What Is an Immutable Data Structure?


Heather Bennett

An immutable data structure is a data structure that cannot be modified after it is created. This means that once you create an immutable data structure, you cannot change its values or add or remove elements from it. Instead, any operation that modifies the data structure will create a new copy with the desired changes, leaving the original data structure intact.

Benefits of Immutable Data Structures

Immutable data structures offer several benefits:

  • Thread safety: Since immutable data structures cannot be modified, they can be safely shared across multiple threads without any need for locks or synchronization mechanisms.
  • Predictability: With immutable data structures, you can always rely on the fact that their values will not change. This makes your code more predictable and easier to reason about.
  • Performance optimizations: Immutable data structures can enable various performance optimizations. For example, since they cannot be modified, they can be safely cached and reused without worrying about unexpected changes.

Examples of Immutable Data Structures


A string is an example of an immutable data structure in many programming languages. Once you create a string, you cannot modify its characters individually. Instead, any operation that modifies a string will create a new string with the desired changes.


Sets are another commonly used immutable data structure. Once you create a set with a given set of elements, you cannot add or remove elements from it directly. Instead, operations like adding or removing elements will create new sets with the desired changes.


Tuples are also often implemented as immutable data structures. Once you create a tuple with a specific set of values, you cannot modify those values individually. Instead, any operation that modifies a tuple will create a new tuple with the desired changes.

Considerations for Using Immutable Data Structures

While immutable data structures offer many advantages, there are some considerations to keep in mind:

  • Memory overhead: Since immutable data structures create new copies whenever they are modified, they can consume more memory compared to their mutable counterparts. This can be a concern when working with large data sets.
  • Performance implications: While immutable data structures can enable certain performance optimizations, they can also introduce performance overhead due to the need to create new copies for modifications. It’s important to consider the specific use case and performance requirements.


Immutable data structures provide numerous benefits such as thread safety, predictability, and potential performance optimizations. Examples of immutable data structures include strings, sets, and tuples. However, it’s important to consider memory usage and performance implications when deciding to use immutable data structures in your code.

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