The Importance of Pointer Receivers When Copying Instances

When manipulating data, understanding the nuances of passing values by reference or by value is crucial. In Go, methods can be defined with either value or pointer receivers, and it's essential to comprehend the implications of this choice, especially when copying instances.

Value Receivers

Methods with value receivers operate on a copy of the value they receive. Any modifications made within the method will not affect the original value. This ensures that calling methods on copied instances will not compromise the original data.

Pointer Receivers

Conversely, methods with pointer receivers allow direct access and modification of the original value. Such methods have the potential to mutate the data, which can lead to subtle and unintended side effects. Copying instances with pointer receivers carries the risk of introducing inconsistencies between the original and copied data.

Example: Wrapper Struct

To illustrate the issue, consider a type called Wrapper with fields v (value) and p (pointer to a value):

type Wrapper struct {
    v int
    p *int
}

The Set() method with a pointer receiver modifies both v and the pointed value:

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

Suppose we have a Wrapper instance a:

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

Calling Set() on a will modify both v and *p:

a.Set(1)

Now, if we copy a to create b, we expect that both instances will have consistent values:

b := a

However, subsequent modifications to a using Set() will not propagate to b due to the copying of the pointer p, resulting in inconsistent data:

fmt.Printf("a.v=%d, a.p=%d;  b.v=%d, b.p=%d\n", a.v, *a.p, b.v, *b.p)
a.Set(1)
fmt.Printf("a.v=%d, a.p=%d;  b.v=%d, b.p=%d\n", a.v, *a.p, b.v, *b.p)

Output:

a.v=0, a.p=0;  b.v=0, b.p=0
a.v=1, a.p=1;  b.v=0, b.p=1

Conclusion

When dealing with types that have methods with pointer receivers, it's crucial to avoid copying instances to prevent data inconsistencies. Instead, operate on pointer values to ensure that modifications are reflected in all instances that reference the same underlying data.