Why Does Go\'s `fmt.Println` Produce Unexpected Output with Embedded Types and Multiple `String()` Methods?

Unexpected String() Method Behavior in Go Embedded Types

When using embedded types in Go, the fields and methods of the embedded type are effectively inherited by the embracing type. This can lead to unexpected behavior, especially when dealing with custom String() methods.

Consider this example:

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)
}

In this code, the Engineer type embeds both the Person and TaxPayer types, which define their own String() methods for formatting the respective data. However, if we instantiate an Engineer object and call fmt.Println(engineer), the output is not as expected:

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

This output suggests that the Engineer.String() method is being invoked, but the TaxPayer.String() method is also contributing to the result. This is because the String() method is promoted to the Engineer type when embedded, and both the Person.String() and TaxPayer.String() methods are eligible for invocation.

To clarify this, consider the following scenario:

fmt.Println(engineer.String()) // Compile error: ambiguous selector engineer.String

In this case, the compiler raises an error because engineer has multiple promoted String() methods, resulting in an ambiguous selector. However, fmt.Println(engineer) succeeds because the compiler automatically selects the default formatting for Engineer based on its fields.

The reason for this apparent discrepancy is that the fmt.Println() function essentially delegates the string conversion to the fmt package. When it encounters a value that implements the fmt.Stringer interface (which defines a String() method), it invokes that method to obtain the string representation.

In our example, since both Person.String() and TaxPayer.String() exist, neither is promoted to Engineer and the default formatting is used. However, in the case of fmt.Println(engineer.String()), the compiler encounters the ambiguous selector and raises an error.

In conclusion, embedded types can lead to unexpected String() method behavior when multiple embedded types define such methods. It is important to understand the mechanics of embedding and method promotion to avoid potential confusion and ensure desired output.

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