Go Pointers: What's the Difference Between `&` (Address-of) and `*` (Dereference)?

& and * Pointers

In Go, passing variables as function arguments can sometimes trigger compiler errors, which can be resolved by using pointers. However, the distinctions between & and * pointers often lead to confusion. This article aims to clarify the differences and usage scenarios for both pointer types.

Addressing the Error

In your example, the error is likely due to the mismatch between the expected argument type and the actual variable passed. The Decode function requires an address or pointer to a User struct, but you are passing the value directly. To resolve this, you can use the & operator to obtain the address of the User variable:

if err := decoder.Decode(&u); err != nil {
    http.Error(rw, "could not decode request", http.StatusBadRequest)
    return
}

Understanding Pointers

Pointers are variables that hold addresses of other variables. The & operator returns the address of a variable, while the * operator allows us to access the value at that address.

In the example above, &u gives the address of the User struct, which is then passed to the Decode function that expects a pointer.

However, if you had created the User instance using:

u := new(User)

Then u would already be a pointer, and the & operator would not be necessary. You could also explicitly create a pointer using:

var u *User

Difference between & and *

The key difference between & and * is that * represents a redirection to the value stored at the address, while & returns the address itself.

Example:

var y int
var pointerToY *int
var pointerToPointerToInt **int

y = 10
pointerToY = &y
pointerToPointerToInt = &pointerToY

Now:

• y holds the value 10.
• *pointerToY redirects to y and prints its value 10.
• &y returns the address of y.
• &pointerToY returns the address of pointerToY.
• &pointerToPointerToInt returns the address of pointerToPointerToInt.
• **pointerToPointerToInt prints the value 10 since it redirects twice to get to y.

Usage Scenarios

& (Address-of Operator):

• To pass the address of a variable to a function.
• To obtain the address before assigning to a pointer.
• To compare two pointers.

Example:

func swap(x, y *int) {
    *x, *y = *y, *x
}

* (Dereference Operator):

• To access the value stored at an address.
• To assign a value to a pointer.
• To compare two pointers.

Example:

var p *int

*p = 10

Conclusion:

Understanding & and pointers is crucial in Go for efficient variable handling and addressing compiler errors. The key distinction lies in the 'redirect' behavior of , while & returns the actual address of the variable. By carefully considering the intended usage scenarios, you can effectively leverage pointers in your Go code for optimal performance and correctness.

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