**Should You Copy Instances of Go Types with Pointer Receivers?**

Copy of Instance Types with Methods

In Go, it is important to understand the difference between value receivers and pointer receivers when defining methods. When a method uses a value receiver, a copy of the receiver is passed to the function, preventing the method from changing the original value. However, if the method uses a pointer receiver, it has direct access to the original value and can modify it.

Copying Instances with Only Value Receivers

If a type has only methods with value receivers, it is safe to copy instances of that type. This is because any method call on the copy will only affect the copy, not the original value. As the receiver is passed by value, any modifications made within the method will be limited to the copy.

Copying Instances with Pointer Receivers

However, copying instances of a type that has methods with pointer receivers can lead to problems. This is because a method with a pointer receiver can modify the original value. Consider the following example:

<code class="go">type Wrapper struct {
    v int
    p *int
}

func (w *Wrapper) Set(v int) {
    w.v = v
    *w.p = v
}

func main() {
    a := Wrapper{v: 0, p: new(int)}
    b := a

    a.Set(1)
}</code>

In this example, the Wrapper type has a method with a pointer receiver, Set. This means that when this method is called, it can modify the original value of Wrapper. However, if we copy the value of a to b and then call Set on a, it will modify both the original value and the copied value. This is because the variable b is referencing the same underlying memory location as a, and any changes made to either will affect the other.

Avoid Copying Instances with Pointer Receivers

To avoid unpredictable behavior, it is generally best to avoid copying instances of a type that has methods with pointer receivers. Instead, use pointers to work with these instances. By working with pointers, you can ensure that any changes made within a method are applied to only the intended object.

Conclusion

Understanding the difference between value receivers and pointer receivers is crucial for writing robust and efficient Go code. If a type has only methods with value receivers, it is safe to copy instances of that type. However, if any method has a pointer receiver, it is important to avoid copying instances and instead use pointers to work with these objects.

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