Solidity constructor syntax

Question

Looking through an OpenZeppelin tutorial, I came across this code snippet:

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract GameItem {
    constructor() ERC721("GameItem", "ITM") {}

What is the syntax in the constructor that allows a class instance to be passed in after constructor()? I can’t seem to find any reference to this in the Solidity docs

Answer 1

It's invoking a parent constructor, in this case the parent class is named ERC721. It's the same as calling super.constructor() or parent.constructor() in some other languages.

In order to call the parent constructor, your contract needs to actually inherit from it:

contract GameItem is ERC721 {

---

Solidity supports inheritance from multiple parents. That's why you cannot just use an ambiguous keyword such as parent. By explicitly stating the parent class name, you can specify which values you want to pass to which parent:

pragma solidity ^0.8;

contract Parent1 {
    constructor(string memory message1) {}
}

contract Parent2 {
    constructor(string memory message2) {}
}

contract Child is Parent1, Parent2 {
    constructor() Parent1("hello") Parent2("world") {}
}

Docs: https://docs.soliditylang.org/en/v0.8.13/contracts.html#arguments-for-base-constructors