Go Academy · Lesson

Inspecting Struct Fields and Tags

Iterating fields and reading struct tags

Inspecting Struct Fields and Tags is a free Go Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

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?

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

Frequently asked questions

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

Yes — the full text of “Inspecting Struct Fields and Tags” is free to read here on the web, and the Go Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Go Academy course, upgrade to CoddyKit PRO.

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.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

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

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Go Academy lesson?

Yes. Every Go Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Go Academy