How Do Interface Pointers Behave When Calling Methods in Go?

Calling Methods on Interface Pointers in Go

If you're working with the Gorp library, which implements the SqlExecutor interface, you might find yourself running into issues when trying to call methods on pointers to interface values. This confusion arises because Go doesn't strictly follow the concept of "call by reference."

In Go, interfaces are used to represent a group of objects that have common methods. When you assign a value to an interface, you're not actually storing a reference to that object, but rather a pointer to the object's value. This means that if you call a method on an interface value, you're actually calling it on the underlying object.

Consider this example:

package main

import (
    "fmt"
)

type Person struct {
    Name string
}

func (p *Person) Greet() {
    fmt.Println("Hello, my name is", p.Name)
}

func main() {
    // Create a person object
    p := Person{Name: "John"}

    // Create an interface value that points to the person object
    var person interface{} = p

    // Call the Greet method on the interface value
    person.Greet() // Output: Hello, my name is John
}

In this example, we create a Person object and assign it to the person interface value. When we then call the Greet method on the person interface value, it correctly calls the Greet method on the underlying Person object. This is because the interface value is actually pointing to the Person object.

When it comes to pointers to interface values, things can be a bit more confusing. In Go, it's generally not necessary to use pointers to interface values. The only scenario where it might be necessary is when you need to modify the interface value itself. For example, if you wanted to change the object that the interface value is pointing to, you would need to use a pointer to the interface value.

Here's an example:

package main

import (
    "fmt"
)

type Person struct {
    Name string
}

func (p *Person) Greet() {
    fmt.Println("Hello, my name is", p.Name)
}

func main() {
    // Create a person object
    p := Person{Name: "John"}

    // Create a pointer to the person object
    pPtr := &p

    // Create an interface value that points to the person object
    var person interface{} = pPtr

    // Change the object that the interface value is pointing to
    person = &Person{Name: "Jane"}

    // Call the Greet method on the interface value
    person.Greet() // Output: Hello, my name is Jane
}

In this example, we create a pointer to the Person object and assign it to the person interface value. When we then change the object that the person interface value is pointing to, the Greet method is called on the new object. This is because we are modifying the interface value itself, not the underlying object.

In general, you shouldn't need to use pointers to interface values in your Go code. However, if you do need to use them, it's important to remember that they behave differently from pointers to regular values.

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