Why Doesn't fmt.Println Invoke My Stringer Interface's String Method When Passing a Value-Based Object?

Stringer Interface Confusion with Println: Understanding Value-Based vs. Pointer-Based Objects

Question:

In a scenario where an object implements the Stringer interface, why doesn't the object's String method get invoked when using fmt.Println if the object is value-based?

Code Example:

Consider the following Go code:

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)
}

When myCar is a pointer, the String method gets invoked as expected. However, when myCar is a value, the default Go formatting is used instead.

Answer:

The reason for this behavior lies in the way Go interfaces work. When you specify a type that implements an interface (Stringer, in this case), Go expects that type to be the exact type of the interface. When you pass a value of type Car to fmt.Println, it is implicitly converted to interface{}, and there is no type Car in the interface{} type system. Instead, it's a type *Car (a pointer to Car).

The fmt.Println function uses a type switch to determine how to print the value based on its type. For a Stringer interface, it checks if the value implements the String method. Since Car (value-based) doesn't implement String, the default formatting is used. However, when you call myCar.String() explicitly, the compiler automatically converts it to (&myCar).String(), which has the correct *Car type and invokes the desired formatting method.

To ensure that the object is formatted as desired regardless of its type, you have two options:

1. Implement String on Car (value-based): This requires creating a separate String method for Car that operates on value receivers, which would incur unnecessary object copying.
2. Always pass a pointer to fmt.Println: By using fmt.Println(&myCar), you ensure that the passed value has the correct *Car type and the String method will be invoked as expected.

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