README.md

Lesson 4 - Interfaces and external calls

External calls

• What is a “MethodID”
• Fallback and receive functions
• External calls

References

https://docs.soliditylang.org/en/latest/control-structures.html#external-function-calls

https://solidity-by-example.org/function-selector/

Using interfaces

• Fallback and receive functions
• External calls
• Attaching “mismatched” contracts
• Why some functions work and others don’t because of the “MethodID”

Restricting access to functions

• Wrapping up contents
  • Modifier
  • Assertion inside modifiers
  • Message Sender
  • Visibility
  • Mutability
• Implementing basic access control on setText

References

https://docs.soliditylang.org/en/latest/common-patterns.html#restricting-access

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;

contract HelloWorld {
    string private text;
    address public owner;

    constructor() {
        text = "Hello World";
        owner = msg.sender;
    }

    function helloWorld() public view returns (string memory) {
        return text;
    }

    function setText(string calldata newText) public onlyOwner {
        text = newText;
    }

    function transferOwnership(address newOwner) public onlyOwner {
        owner = newOwner;
    }

    modifier onlyOwner()
    {
        require (msg.sender == owner, "Caller is not the owner");
        _;
    }
}

Contract interaction

• Interacting with previously deployed contracts using interfaces
• Inspecting “MethodId” mismatch and fallback functions
• Inspecting execution errors based on assertions
• Fixing the interfaces
• Interacting with other peoples contract

Extra reference material

• Handy reference sheet: Solidity Cheatsheet
• Overall style guide: Solidity Style Guide
• Solidity quick guide: Learn Solidity in Y minutes

Homework

• Create Github Issues with your questions about this lesson
• Read the references