What's the Difference Between Nil, Empty, and Non-Nil Slices in Go?

Nil, Empty, and Non-Nil Slices in Go

As a beginner in Go programming, it's crucial to understand the nuances of slices, including nil, non-nil, and empty slices. In this article, we will delve into their differences and provide insights on detecting whether a slice is empty.

Observable Behavior

While nil and empty slices (with zero capacity) share similar observable behavior, they are distinct. Both types can be passed to the len() and cap() functions and traversed with for range loops (with zero iterations). Additionally, they can be sliced without violating slice expression restrictions.

Under the Hood

Internally, a slice's value is represented by a reflect.SliceHeader struct containing fields for data, length, and capacity.

• Nil slice: The SliceHeader struct is initialized with zero values (all fields equal to 0), representing an empty slice with no underlying array.
• Non-nil slice with cap = len = 0: While the Length and Cap fields are zero, the Data pointer may not be. This slice points to a zero-sized underlying array.
• Empty slice (either nil or non-nil): An empty slice has a length of 0, regardless of its capacity.

Testing for Emptiness

To determine if a slice is empty, simply compare its length to 0: len(s) == 0. This condition holds true for both nil and non-nil slices with zero capacity.

Additional Note

Although nil and empty slices behave similarly in most cases, some packages like encoding/json and fmt may treat them differently. Therefore, it's recommended to differentiate nil and empty slices when interacting with such packages.

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