What's the Best Way to Initialize an Empty Slice in Go?

Understanding Empty Slice Initialization in Go: Comparing Different Approaches

When creating an empty slice in Go, developers may encounter the following options: using make([]int, 0), using []int{}, or simply leaving it as a nil value. It's essential to understand the nuances and implications of each approach.

make([]int, 0) vs. []int{}

Both make([]int, 0) and []int{} result in semantically identical outcomes. They both produce an empty slice of type []int. However, there's a subtle difference under the hood. make([]int, 0) triggers an internal call to runtime.makeslice in Go 1.16 and later versions, while []int{} is optimized by the compiler.

Advantages of nil Values

Another option is to leave the slice as a nil value, initialized with var, such as var myslice []int. A nil slice is functionally equivalent to a zero-length slice, but it points to nothing. It has a length of zero and can be appended to without requiring any allocation.

Marshaling Differences

It's worth noting that while make([]int, 0) and []int{} represent empty slices, there is a distinction when marshaling them into JSON using json.Marshal. An empty slice will be marshaled into "[]", while a nil slice will be marshaled into "null".

Allocation Considerations

Contrary to popular belief, none of the aforementioned methods will cause any memory allocation. Empty slices are stored inline, so they consume no extra memory beyond what is necessary for their header.

Conclusion

When choosing between make([]int, 0), []int{}, or nil, the specific requirements of the application should be considered. []int{} is the most straightforward and optimized approach, while nil values provide convenience when marshaling into JSON or working with zero-length slices.

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