The reflect package

Learn the basics of the reflect package.

The reflect package

The reflect package lets you inspect the type and contents of a value at run time. You can ask what an any value really is when its type is unknown at compile time.

var x any = 3.14
fmt.Println(reflect.TypeOf(x))  // float64

Packages such as encoding/json and fmt read struct fields this way.

The cost: speed and type safety

Reflection is powerful but has two costs: it is slower than normal code, and mistakes show up as a panic at run time instead of a compile error. That is why interfaces and generics come first; reflect is used when writing truly generic tools (serializers, ORMs, validators).

Exercise Print the type

Declare var x any = 42 and print the result of reflect.TypeOf(x) with fmt.Println.

Show example solution

package main

import (
    "fmt"
    "reflect"
)

func main() {
    var x any = 42
    fmt.Println(reflect.TypeOf(x))
}

Test yourself

When is the reflect package needed most?
- To read files
- When inspecting values whose type is unknown at compile time
- To make loops faster
- To start goroutines
Show answer

When inspecting values whose type is unknown at compile time — reflect reads the real type and fields of a value passed as any at run time; JSON encoders rely on it.

Query the type

var x any = "Go"
fmt.Println(reflect.____(x))

Show answer

var x any = "Go"
fmt.Println(reflect.TypeOf(x))

reflect concepts

reflect.TypeOf

Returns the type
reflect.ValueOf

Wraps the value
any

Can hold any type
panic

Happens on misuse of reflect

Next lesson: reflect.Type

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