Constructor

The init function is a specialized entry point used to run logic exactly once. This is the primary way to initialize contract’s state, set up configurations, or mint initial objects.

Requirements

To be recognized as a constructor, a function must meet the following criteria:

1. Naming: The function name must be exactly init.
2. Visibility: The function must be private.
3. Signature: It takes a single argument, a reference to the TxContext, and has no return values.

Important

If any of the above requirements is not met, the compiler will throw an error.

Implementation Example

The following snippet demonstrates a classic init implementation. When the function is called, it creates and shares a Foo object.

module test::constructor;

use stylus::{
    tx_context::TxContext,
    object::{Self, UID},
    transfer::{Self}
};

public struct Foo has key {
    id: UID,
    value: u64
}

entry fun init(ctx: &mut TxContext) {
  let foo = Foo {
    id: object::new(ctx),
    value: 101,
  };

  transfer::share_object(foo);
}

Technical Enforcement

The framework ensures the “once-only” execution of the init function through a persistent storage flag.

• During compilation, a deterministic storage slot is assigned to the initialization flag. This slot is calculated as the Keccak256 hash of the string “init_key”.

• When the function is executed, the runtime checks this storage slot. If the flag is false, the init function executes. Once successful, the flag is set to true.

• This prevents anyone from re-triggering the initialization logic after the contract is live, ensuring the module’s setup and configuration remain immutable.

ABI Representation

In the generated ABI, the init function is represented as follows:

function constructor() external;