Why Do Go Struct Setters Need Pointers to Modify the Original Struct?

Using Struct Setters: Understanding the Importance of Pointers

When working with structs in Go, the use of setters can modify the struct's properties. However, there can be occasions where the desired modifications are not reflected as expected. This is due to the way structs are passed into functions.

In the example code provided:

type T struct { Val string }

func (t T) SetVal(s string) {
    t.Val = s
}

func (t *T) SetVal2(s string) {
    (*t).Val = s
}

The provided SetVal function doesn't modify the original struct as anticipated because a copy of the struct is passed as a value into the function. Modifications made to this copy do not affect the original struct.

Alternatively, the SetVal2 function expects a pointer to a struct, allowing it to modify the original struct. This is because Go passes pointers to structs by value, meaning the function receives a reference to the same memory location as the original struct.

To demonstrate this behavior, consider the following code:

v := T{"abc"}
fmt.Println(v)        // {abc}
v.SetVal("pdq")
fmt.Println(v)        // {abc}
v.SetVal2("xyz")
fmt.Println(v)        // {xyz}

In this example, the original struct v is not modified by SetVal, but it is modified by SetVal2. This is evident by the difference in output.

The key takeaway is that in Go, when using setters for structs, it is important to understand the difference between passing by value and passing by pointer. Using pointers allows functions to modify the original struct, while passing by value creates a copy of the struct within the function.

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