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26 - Reflection (reflect)

Go is statically typed. Reflection allows a program to inspect and manipulate its own types and variables at runtime. The standard library uses it heavily for things like encoding/json and fmt.Printf.

1. reflect.Type and reflect.Value

  • reflect.TypeOf(x) gives you metadata: the name, the kind (Struct, String, Slice), and struct tags.
  • reflect.ValueOf(x) gives you the actual runtime value, allowing you to read it or mutate it.

The Production Danger

Reflection comes with three enormous penalties:

  1. Loss of Compile-Time Safety: If you try to call v.SetString("hello") on an int, it will cause a structural panic at runtime, crashing your app.
  2. Performance Devastation: Reflection defies compile-time optimizations like inlining. It is orders of magnitude slower than native code.
  3. Obscure Code: Heavy reflection creates code that is impossible to navigate with an IDE.

2. When to Use It (And When NOT To)

When to use it:

  • Building generic decoders like json.Unmarshal.
  • Validating structs dynamically via Struct Tags (validate:"required").

When NOT to use it:

  • If an interface works, use an interface.
  • If Generics ([T any]) solve the problem mathematically at compile time, use Generics.

3. How to Mutate Values

To change a value via reflection, you MUST pass a pointer to it. v := reflect.ValueOf(&myStruct).Elem() If you do not call .Elem() on a pointer, you cannot use .Set(), and the app will panic.

Exercises

  • ex01_validator.go
  • ex02_copier.go
  • ex03_refactor.go