Struct and Enum Visibility

Since language version 2.4

By default, struct and enum construction, destruction, and field access are restricted to the defining module. Move 2.4 introduces explicit visibility modifiers that allow external modules to perform these operations.

Visibility Levels

Three modifiers are available, mirroring the function visibility keywords:

Modifier	Accessible from
public	Any module
package	All modules in the same package
friend	Modules declared as friend in the defining module

public(package) and public(friend) are accepted as aliases for package and friend respectively, but are discouraged and will be deprecated. Prefer the shorthand forms.

Performance Consideration

Cross-module type operations are currently compiled into function calls rather than direct bytecode instructions. Thus, they are not a zero-cost abstraction yet: they are more expensive than the equivalent operations performed within the defining module. This is expected to change in the future with VM improvements. In the meantime, use visibility modifiers only when the cross-module access is genuinely needed.

Restrictions

Types with the key ability cannot have visibility modifiers. Types stored as roots in global storage must remain module-private to preserve global storage access control.

// ERROR: types with the key ability cannot be public, package, or friend
public struct Resource has key { value: u64 }
public enum VersionedData has key { V1 { name: vector<u8> } }

Global storage operations remain module-only. Even for public types, move_to, move_from, borrow_global, and borrow_global_mut are restricted to the defining module.

Transaction Arguments

public structs and enums can be passed as entry and view function arguments if the type has the copy ability and does not have the key ability. All field types must themselves be valid argument types, recursively.

module 0x42::types {
    public struct Point has copy, drop {
        x: u64,
        y: u64,
    }

    public enum Direction has copy, drop {
        North, South, East, West,
    }

    // Both Point and Direction can be passed directly as transaction arguments
    entry fun move_to_point(s: &signer, destination: Point) { .. }
    entry fun move_player(s: &signer, dir: Direction) { .. }
}

Upgradability

Visibility can be broadened in a package upgrade but not narrowed:

• A private type can be upgraded to package, friend, or public.
• A friend or package type can be upgraded to public.
• A friend or package type can be downgraded back to private.