> ## Documentation Index
> Fetch the complete documentation index at: https://docs.theblockchainlibrary.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Immutable Variable

> A smart-contract variable assigned during deployment and then fixed for the contract's lifetime.

# Immutable Variable

A smart-contract variable assigned during deployment and then fixed for the contract's lifetime.

## Definition

A smart-contract variable assigned during deployment and then fixed for the contract's lifetime.

## Why it matters

Smart-contract concepts are essential for understanding programmable blockchain applications.

## How it works

The developer declares a state variable with the 'immutable' keyword within the contract scope. Inside the constructor function, the developer assigns a value to this variable based on inputs or other logic. During compilation, the Solidity compiler replaces references to the variable with the assigned value directly in the runtime code.

## Real-world example

A decentralized exchange deploying a factory contract might set the address of a specific fee-collection contract as an immutable variable during deployment.

## Advantages

* Reduced gas costs for read operations
* Guaranteed value persistence after deployment
* Prevents accidental state modification

## Limitations

* Cannot be changed after deployment
* Limited to specific data types
* Restricted assignment inside constructor only

## Common misconceptions

* People often think immutable variables are the same as constants, but constants must be fixed at compile-time.
* Many users assume immutable variables can be updated via administrative functions, but they are physically locked.

## Related knowledge

* [ABI](/generated/v2/glossary/abi) — term
* [Smart Contract](/generated/v2/glossary/smart-contract) — term
* [Solidity](/generated/v2/resources/solidity) — term

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**Canonical knowledge ID:** `glossary:immutable-variable`