How does the reflect package work in Go and when should it be used?

The reflect package provides runtime type introspection. It lets you inspect types and values at runtime, set values dynamically, and call methods whose signatures are not known at compile time. It is the foundation of JSON marshalling, ORM field mapping, and dependency injection frameworks.

import "reflect"

// reflect.TypeOf and reflect.ValueOf
type Person struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}
p := Person{"Alice", 30}

t := reflect.TypeOf(p)
v := reflect.ValueOf(p)

fmt.Println(t.Name())       // "Person"
fmt.Println(t.Kind())       // struct
fmt.Println(v.Field(0))     // Alice
fmt.Println(v.Field(0).Type()) // string

// Iterating struct fields
for i := 0; i < t.NumField(); i++ {
    field := t.Field(i)
    val   := v.Field(i)
    tag   := field.Tag.Get("json")
    fmt.Printf("%s (%s) [json:%s] = %v\n", field.Name, field.Type, tag, val)
}

// Setting values via reflect — requires addressable value
vp := reflect.ValueOf(&p).Elem() // dereference pointer to get addressable value
vp.FieldByName("Name").SetString("Bob")
fmt.Println(p.Name) // Bob

// Calling methods dynamically
method := v.MethodByName("String") // if Person has a String() method
if method.IsValid() {
    results := method.Call(nil)
    fmt.Println(results[0])
}

// reflect.DeepEqual — structural equality (used in tests)
fmt.Println(reflect.DeepEqual([]int{1,2,3}, []int{1,2,3})) // true

Performance warning: reflection is 10–100× slower than direct type-specific code because it bypasses compiler optimisations. Use reflection in framework/library code that genuinely needs runtime type introspection; avoid it in hot paths. Go 1.18+ generics often replace the need for reflection in type-agnostic algorithms.

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