Why Doesn\'t Go Detect Ambiguity in String() Method Invocation for Embedded Structs at Compile Time?

Weird Method Invocation for Embedded Structs in Go: Understanding String()

In Go, embedded structs inherit the methods of their embedded types. However, when multiple embedded types define methods with the same name, ambiguities arise. Let's explore this behavior focusing on the String() method in particular.

In the example code provided:

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)
}

When the Engineer struct is printed using fmt.Println(engineer), the output varies depending on the presence of the String() methods in the embedded types.

With Person.String():

• fmt calls Person.String() because it has the shallowest depth in the Engineer type.
• The output is: "{name: John Doe, age: 35 3 Construction}"

Without Person.String():

• fmt calls TaxPayer.String() because it's the only promoted String() method.
• The output is: "3"

Without Both String() Methods:

• No String() method can be promoted from the embedded types.
• fmt prints the default field values: "{{John Doe 35} {3} Construction}"

These scenarios highlight the depth rule and ambiguity resolution for promoted methods in Go. However, the question arises why the ambiguity is not detected at compile time when multiple String() methods with a depth of zero are present.

Ambiguous Selector Check:

Normally, when attempting to invoke a method with an ambiguous selector, such as engineer.Foo(), a compile-time error occurs. However, this does not happen with methods named String().

Reason:

When printing a value without explicitly calling its String() method, the fmt.Println function checks if the value implements fmt.Stringer. It then calls the implemented String() method. Since all Go types implicitly implement Stringer by default (https://golang.org/doc/go1.19#fmt), there is always a promoted String() method for any type.

Conclusion:

The ambiguity in method invocation for embedded structs arises due to the depth rule and the special handling of the String() method for printing values. By understanding these rules and the subtle differences in method promotion, developers can avoid unexpected behavior and maintain code clarity in Go programs.

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