Abilities - Drop

The drop ability - the simplest of them - allows the instance of a struct to be ignored or discarded. In many programming languages this behavior is considered default. However, in Move, a struct without the drop ability is not allowed to be ignored. This is a safety feature of the Move language, which ensures that all assets are properly handled. An attempt to ignore a struct without the drop ability will result in a compilation error.

module book::drop_ability {

    /// This struct has the `drop` ability.
    public struct IgnoreMe has drop {
        a: u8,
        b: u8,
    }

    /// This struct does not have the `drop` ability.
    public struct NoDrop {}

    #[test]
    // Create an instance of the `IgnoreMe` struct and ignore it.
    // Even though we constructed the instance, we don't need to unpack it.
    fun test_ignore() {
        let no_drop = NoDrop {};
        let _ = IgnoreMe { a: 1, b: 2 }; // no need to unpack

        // The value must be unpacked for the code to compile.
        let NoDrop {} = no_drop; // OK
    }
}

The drop ability is often used on custom collection types to eliminate the need for special handling of the collection when it is no longer needed. For example, a vector type has the drop ability, which allows the vector to be ignored when it is no longer needed. However, the biggest feature of Move's type system is the ability to not have drop. This ensures that the assets are properly handled and not ignored.

A struct with a single drop ability is called a Witness. We explain the concept of a Witness in the Witness and Abstract Implementation section.

Types with the drop Ability

All native types in Move have the drop ability. This includes:

• bool
• unsigned integers
• vector
• address

All of the types defined in the standard library have the drop ability as well. This includes:

• Option
• String
• TypeName