How to Remove Elements from Type-Asserted Interface Slices in Go?

Type Assertions in Golang: Removing Elements from Interface Slices

In Golang, type assertions allow us to extract the underlying type of an interface value. However, after type assertion, how can we manipulate the underlying value?

Consider the following example, which attempts to remove an element from a type-asserted slice of interfaces:

value := []interface{}{0, "one", "two", 3, 4}
i := 2
value.([]interface{}) = append(value.([]interface{})[:i], value.([]interface{})[i+1:]...)

The above code results in the error "cannot assign to value ([]interface{})". This error stems from the fact that interfaces store a copy of the underlying value, and type assertions do not modify the value within the interface.

Solution

To modify the underlying value, we must store a pointer to the slice in the interface instead. For example:

var value interface{} = &[]interface{}{0, "one", "two", 3, 4}

Now, we can dereference the pointer and modify the slice as follows:

sp := value.(*[]interface{})
i := 2
*sp = append((*sp)[:i], (*sp)[i+1:]...)

Output:

fmt.Println(value) // &[0 one 3 4]

As you can see, the element at index 2 ("two") has been removed from the slice. This approach successfully modifies the underlying value because the interface stores a pointer to the slice, rather than a copy of the slice itself.

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