How Can I Modify a Slice Passed as an Argument in a Go Function?

Passing Slices as Arguments with Original Slice Modifications

Passing variables by value in Go limits the effects of function modifications to the function's scope. In cases where a slice needs to be modified outside the function, passing a slice directly as an argument is insufficient.

The Slice Append Function Behavior

The append function allocates a new slice and copies the elements from the existing slice, including any appends made within the function. However, the original slice remains unaffected.

Example: Recursion on an Accumulating Slice

The code snippet provided initially attempts to accumulate validation messages in a slice passed as an argument to a recursive function. However, the function's modifications are not reflected in the original slice.

Solutions

There are two main solutions to this issue:

1. Pass a Pointer to the Slice: By passing a pointer to the original slice, any modifications made within the function will directly affect the original.

func myAppend(list *[]string, value string) {
    *list = append(*list, value)
}

2. Return the Modified Slice from the Function: Instead of passing an accumulator, the recursed function can return the modified slice, eliminating the need to pass an accumulator variable.

func validate(obj Validatable, messages []ValidationMessage) []ValidationMessage {
    //...Processing

    return messages
}

Idiomatic Go Approach

Passing pointers to slices is an idiomatic Go approach when the function needs to modify the original slice. As demonstrated in the returning slice approach, it is also possible to circumvent the need for accumulators by returning the modified slice.

Performance Considerations

The performance impact of passing a pointer versus returning a slice is negligible in most scenarios. However, the precise behavior may vary depending on compiler optimizations and specific code usage.

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