Ethereum: When to use `constant` and `immutable`?

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When to use «constant» and «immutable» in Ethereum

When writing smart contracts on the Ethereum blockchain, developers should carefully consider when to use the keywords «constant» and «immutable». In this article, we will explore the differences between these two keywords and provide scenarios where one or both are appropriate.

continuous keyword

The «Constant» keyword is used to declare variables that should not be changed after initialization. These variables can be used for continuous computations, caching, or other purposes where the value remains unchanged for the duration of the contract.

// uint256 public constant MINIMUM_USD = 50 * 1e18;

Here are some scenarios where «standard» is useful:

• Constant computation: If you need to compute a specific value that does not change over time, use a «constant».

• Caching data: If your contract needs to cache frequently used values, use «constant».

• Returning constants: When returning constants from functions, use constant.

immutable keyword

The immutable keyword is used to declare variables that can never be changed after initialization. These variables have an immutable structure and should not be changed after creation.

// public uint256 immutable MINIMUM_USD;

Here are some scenarios where «immutable» is useful:

• Creating constants: If you want to create constants with a specific value, use immutable.

• Creating immutable structures: If you are using a structure and want to ensure that its fields cannot be changed after creation, use immutable.

• Protecting variables: Use immutable variables when protecting them to prevent accidental changes.

Key differences

Here are the key differences between «constant» and «immutable»:

|
Keyword |
Purpose

|
Use case |

| — | — | — |

| standard | Calculations, cache, constant return | Calculating constant values ​​that do not change over time. |

| «unchanged» | Immutable variables that cannot be changed after initialization | Create immutable structures or protect variables to prevent accidental changes. |

Finally, use the «constant» keyword when calculating constants or caching data that will remain unchanged for the life of the contract. Use the immutable keyword when creating immutable variables that should not be modified after initialization.

By following these guidelines and best practices for using «constant» and «immutable», you can write smart contracts that are more efficient, readable, and maintainable on the Ethereum blockchain.