Why does `fmt.Println()` behave differently with embedded types in Go when the embedded types have conflicting `String()` methods?

Ambiguous Behavior of String() Method with Embedded Types in Go

When dealing with embedded types in Go, understanding the behavior of the String() method can be puzzling. Let's delve into this behavior based on a specific code sample.

Consider the following code:

type Engineer struct {
    Person
    TaxPayer
    Specialization string
}

type Person struct {
    Name string
    Age  int
}

func (p Person) String() string {
    return fmt.Sprintf("name: %s, age: %d", p.Name, p.Age)
}

type TaxPayer struct {
    TaxBracket int
}

func (t TaxPayer) String() string {
    return fmt.Sprintf("%d", t.TaxBracket)
}

func main() {
    engineer := Engineer{
        Person: Person{
            Name: "John Doe",
            Age:  35,
        },
        TaxPayer: TaxPayer{3},
        Specialization: "Construction",
    }
    fmt.Println(engineer)
}

The output of this code is:

{name: John Doe, age: 35 3 Construction}

However, if Person.String() is removed, the output becomes:

3

And if TaxPayer.String() is also removed, the output changes to:

{{John Doe 35} {3} Construction}

Initially, it seems that there must be an implicit String() method for the Engineer struct. However, this is not the case.

The String() Method in Embedded Types

When types are embedded within a struct, their fields and methods become accessible through the embedding type. This "promotion" of methods can lead to ambiguity if multiple embedded types define a method with the same name, such as String().

In the given code sample, since both Person and TaxPayer have a String() method, the promotion of these methods to the Engineer type causes ambiguity. This is why Engineer.String() results in a compilation error.

Why no Ambiguity Error When Using fmt.Println()

Despite the ambiguity in Engineer's method set, fmt.Println(engineer) does not result in a compilation error. This is because fmt.Println() calls fmt.Fprint(os.Stdout, engineer).

fmt.Fprint() checks if the passed value implements the fmt.Stringer interface, which includes a String() method. If it does, String() is used to produce a string representation of the value.

In this case, since neither Person nor TaxPayer implements fmt.Stringer, the default formatting (struct fields) is used instead. This results in the output we see when fmt.Println(engineer) is called.

Conclusion

Understanding the behavior of embedded types and the promotion of their methods is crucial in Go. When multiple embedded types define a method with the same name, it can lead to ambiguity, resulting in compilation errors. However, when using fmt.Println(), the default formatting is used when the passed value does not implement fmt.Stringer.

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