Variable Scope: State & Local variables

Variable Scope

Storage variables (aka state variables)

• Stored in the blockchain and their values are persistent and publicly accessible.

• Declared outside a function.

contract MyContract {    
    uint public balance; // Declares a state variable
}

Local variables

• Declared within a function

• Not stored on the blockchain

contract MyContract {    

    function addOne() public {        
        uint256 localNumber = 2;        
    }
}

Local variables are useful for storing temporary values or intermediate results and can be declared in the same way as any other type of variable.

Contract layout

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract Variables {
    
    // State variables are stored on the blockchain.
    string public text = "Hello";
    uint public num = 123;

    function doSomething() public {
        // Local variables are not saved to the blockchain.
        uint i = 456;

        // Here are some global variables
        uint timestamp = block.timestamp;  // Current block timestamp
        address sender = msg.sender;       // address of the caller
    }
}

Visibility

State variables only have three possible visibility modifiers:

1. public (accessed internally as well as via external function calls)

2. internal (only accessed internally and inheriting contracts)

3. private (only within the contract it is defined)

When visibility is not specified, state variables have the default value of internal.

Updating State Variables

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract SimpleStorage {
    // State variable to store a number
    uint public num;

    // You need to send a transaction to write to a state variable.
    function set(uint _num) public {
        num = _num;
    }

    // You can read from a state variable without sending a transaction.
    function get() public view returns (uint) {
        return num;
    }
}

• To write or update a state variable you need to send a transaction.

• On the other hand, you can read state variables for free, as it does not involve a transaction.

If you have public state variables in your contract, the compiler will create getter functions for these automatically. --> get() is not required