Interfaces are one of the fundamental tools for abstraction in Go. Interfaces store type information when assigned a value. Reflection is a method of examining type and value information at runtime.

Go implements reflection with the reflect package which provides types and methods for inspecting portions of the interface structure and even modifying values at runtime.

With this post I hope to illustrate how parts of the interface structure relate to the reflect API and ultimately make using the reflect package more approachable!

Assigning a Value to an Interface

An interface encodes three things: a value, a method set, and the type of the stored value.

The structure for an interface looks like the following:

We can clearly see the three parts of the interface in that diagram: the _type is type information, *data is a pointer to the actual value, and the itab encodes the method set.

When a function accepts an interface as a parameter, passing a value to that function packs the value, method set, and type into the interface.

Examining Interface Data At Runtime with the Reflect Package

Once a value is stored in an interface, you can use the reflect package to examine its parts. We can’t examine the interface struct directly; instead the reflect package maintains its own copies of the interface structure to which we do have access.

Even though we’re accessing the interface via reflect objects, there’s a direct correlation to the underlying interface.

The reflect.Type and reflect.Value types provide methods to access portions of the interface.

reflect.Type focuses on exposing data about types and is therefore confined to the _type portion of the structure while reflect.Value has to combine type information with the value to allow programmers to examine and manipulate values and therefore has to peek into the _type as well as the data.

reflect.Type – Examining Types

The reflect.TypeOf() function is used to extract type information for a value. Since its only parameter is an empty interface, the value passed to it gets assigned to an interface and therefore the type, methodset, and value become available.

reflect.TypeOf() returns a reflect.Type which has methods that allow you to example the value’s type.

Below are a few of the Type methods available and their corresponding bits of the interface that they return.

An Example reflect.Type Usage

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package main

import (
	"log"
	"reflect"
)

type Gift struct {
	Sender    string
	Recipient string
	Number    uint
	Contents  string
}

func main() {
	g := Gift{
		Sender:    "Hank",
		Recipient: "Sue",
		Number:    1,
		Contents:  "Scarf",
	}

	t := reflect.TypeOf(g)

	if kind := t.Kind(); kind != reflect.Struct {
		log.Fatalf("This program expects to work on a struct; we got a %v instead.", kind)
	}

	for i := 0; i < t.NumField(); i++ {
		f := t.Field(i)
		log.Printf("Field %03d: %-10.10s %v", i, f.Name, f.Type.Kind())
	}
}

The purpose of this program is to print the fields in our Gift struct. When the g value is passed to reflect.TypeOf(), g is assigned to an interface which the compiler populates with type and method set information. This allows us to walk the []fields of the type portion of the interface structure and we get the following:

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2018/12/16 12:00:00 Field 000: Sender     string
2018/12/16 12:00:00 Field 001: Recipient  string
2018/12/16 12:00:00 Field 002: Number     uint
2018/12/16 12:00:00 Field 003: Contents   string

reflect.Method - Examining the itab/Method-Set

The reflect.Type type also allows you to access portions of the itab to extract method information from the interface.

Examining Methods with Reflect

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package main

import (
	"log"
	"reflect"
)

type Reindeer string

func (r Reindeer) TakeOff() {
	log.Printf("%q lifts off.", r)
}

func (r Reindeer) Land() {
	log.Printf("%q gently lands.", r)
}

func (r Reindeer) ToggleNose() {
	if r != "rudolph" {
		panic("invalid reindeer operation")
	}
	log.Printf("%q nose changes state.", r)
}

func main() {
	r := Reindeer("rudolph")

	t := reflect.TypeOf(r)

	for i := 0; i < t.NumMethod(); i++ {
		m := t.Method(i)
		log.Printf("%s", m.Name)
	}
}

This code quite literally iterates over the function data stored in the itab and displays the name of each method:

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2018/12/16 12:00:00 Land
2018/12/16 12:00:00 TakeOff
2018/12/16 12:00:00 ToggleNose

reflect.Value – Examining Values

So far we’ve only talked about type information – fields, methods, etc. reflect.Value gives us information about the actual value stored by an interface.

