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Abilities: Introduction

Move has a unique type system that allows customizing type abilities. In the previous section, we introduced the struct definition and how to use it. However, the instances of the Artist and Record structs had to be unpacked for the code to compile. This is the default behavior of a struct without abilities.

Throughout the book, you will see chapters named Ability: <name>, where <name> is the name of the ability. These chapters will cover the ability in detail, how it works, and how to use it in Move.

What are Abilities?​

Abilities allow certain behaviors for a type. They are part of the struct declaration and define which behaviors are allowed for the instances of the struct.

Abilities Syntax​

Abilities are set in the struct definition using the has keyword followed by a list of abilities. The abilities are separated by commas. Move supports 4 abilities: copy, drop, key, and store, each defining a specific behavior for the struct instances.

/// This struct has the `copy` and `drop` abilities.
struct VeryAble has copy, drop {
    // field: Type1,
    // field2: Type2,
    // ...
}

Overview​

A quick overview of the abilities:

All built-in types, except references, have copy, drop, and store abilities. References have copy and drop.

  • copy - allows the struct to be copied. Explained in the Ability: Copy chapter.
  • drop - allows the struct to be dropped or discarded. Explained in the Ability: Drop chapter.
  • key - allows the struct to be used as a key in storage. Explained in the Ability: Key chapter.
  • store - allows the struct to be stored in structs with the key ability. Explained in the Ability: Store chapter.

While it is important to mention them here, we will go into detail about each ability in the following chapters and provide proper context on how to use them.

No Abilities​

A struct without abilities cannot be discarded, copied, or stored in storage. We call such a struct a Hot Potato. It is a joke, but it is also a good way to remember that a struct without abilities is like a hot potato - it can only be passed around and requires special handling. Hot Potato is one of the most powerful patterns in Move, and we will go into detail about it in the Hot Potato chapter.

Further Reading​

Understanding Move Resource Abilities​

In Move, abilities are represented as bit flags. The value 8 in the abilities field corresponds to specific capability flags. Let me break this down:

Ability Bit Flags

abilities: 8 means:
1 = copy  (0b0001)
2 = drop  (0b0010)
4 = store (0b0100)
8 = key   (0b1000)