Reflection-Golang

REFLECTION

SUMMARY

• This is a form of meta programming
• This can be viewed as the ability of a program to introspect its own structures
  • Its a form of metaprogramming
  • The ability of a program to manipulate, read and analyse other programs or itself.
  • Ability to change its behaviour at runtime

All programs are a combination of data and instructions and reflection gives us the ability to inspect the data that the program uses. Each variable in Go is an interface value which contains the value and the type of the value.

• We can also use type aliases where we reassign types

type bigNumber int

• The golang language cannot know those types in advance so we have a general purpose objects that contain the reflection values and types
• these are reflection types, they are an abstracted layer on top of every possible type in the language.
• The first law of reflection is
  • You can go from interface value to reflection object
  • You can go from reflection object to interface value.
  • We can check if the value can be set by using reflectVal.Canset()
  • Not all objects can be settable so you always need to check.

var x float64 = 3.14
reflectObjVal := reflect.ValueOf(x)
reflectObjType := reflect.TypeOf(x)

NB: If we pass by value, the reflection value will not be able to be set

var x float64 = 3.14
reflectObjVal := reflect.ValueOf(&x)
reflectObjType := reflect.TypeOf(&x)

//Get pointer value
refObjVal := reflectObjValPtr.Elem()
//Check if we can set a value 
refObjVal.CanSet()

We can dereferrence a pointer to its value by using the folowing

refObjVal := reflectObjValPtr.Elem()
//Same as
refObjVal := *reflectObjValPtr

We can now use the set method to reassign a value to a reflection value.

refObjVal.Set(reflect.ValueOf(5.124))

Set assigns x to the value v. It panics if CanSet returns false. As in Go, x's value must be assignable to v's type. We can check the kind of a reflection item.

reflectObjType.Kind()

Custom tags can be defined and used acccordingly from the following manner

type City struct{
Name string `json:"name"`
Postcode string `json:"postcode"`
Mayor string `json:"mayor"`
}

tag := structFieldRefObjType.Tag.Get("json") // Gets the json tag - Very handy