What is Copy On Write(COW) in Swift?

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Copy-On-Write (COW) is a memory management technique used in Swift programming language to optimize the performance of memory allocation and deallocation operations. In COW, whenever a new instance of a data structure is created, the original data structure is not modified, instead, a new copy of the data structure is created in memory and modifications are made to the new copy. Copy-on-write is a highly used strategy in Swift for optimising memory usage. The main idea of COW is that when multiple callers want to access the resources which are same, you can put them pointing to the same resource. The state of the resource will be maintained until a caller tries to modify its “copy” of the resource. The main advantage is that if a caller never makes any modifications, no true copy need ever be created. Don’t confuse copy on right with reference type.

This technique helps to reduce the time and memory overhead involved in creating and managing multiple copies of large data structures. Benefits of using COW in Swift include:

  1. Improved Performance: COW reduces the time and memory overhead involved in creating and managing multiple copies of large data structures, leading to improved performance.
  2. Reduced Memory Usage: Since COW creates a new copy of a data structure only when it needs to be modified, it helps to reduce the overall memory usage in an application.
  3. Ease of Use: Swift implements COW automatically for data structures such as arrays and dictionaries, making it easy for developers to use and take advantage of this technique.

COW is a powerful technique for optimizing the performance and memory usage of Swift applications, and helps to ensure that data structures are manipulated safely and efficiently. Copy on Write is adapted to arrays and dictionaries in Swift which are value types.
Those value types initially point as reference types until some caller tries to modify them. The true copy will be created only when the caller modifies that copy.

var firstFewNumbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]
var secondFewNumbers = firstFewNumbers
var thirdFewNumbers = firstFewNumbers

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For example, I have an array called firstFewNumbers. After that I copy the same array in secondFewNumbers and thirdFewNumbers. Without copy on write availability, Swift has to copy all elements of firstFewNumbers to other arrays even that array never modified. Thanks to copy on write, secondFewNumbers and thirdFewNumbers will not use any memory at this time.


Now, I have appended an item in the secondFewNumbers and as it is modified it will make a true copy which will consume memory. Thatโ€™s how Swift can ensure that no memory resource is wasted when I make a copy of array or dictionaries that never modified.

โœ๏ธ Written by Ishtiak Ahmed

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