Go Academy · Lesson

Inspecting Struct Fields and Tags

Iterating fields and reading struct tags

Lesson 2 of 413 steps

Inspecting Struct Fields and Tags is a free Go Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, and your progress syncs across the web and the CoddyKit app. The Go Academy course includes 4 lessons in total.

StructField type

reflect.StructField describes one field of a struct: Name, Type, Tag, Index, Offset, Anonymous, and IsExported.

Iterating fields

Loop over all fields of a struct type:

t := reflect.TypeOf(struct{ ID int; Name string }{})
for i := 0; i < t.NumField(); i++ {
    f := t.Field(i)
    fmt.Println(f.Name, f.Type, f.IsExported())
}

Reading struct tags

Tags are accessed via StructField.Tag.Get("key"):

type User struct {
    ID   int    `json:"id" db:"user_id"`
    Name string `json:"name,omitempty"`
}

f, _ := reflect.TypeOf(User{}).FieldByName("Name")
fmt.Println(f.Tag.Get("json")) // "name,omitempty"

Parsing tag options

Tags may contain comma-separated options. Parse them with strings.Split:

tag := f.Tag.Get("json") // "name,omitempty"
parts := strings.Split(tag, ",")
name := parts[0] // "name"
omitempty := len(parts) > 1 && parts[1] == "omitempty"

FieldByName and FieldByIndex

Look up a field by name or index. FieldByName searches exported fields in the struct and embedded structs:

f, ok := reflect.TypeOf(User{}).FieldByName("Name")
if ok { fmt.Println(f.Tag.Get("json")) }

Embedded struct fields

For embedded structs, Anonymous is true. Use Type.FieldByIndex([]int{0,1}) to access nested fields.

Unexported fields

Unexported (lowercase) fields can be inspected for their type and tag but their values cannot be read or set via reflection — a panic results.

Building a field cache

Parsing struct tags on every call is expensive. Cache the result in a sync.Map keyed by reflect.Type:

var fieldCache sync.Map
func getFields(t reflect.Type) []fieldInfo {
    if v, ok := fieldCache.Load(t); ok { return v.([]fieldInfo) }
    fields := parseFields(t)
    fieldCache.Store(t, fields)
    return fields
}

Use case: simple ORM

Walk all fields with a "db" tag to build an INSERT column list dynamically:

t := reflect.TypeOf(u)
v := reflect.ValueOf(u)
for i := 0; i < t.NumField(); i++ {
    col := t.Field(i).Tag.Get("db")
    val := v.Field(i).Interface()
    // build query...
}

Use case: config loader

Walk struct fields with an "env" tag and populate them from environment variables:

if env := f.Tag.Get("env"); env != "" {
    if val := os.Getenv(env); val != "" {
        v.Field(i).SetString(val)
    }
}

StructTag.Lookup

Tag.Lookup("key") returns the tag value and a boolean indicating whether the key was present at all (unlike Get, which returns "" for absent keys).

val, ok := f.Tag.Lookup("json")
if !ok { /* no json tag */ }

Quick Check

How do you read the value of the "json" struct tag from a reflect.StructField?

Answer it in the interactive lesson

Recap: Struct Fields and Tags

Key points:

reflect.TypeOf → .NumField(), .Field(i) for field metadata
StructField.Tag.Get("key") to read tag values
Cache field metadata in sync.Map to avoid repeated parsing
Use Tag.Lookup when absent tag vs empty string matters

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Frequently Asked Questions

Is the “Inspecting Struct Fields and Tags” lesson free?

Yes — any 3 lessons of the Go Academy learning path, including “Inspecting Struct Fields and Tags”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Go Academy course includes 4 lessons in total.

What will I learn in “Inspecting Struct Fields and Tags”?

Iterating fields and reading struct tags You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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How long does the “Inspecting Struct Fields and Tags” lesson take?

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