How can I change the pointer type and value of an interface variable using reflection in Go?

Changing Pointer Type and Value under Interface with Reflection

Question

How can we change the pointer type and value of a variable defined by an interface using reflection?

Answer

In Go, everything is passed by value, including interfaces. When assigning an interface value, a copy is made, limiting the modification potential.

To modify the value stored in an interface variable, its address must be used. By accessing the variable's address through reflect.ValueOf(&varName).Elem(), we can set a new pointer value into it.

Example

var a fmt.Stringer // Interface variable
a = &Greeter{"John"}

v := reflect.ValueOf(&a).Elem() // Access variable's address
v.Set(reflect.ValueOf(&Greeter2{"Jack"})) // Set new pointer value

fmt.Println(a.String()) // Hello2, My name is Jack (Greeter2.String() called)

Key Points

• Start from the address using reflect.ValueOf(&varName).
• Set a pointer value into it, as interfaces implemented by pointers cannot be set into copies.

Limitations

• Modifying a copy is impossible due to Go's pass-by-value nature. Only by passing a pointer can the pointed value be modified, effectively changing the variable's value.
• Changing the interface type of a variable is not possible, as the variable's actual type is determined at compile time.

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