Fallback function (sol v 0.8)

Fallback()

• The fallback() function is a special function in Solidity that is used to define a default behavior if no other functions have been called.

• It is triggered when a contract receives a transaction or message call and no other functions match the given signature.

It's mainly used to enable the contract to receive ETH.

// This fallback function will keep all the Ether

fallback() external payable {
    balance[msg.sender] += msg.value;
} 

• must be marked external payable

• cannot take any parameters

Receive v Fallback v payable

• To handle these scenarios when someone sends ETH directly to a contract, and does not call payable functions, we use receive and fallback functions.

• If someone sends ETH to a smart contract without passing calldata and there is a receive function, the receive function is executed.

• If there is no receive function, then the fallback is executed.

• If there is no receive or fallback, then the transaction reverts.

• receive is for receiving ether without calling data.

// msg.data is empty
address.call{value : 1 ether}("");   

• payable function is for receiving ether with data

address.call{value : 1 ether}(abi.encodeWithSignature('fundme()');

If you want your contract to receive Ether, you should implement receive.

Using fallback for receiving Ether is not recommended, since it would catchall, and not fail on interface confusions.