Defining Initializer Functions

This page explains how to write an initializer function, also known as a constructor.

Initializers are regular functions that set an "initialized" flag (a nullifier) for the contract. A contract can only be initialized once, and contract functions can only be called after the contract has been initialized, much like a constructor. However, if a contract defines no initializers, it can be called at any time. Additionally, you can define as many initializer functions in a contract as you want, both private and public.

Annotate with #[initializer]

Define your initializer like so:

#[initializer]
fn constructor(){
    // function logic here
}

Public or private

Aztec supports both public and private initializers. Use the appropriate macro, for example for a private initializer:

#[private]
#[initializer]
fn constructor(){
    // function logic here
}

Initializer with logic

Initializers are commonly used to set an admin, such as this example:

constructor

#[public]
#[initializer]
fn constructor(admin: AztecAddress, name: str<31>, symbol: str<31>, decimals: u8) {
    assert(!admin.is_zero(), "invalid admin");
    storage.admin.write(admin);
    storage.minters.at(admin).write(true);
    storage.name.initialize(FieldCompressedString::from_string(name));
    storage.symbol.initialize(FieldCompressedString::from_string(symbol));
    storage.decimals.initialize(decimals);
}

^{_{Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L92-L105}}

Here, the initializer is writing to storage. It can also call another function. Learn more about calling functions from functions here.

Multiple initializers

You can set multiple functions as an initializer function simply by annotating each of them with #[initializer]. You can then decide which one to call when you are deploying the contract.

Calling any one of the functions annotated with #[initializer] will mark the contract as initialized.

See this page of the protocol specs for more info about what marking a function as initialized means.

To see an initializer in action, follow the Counter codealong tutorial.