Fallback function (sol v 0.6)

is executed if none of the other functions match the function identifier or no data was provided with the function call.

Only one unnamed function can be assigned to a contract and it is executed whenever the contract receives plain Ether without any data.

// This fallback function will keep all the Ether

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

Properties of a fallback function:

• Has no name or arguments.

• If it is not marked payable, the contract will throw an exception if it receives plain ether without data.

• Cannot return anything.

• Can only be defined once per contract.

• It is also executed if the caller meant to call a function that is not available.

• Mandatory to mark it external.

• Limited to 2300 gas when called by another function - so as to make this function call as cheap as possible.

Receiving ETH

Contracts receiving ETH without a payable function call and without a fallback function will throw an exception. Therefore, cannot receive ETH.

However, there are some exceptions to this; You cannot completely avoid receiving ETH:

1. Some other SC selfdestructs and names you as beneficiary

2. Miner reward sets your SC address as beneficiary

Prior Solidity 0.6 the fallback function was simply an anonymous function that looked like this:

function () external {}

It's now two different functions. receive() to receive money and fallback() to just interact with the Smart Contract without receiving Ether.