[Solidity] What is msg.sender?

This section describes msg.sender, which is often found in Solidity code.

msg.sender means "caller address"

You can get the caller's account address by writing msg.sender.

Seeing is believing, so let's take a look.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
 
contract TestContract {

    function whoAmI() public view returns(address) {
        return msg.sender;
    }

}

The value of msg.sender can be obtained by calling the above function.

Looking at the output, we can see that the caller's account address (0x5B38Da6a701c568545dCfcB03FcB875f56beddC4) is displayed.

Usage of msg.sender

There are many ways to use msg.sender, but the following two are typical.

• Check if the caller is the owner
• Storing data using the caller's address as a key

Check if the caller is the owner

The sample code below compares the address of the contract owner with the address of the caller and outputs the characters only if they match (only if the caller is the owner).

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
 
contract TestContract {

    address private owner;

    constructor() {
        owner = msg.sender;
    }

    function sayHiToOwner() public view returns(string memory) {
        require(msg.sender == owner);
        return "Hi. Welcome back.";
    }

}

Storing data using the caller's address as a key

The sample code below uses the caller's account address as the key to store the data.

This is a common method used to store user-specific data.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
 
contract TestContract {

    mapping (address => string) public userNames;

    function setUserName(string memory _userName) public {
        userNames[msg.sender] = _userName;
    }

    function getUserName() public view returns(string memory) {
        return userNames[msg.sender];
    }

}