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Immutable Variable

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

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Definition

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

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Why it matters

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

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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.

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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.

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Advantages

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

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Limitations

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

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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.
Canonical knowledge ID: glossary:immutable-variable