How Can I Clone a Go Struct with Unexported Fields?

How to Clone a Structure with Unexported Field?

In Go, structures with unexported fields pose challenges when cloning objects. If a structure's field is unexported, it cannot be accessed or modified outside the declaring package.

Problem:

Consider the following T structure:

type T struct {
    S  string
    is []int
}

Assigning one T object to another will not create an independent copy because the unexported field is is shared.

Solution 1: Clone Method

The most straightforward solution is to provide a Clone method in the package where T is defined. This method can explicitly copy the fields of T, including the unexported field is.

Solution 2: Setters

If providing a Clone method is not feasible, providing a setter function for the unexported field, such as SetIs(), allows for controlled modification of the field.

Limitation:

It's important to note that these solutions rely on the availability of the source code for the type. If working with third-party packages, cloning structures with unexported fields may not be possible without modifications to the original package.

Unsafe Approach (Not Recommended):

While using the unsafe package can bypass access restrictions to unexported fields, this approach is strongly discouraged as it violates Go's encapsulation principles and can lead to runtime errors.

Alternative Option:

If modifying unexported fields is not essential, consider extracting the desired data into a separate, cloned structure. For example, the unexported is field could be extracted using reflection, preserving its initial value.

var r T
s := T{S: p.S}

Conclusion:

Cloning structures with unexported fields requires careful consideration and may involve package-specific solutions. It's crucial to prioritize code safety and encapsulation while evaluating the options available.

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