带有地址参数的 Solidity 默认构造函数

Question

As I'm learning Solidity I'm struggling to understand how the isApprovedForAll function below actually works.

In particular I'd like to understand how calling the default ProxyRegistry constructor with an address parameter creates an initial mapping , what that initial mapping looks like, and why I should expect the if statement to ever return true.

The code is taken from an official OpenSea api example.

contract OwnableDelegateProxy {}

contract ProxyRegistry {
    mapping(address => OwnableDelegateProxy) public proxies;
}

contract SomeContract {

    address proxyRegistryAddress;

    constructor(
        address _proxyRegistryAddress
    ) {
        proxyRegistryAddress = _proxyRegistryAddress;
    }

    function isApprovedForAll(address owner, address operator)
        public
        view
        returns (bool)
    {
        ProxyRegistry proxyRegistry = ProxyRegistry(proxyRegistryAddress);
        if (address(proxyRegistry.proxies(owner)) == operator) {
            return true;
        }
        return false;
    }
}

Answer 1

ProxyRegistry proxyRegistry = ProxyRegistry(proxyRegistryAddress);

This line does not call the ProxyRegistry constructor. It creates a helper object (named proxyRegistry ) pointing to the proxyRegistryAddress address, and assumes there's a contract implementing functions defined in the ProxyRegistry contract.

In this case, it might be less confusing to just define interface ProxyRegistry { instead of the contract ProxyRegistry { .

But if you wanted to deploy the ProxyRegistry contract to a new address and call its constructor, you'd need use the new keyword:

new ProxyRegistry(<constructor_params>);

---

if (address(proxyRegistry.proxies(owner)) == operator) {
    return true;
}

This snippet calls the autogenerated getter function proxies(address) on the remote contract, retrieving the mapping value by its key. And if the remote mapping value equals the operator value, it returns true .