are going to send / push the different contracts

simpleStorageArray.push(simpleStorage) //pushing the contracts from the simpleStorage variable

This is how I understand it , i hope it makes sense

[p0lpixel]

Thanks. I understand it a bit more.

SimpleStorage simpleStorage = new SimpleStorage();

In the above^, is SimpleStorage the array type, simpleStorage a variable, and SimpleStorage() the new object/contract?

[alymurtazamemon]

@p0lpixel Yes.

[krakxn]

I will keep this as simple as possible for your ease:

Your doubts: "What is happening in the line SimpleStorage[] public simpleStorageArray? What does the line SimpleStorage simpleStorage = new SimpleStorage(); doing?"

Note: if you are comfortable with OOP languages, treat Contracts like Classes

• So, we initialize an ARRAY with name simpleStorageArray. Further, it is a type of SimpleStorage - which means that our array will store instances of the contract SimpleStorage.

• Now, there is a typo in your second line, it should be SimpleStorage simpleStorage = new SimpleStorage();

• What this means is that we are instantiating/creating an object of SimpleStorage and storing it in a new VARIABLE simpleStorage (of type SimpleStorage, so it can store them). And empty brackets here means we are calling a default constructor of SimpleStorage contract which assigns default values to the newly created object in this line. Note: every object creation requires a constructor call (it is automatic) to assign default values to the newobject

• Put simply, simpleStorageArray (of type SimpleStorage) is an array of SimpleStorage contracts, whereas simpleStorage stores new objects of SimpleStorage when the function is called.

• So, when we push the object into our original array, it stores the newly created objects in the function.

Hope this helps!