Solidity 101

Recently, I have been revisiting Solidity, consolidating the finer details, and writing "WTF Solidity" tutorials for newbies.

Twitter: @0xAA_Science | @WTFAcademy_

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Codes and tutorials are open source on GitHub: github.com/AmazingAng/WTF-Solidity

In this section, we will introduce two keywords to restrict modifications to their state in Solidity: constant and immutable. If a state variable is declared with constant or immutable, its value cannot be modified after contract compilation.

Value-typed variables can be declared as constant and immutable; string and bytes can be declared as constant, but not immutable.

constant and immutable

constant

The constant variable must be initialized during declaration and cannot be changed afterwards. Any modification attempt will result in an error at compilation.

// The constant variable must be initialized when declared and cannot be changed after that
    uint256 constant CONSTANT_NUM = 10;
    string constant CONSTANT_STRING = "0xAA";
    bytes constant CONSTANT_BYTES = "WTF";
    address constant CONSTANT_ADDRESS = 0x0000000000000000000000000000000000000000;

Immutable

The immutable variable can be initialized during declaration or in the constructor, which is more flexible.

// The immutable variable can be initialized in the constructor and cannot be changed later
    uint256 public immutable IMMUTABLE_NUM = 9999999999;
    address public immutable IMMUTABLE_ADDRESS;
    uint256 public immutable IMMUTABLE_BLOCK;
    uint256 public immutable IMMUTABLE_TEST;

You can initialize the immutable variable using a global variable such as address(this), block.number, or a custom function. In the following example, we use the test() function to initialize the IMMUTABLE_TEST variable to a value of 9:

// The immutable variables are initialized with the constructor, that is:
    constructor(){
        IMMUTABLE_ADDRESS = address(this);
        IMMUTABLE_BLOCK = block.number;
        IMMUTABLE_TEST = test();
    }

    function test() public pure returns(uint256){
        uint256 what = 9;
        return(what);
    }

Verify on Remix

1. After the contract is deployed, You can obtain the values of the constant and immutable variables through the getter function.

   

2. After the constant variable is initialized, any attempt to change its value will result. In the example, the compiler throws: TypeError: Cannot assign to a constant variable.

   

3. After the immutable variable is initialized, any attempt to change its value will result. In the example, the compiler throws: TypeError: Immutable state variable already initialized.

   

Summary

In this section, we introduced two keywords to restrict modifications to their state in Solidity: constant and immutable. They keep the variables that should not be changed unchanged. It will help to save gas while improving the contract's security.