Attribute Macro trait_definition

#[trait_definition]

Marks trait definitions to ink! as special ink! trait definitions.

There are some restrictions that apply to ink! trait definitions that this macro checks. Also ink! trait definitions are required to have specialized structure so that the main #[ink::contract] macro can properly generate code for its implementations.

Example

Trait definition:

#[ink::trait_definition]
pub trait Erc20 {
    /// Returns the total supply of the ERC-20 smart contract.
    #[ink(message)]
    fn total_supply(&self) -> Balance;

    /// Transfers balance from the caller to the given address.
    #[ink(message)]
    fn transfer(&mut self, amount: Balance, to: AccountId) -> bool;

    // etc.
}

Trait implementation

Given the above trait definition you can implement it as shown below:

#[ink::contract]
mod base_erc20 {
    #[ink(storage)]
    pub struct BaseErc20 {
        total_supply: Balance,
    }

    impl BaseErc20 {
        #[ink(constructor)]
        pub fn new(initial_supply: Balance) -> Self {
            Self { total_supply: initial_supply }
        }
    }

    impl Erc20 for BaseErc20 {
        /// Returns the total supply of the ERC-20 smart contract.
        #[ink(message)]
        fn total_supply(&self) -> Balance {
            self.total_supply
        }

        #[ink(message)]
        fn transfer(&mut self, amount: Balance, to: AccountId) -> bool {
            unimplemented!()
        }
    }
}

Header Arguments

The #[ink::trait_definition] macro can be provided with some additional comma-separated header arguments:

• namespace: String

  The namespace configuration parameter is used to influence the generated selectors of the ink! trait messages. This is useful to disambiguate ink! trait definitions with equal names.

  Usage Example:

  #[ink::trait_definition(namespace = "foo")]
  pub trait TraitDefinition {
      #[ink(message)]
      fn message1(&self);
  
      #[ink(message, selector = 42)]
      fn message2(&self);
  }

  Default value: Empty.

• keep_attr: String

  Tells the ink! code generator which attributes should be passed to call builders. Call builders are used to doing cross-contract calls and are automatically generated for contracts.

  Usage Example:

  #[ink::trait_definition(keep_attr = "foo, bar")]
  pub trait Storage {
      #[ink(message)]
  //  #[foo]
      fn message1(&self);
  
      #[ink(message)]
  //  #[bar]
      fn message2(&self);
  }

  Allowed attributes by default: cfg, cfg_attr, allow, warn, deny, forbid, deprecated, must_use, doc, rustfmt.