Reflection

Remarks#

The reflect docs are a great reference. In general computer programming, reflection is ability of a program to examine the structure and behavior of itself at runtime.

Based on its strict static type system Go lang has some rules (laws of reflection)

Basic reflect.Value Usage

import "reflect"

value := reflect.ValueOf(4)

// Interface returns an interface{}-typed value, which can be type-asserted
value.Interface().(int) // 4

// Type gets the reflect.Type, which contains runtime type information about
// this value
value.Type().Name() // int

value.SetInt(5) // panics -- non-pointer/slice/array types are not addressable

x := 4
reflect.ValueOf(&x).Elem().SetInt(5) // works

Structs

import "reflect"

type S struct {
    A int
    b string
}

func (s *S) String() { return s.b }

s := &S{
    A: 5,
    b: "example",
}

indirect := reflect.ValueOf(s) // effectively a pointer to an S
value := indirect.Elem()       // this is addressable, since we've derefed a pointer

value.FieldByName("A").Interface() // 5
value.Field(2).Interface()         // "example"

value.NumMethod()    // 0, since String takes a pointer receiver
indirect.NumMethod() // 1

indirect.Method(0).Call([]reflect.Value{})              // "example"
indirect.MethodByName("String").Call([]reflect.Value{}) // "example"

Slices

import "reflect"

s := []int{1, 2, 3}

value := reflect.ValueOf(s)

value.Len()                // 3
value.Index(0).Interface() // 1
value.Type().Kind()        // reflect.Slice
value.Type().Elem().Name() // int

value.Index(1).CanAddr()   // true -- slice elements are addressable
value.Index(1).CanSet()    // true -- and settable
value.Index(1).Set(5)

typ := reflect.SliceOf(reflect.TypeOf("example"))
newS := reflect.MakeSlice(typ, 0, 10) // an empty []string{} with capacity 10

reflect.Value.Elem()

import "reflect"

// this is effectively a pointer dereference

x := 5
ptr := reflect.ValueOf(&x)
ptr.Type().Name() // *int
ptr.Type().Kind() // reflect.Ptr
ptr.Interface()   // [pointer to x]
ptr.Set(4)        // panic

value := ptr.Elem() // this is a deref
value.Type().Name() // int
value.Type().Kind() // reflect.Int
value.Set(4)        // this works
value.Interface()   // 4

Type of value - package “reflect”

reflect.TypeOf can be used to check the type of variables when comparing

package main
    
    import (
        "fmt"
        "reflect"
    )
    type Data struct {
     a int
    }
    func main() {
        s:="hey dude"
        fmt.Println(reflect.TypeOf(s))
        
        D := Data{a:5}
        fmt.Println(reflect.TypeOf(D))
        
    }

Output :
string
main.Data