What is the purpose of a field named "_" (underscore) containing an empty struct?

Question

I've seen two pieces of Go code using this pattern:

type SomeType struct{   Field1 string   Field2 bool   _      struct{}    // <-- what is this? } 

Can anyone explain what this code accomplishes?

Answer 1

This technique enforces keyed fields when declaring a struct.

For example, the struct:

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

can only be declared with keyed fields:

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

One reason for doing this is to allow additional fields to be added to the struct in the future without breaking existing code.