Why Does a '_'-Prefixed Field in a Go Struct Enforce Keyed Initialization?

Uncovering the Enigma of the "_"-Prefixed Field: Enforcing Keyed Struct Declarations

In the realm of Go programming, a peculiar syntax has emerged: the "_"-prefixed field within a struct. This cryptic notation has left many puzzled about its purpose. Let's unravel the mystery behind this structure.

The "_"-prefixed field is an empty struct, effectively a placeholder that influences how the struct is declared. Consider the following code snippet:

type SomeType struct {
  Field1 string
  Field2 bool
  _      struct{}
}

This code introduces a struct named "SomeType" with three fields: "Field1," "Field2," and "_." The empty struct denoted by "_" serves a specific function.

When constructing a variable of the "SomeType" struct, fields must be explicitly assigned with keys. For instance:

// ALLOWED:
bar := SomeType{Field1: "hello", Field2: true}

// COMPILE ERROR:
foo := SomeType{"hello", true}

Without a "_"-prefixed field, default initialization of a struct would be possible as seen in "foo," but in this case, the compiler prompts an error.

This mechanism ensures that all fields are accessed using keys, allowing for the modification and addition of fields in the future without compromising existing code that relies on key-based field access. Thus, the "_"-prefixed empty struct becomes a syntactic safeguard for future extensibility and interoperability.

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