Why Doesn't `fmt.Println` Call My Stringer Interface Method When Using a Non-Pointer Object?

Why Isn't My Stringer Interface Method Getting Invoked When I Use fmt.Println?

When working with Go's Stringer interface, users may encounter situations where their String() method is not called when passing non-pointer objects to fmt.Println. To shed light on this behavior, let's examine the following code:

package main

import "fmt"

type Car struct {
    year int
    make string
}

func (c *Car) String() string {
    return fmt.Sprintf("{make:%s, year:%d}", c.make, c.year)
}

func main() {
    myCar := Car{year: 1996, make: "Toyota"}
    fmt.Println(myCar)
}

Here, we define a Car struct and implement the String() method as a value method. However, when we pass myCar as the argument to fmt.Println, the object is converted to an interface{}. This causes the fmt package to use its own default formatting, bypassing our String() method.

The key to understanding this behavior lies in the fact that we have implemented the String() method on a pointer receiver (*Car). When passing a value of type Car, the compiler does not automatically convert it to a pointer. Therefore, the fmt package cannot match our String() method to the value we passed.

Instead, to invoke our String() method when passing a non-pointer object, we need to manually convert the value to a pointer using the & operator:

fmt.Println(&myCar)

By doing so, we pass a pointer to the Car object to fmt.Println. This allows the value receiver String() method to be invoked and our custom formatting to be applied.

In summary, when working with the Stringer interface, it's important to remember that the method must be defined on the correct receiver type. For pointers, a pointer receiver is appropriate, while for value types, a value receiver can be used. If manually passing a pointer is not desirable, one can consider implementing the String() method as a pointer method on the actual Car struct.

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