What is the third parameter of a Go struct field?

Question

type Config struct {
    CommitIndex uint64 `json:\"commitIndex\"`
    // TODO decide what we need to store in peer struct
    Peers []*Peer `json:\"peers\"`
}
         


        

Answer 1

It's called a struct tag, they can be parsed using the reflect package at runtime.

From https://golang.org/ref/spec#Struct_types:

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration.

The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.

Some packages that use reflection like json and xml use tags to handle special cases better.