Can You Change Both Pointer Type and Value of an Interface Variable Using Reflection in Go?

Changing Pointer Type and Value of Interface Variable Using Reflection

In Go's type system, interfaces define contracts that types can implement to expose specific behaviors. When working with interface values, it's important to understand how value passing and reflection come into play.

Value Passing

In Go, all values are passed by value, including interface values. When you assign an interface value to a variable or pass it to a function, a copy of the interface value is created. This copy contains a reference to the underlying concrete value and the type of that value.

Changing Pointer Value

You mentioned that you can change the pointer value of an interface variable using reflection:

v.Elem().Set(reflect.ValueOf(&Greeter{"Jack"}).Elem())

This works because reflection provides access to the underlying value stored in the interface. By calling v.Elem(), you get a reflection Value for the concrete value, which you can then modify using Set.

Changing Pointer Type

However, your question is about changing both the pointer type and value of an interface variable. Unfortunately, this is not possible using reflection in the way you have described:

v.Elem().Set(reflect.ValueOf(&Greeter2{"Jack"}).Elem())

When you attempt this, you will receive a panic error because Go does not allow assigning a value of a different type to an interface value. Specifically, Go requires that the type of the value being assigned to an interface matches the type of the interface itself.

Solution Using Pointer

To change both the pointer type and value of an interface variable, you need to first create a variable of the desired type and pass its address to the reflection Value. This allows you to modify the value stored in the variable through the pointer.

var newGreeter2 = &Greeter2{"Jack"}
reflect.ValueOf(&&a).Elem().Set(reflect.ValueOf(newGreeter2))

By using this approach, you can effectively change both the type and value of the interface variable. However, it's important to note that you are still working with a copy of the interface value, not the original variable itself.

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