basics

Set a Slice to Struct Field

Let's suppose we have a slice or a struct with a field of type slice, and we want to assign to the slice with another slice or append with an item. This example shows how we can do that with Go's reflect package.

Setting a Variable

When we are setting a reflect.Value object (or rval) with a value, the rval must be addressable. This means that we should be initializing a rval with a pointer and not with a value, so that we can mutate the actual referenced value and not the copied value.

var n int

// The following will not work.
// Panic: reflect.Value.Set using unaddressable value
namesVal := reflect.ValueOf(n)
namesVal.Set(reflect.ValueOf(4))

// The following will work:
// We need get a reflect.Value of the pointer to the variable n and then expose the
// actual reflect.Value using the method Elem().
namesVal := reflect.ValueOf(&n).Elem()
namesVal.Set(reflect.ValueOf(4))

Setting the Struct Field

What if we want to set a field in a struct?

type Person struct {
	Age int
}

person := Person{
	Age: 15,
}

We first need to get a reflect.Value of the Person pointer so that we can actually set the actual Person object and not the copy. Through the reflect package, we use the method FieldByName() to retrieve the reflect.Value of the Age field. Finally, we call Addr() on the field rval object to get its address. In other words:

fieldVal := reflect.ValueOf(&person).Elem().FieldByName("Age").Addr().Elem()
fieldVal.Set(reflect.ValueOf(30))
fmt.Println(person) // It should print 30

Note that we have to call Elem() twice in this context. Once on the struct itself since we are using a pointer to the struct. Second, on the field itself, since we called Addr().

Setup

1. Run the program

   $ make run

Reference

• The Laws of Reflection