View and Pure Functions in Solidity

So far, we have mostly interacted and modified state variables from our Smart Contract. For example, when we write the owner, we modify a state variable. When we update the balance, we modify a state variable (balanceReceived).

For this, we needed to send a transaction. That works very transparently in Remix and also looks instantaneous and completely free of charge, but in reality it isn't. Modifying the State costs gas, is a concurrent operation that requires mining and doesn't return any values.

Reading values, on the other hand, is virtually free and doesn't require mining.

There are two types of reading functions:

1. view: Accessing state variables
2. pure: Not accessing state variables

View Function

Let's make our owner-variable private and instead add a simple getter function function

//SPDX-License-Identifier: MIT

pragma solidity 0.8.3;

contract FunctionsExample {

    mapping(address => uint) public balanceReceived;

    address payable owner;

    constructor() {
        owner = payable(msg.sender);
    }

    function getOwner() public view returns(address) {
        return owner;
    }

    function destroySmartContract() public {
        require(msg.sender == owner, "You are not the owner");
        selfdestruct(owner);
    }

    function receiveMoney() public payable {
        assert(balanceReceived[msg.sender] + msg.value >= balanceReceived[msg.sender]);
        balanceReceived[msg.sender] += msg.value;
    }

    function withdrawMoney(address payable _to, uint _amount) public {
        require(_amount <= balanceReceived[msg.sender], "not enough funds.");
        assert(balanceReceived[msg.sender] >= balanceReceived[msg.sender] - _amount);
        balanceReceived[msg.sender] -= _amount;
        _to.transfer(_amount);
    } 

    receive() external payable {
        receiveMoney();
    }
}

Let's give this a try: 1. Deploy the Smart Contract 2. click on "getOwner" 3. Observe the Transaction Log

You can observe that the deployment is a transaction, marked by the little green checkmark (1), while the reading operation is a call (2). In reality a transaction would need to get signed by a private key and mined by the network, while a reading operation does not need to get signed.

Pure Functions

So, view functions are reading functions - what are pure functions?

Pure functions are functions that are not accessing any state variables. They can call other pure functions, but not view functions.

Let's do a quick example:

//SPDX-License-Identifier: MIT

pragma solidity 0.8.3;

contract FunctionsExample {

    mapping(address => uint) public balanceReceived;

    address payable owner;

    constructor() {
        owner = payable(msg.sender);
    }

    function getOwner() public view returns(address) {
        return owner;
    }

    function convertWeiToEth(uint _amount) public pure returns(uint) {
        return _amount / 1 ether;
    }

    function destroySmartContract() public {
        require(msg.sender == owner, "You are not the owner");
        selfdestruct(owner);
    }

    function receiveMoney() public payable {
        assert(balanceReceived[msg.sender] + msg.value >= balanceReceived[msg.sender]);
        balanceReceived[msg.sender] += msg.value;
    }

    function withdrawMoney(address payable _to, uint _amount) public {
        require(_amount <= balanceReceived[msg.sender], "not enough funds.");
        assert(balanceReceived[msg.sender] >= balanceReceived[msg.sender] - _amount);
        balanceReceived[msg.sender] -= _amount;
        _to.transfer(_amount);
    } 

    receive() external payable {
        receiveMoney();
    }
}

Let's run it:

1. Deploy a new Instance
2. Run "convertWeiToEth" with 100
3. Observe the Transaction log

Do you know why the returned amount is 0? Think about it for a moment, then look into the solution!

Why the amount is 0 solution

The amount is zero, because we return an integer. We divide 100 / 1000000000000000000 = 0.0000000000001. Integers in Solidity are cut off (not even rounded, or always rounded down).

If you enter 1000000000000000000 into the amount field, then the returned amount is 1 (1 Ether)

Alright, that's it, there's nothing much more to say about view and pure functions at this moment. On to the next lecture!

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Last update: March 28, 2022