Solidity fundamentals: Global variables (msg, this & block)

In Solidity, there are certain global variables that are available inside all functions, even if they are not sent as function parameters. Let's review some of them:

msg.sender

This variable refers to the address of the person (or smart contract) who called the current function. Here's an example of using msg.sender and updating a mapping:

mapping (address => string) addressUsernames;

function setMyNumber(string memory _name) public {
  // Update our `addressUsernames` mapping to store the name under `msg.sender`
  addressUsernames[msg.sender] = _name;
}

function getUsername() public view returns (string) {
  // Retrieve the user name using the caller's address
  return addressUsernames[msg.sender];
}

In Solidity, all function executions need to start with an external caller so there will always be a msg.sender.

msg.value

The msg.value global variable contains the amount of ETH that was sent in the transaction. For example if we have built a lottery game and in which users have to pay 0.1ETH per ticket to play, we validate it that like this:

pragma solidity ^0.8.4;


contract MyContract {

  uint256 ticketPrice = 0.1 ether;

  function playLottery(uint256 _number) public payable {
    // validate value sent
    require(msg.value >= ticketPrice, "Minimum value is 0.1 ETH!");
  }
}

this

The this global variable in Solidity refers to the current smart contract and the returned value can be typed to an address. For example if we have a contract that receives ETH from users, we could retrieve the contract's balance like this:

pragma solidity ^0.8.4;


contract MyContract {

  function getContractBalance() public view returns (uint256) {

    return address(this).balance;
  }
}

block.timestamp

The block.timestamp global variable returns the current block timestamp as seconds since unix epoch.

block.difficulty

The block.difficulty global variable returns current block difficulty.