Methods associated with reflect.Values necessarily combine type information with the actual value. For example, in order to extract fields from a struct, the reflect package has to combine knowledge of the layout of the struct – particularly information about the fields and field offsets stored in the _type – with the actual value pointed to by the *data portion of the interface in order to properly decode the struct.

Example of Viewing an Modifying Values

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package main

import (
	"log"
	"reflect"
)

type Child struct {
	Name  string
	Grade int
	Nice  bool
}

type Adult struct {
	Name       string
	Occupation string
	Nice       bool
}

// search a slice of structs for Name field that is "Hank" and set its Nice
// field to true.
func nice(i interface{}) {
	// retrieve the underlying value of i.  we know that i is an
	// interface.
	v := reflect.ValueOf(i)

	// we're only interested in slices to let's check what kind of value v is. if
	// it isn't a slice, return immediately.
	if v.Kind() != reflect.Slice {
		return
	}

	// v is a slice.  now let's ensure that it is a slice of structs.  if not,
	// return immediately.
	if e := v.Type().Elem(); e.Kind() != reflect.Struct {
		return
	}

	// determine if our struct has a Name field of type string and a Nice field
	// of type bool
	st := v.Type().Elem()

	if nameField, found := st.FieldByName("Name"); found == false || nameField.Type.Kind() != reflect.String {
		return
	}

	if niceField, found := st.FieldByName("Nice"); found == false || niceField.Type.Kind() != reflect.Bool {
		return
	}

	// Set any Nice fields to true where the Name is "Hank"
	for i := 0; i < v.Len(); i++ {
		e := v.Index(i)
		name := e.FieldByName("Name")
		nice := e.FieldByName("Nice")

		if name.String() == "Hank" {
			nice.SetBool(true)
		}
	}
}

func main() {
	children := []Child{
		{Name: "Sue", Grade: 1, Nice: true},
		{Name: "Ava", Grade: 3, Nice: true},
		{Name: "Hank", Grade: 6, Nice: false},
		{Name: "Nancy", Grade: 5, Nice: true},
	}

	adults := []Adult{
		{Name: "Bob", Occupation: "Carpenter", Nice: true},
		{Name: "Steve", Occupation: "Clerk", Nice: true},
		{Name: "Nikki", Occupation: "Rad Tech", Nice: false},
		{Name: "Hank", Occupation: "Go Programmer", Nice: false},
	}

	log.Printf("adults before nice: %v", adults)
	nice(adults)
	log.Printf("adults after nice: %v", adults)

	log.Printf("children before nice: %v", children)
	nice(children)
	log.Printf("children after nice: %v", children)
}

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2018/12/16 12:00:00 adults before nice: [{Bob Carpenter true} {Steve Clerk true} {Nikki Rad Tech false} {Hank Go Programmer false}]
2018/12/16 12:00:00 adults after nice: [{Bob Carpenter true} {Steve Clerk true} {Nikki Rad Tech false} {Hank Go Programmer true}]
2018/12/16 12:00:00 children before nice: [{Sue 1 true} {Ava 3 true} {Hank 6 false} {Nancy 5 true}]
2018/12/16 12:00:00 children after nice: [{Sue 1 true} {Ava 3 true} {Hank 6 true} {Nancy 5 true}]

In this last example, we combine what we’ve learned to actually modify a value via a reflect.Value. In this case, someone wrote a function called nice() (probably Hank) that will toggle any struct item in a slice from naughty to nice where the name is “Hank”.

Notice that nice() is able to modify the value of any slice you pass to it and it doesn’t matter exactly what type it receives – as long as it is a slice of a struct that has a Name and Nice field.

Conclusion

Reflection in Go is implemented using interfaces and the reflect package. There’s no magic to it – when you use reflection, you directly access parts of an interface and values stored within.

In this way an interface almost behaves like a mirror, allowing a program to examine itself.

Though Go is a staticly-typed language, reflection and interfaces combine to provide extremely powerful techniques that are usually reserved for dynamic lanugages.

For more information about reflection in Go, definitely read the package documentation as well as the many other great blog posts on the subject.

About the Author

Yo! I’m Ayan and I hope you enjoyed my blog post! Part of my motivation for writing this is to learn more about Go myself. In that spirit, if you have anything comments or suggestions, please feel free to contact me at ayan@ayan.net or @ayangeorge on twitter.