Why Does Appending to a Go Slice Copy Change the Original?

Understanding Slice Behavior in Go: Why Does Appending Change the Original Slice?

In the provided code snippet, we have a function someFunc that operates on a slice of integers. Inside the function, a temporary slice tempA is created and assigned the value of the input slice A. However, upon appending to tempA, the original A is also modified. This behavior may seem counterintuitive, so let's explore why it occurs.

In Go, slices are declared as a data type []T, where T represents the element type. Despite its name, a slice is not a collection of elements stored contiguously in memory. Instead, it is a header structure containing the following information:

• Length: The number of elements in the slice
• Capacity: The number of elements the underlying array can hold
• Pointer: A reference to the underlying array where the slice data is stored

When you assign the value of a slice variable, you are not copying the entire array. Instead, you are creating a new slice header that points to the same underlying array. Thus, when you perform an operation such as appending to tempA, you are also modifying the underlying array and, consequently, the values in the original slice A.

This behavior is essential for ensuring efficient memory management and avoiding unnecessary copying. By using a pointer-based approach, Go slices can share the same underlying array while maintaining separate length and capacity values. This allows for lightweight modifications to slices without the need for expensive copying operations.

For a deeper understanding of slice behavior and the underlying data structures, refer to the following resource: https://blog.golang.org/slices

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