What is the fallback function in Solidity?

Overview

In Solidity, a fallback function is an external function with neither a name, parameters, or return values. It is executed in one of the following cases:

If a function identifier doesn’t match any of the available functions in a smart contract.
If there was no data supplied along with the function call.

Only one such unnamed function can be assigned to a contract.

Properties of the fallback function

They are unnamed functions.
They cannot accept arguments.
They cannot return anything.
There can be only one fallback function in a smart contract.
It is compulsory to mark it external.
It should be marked as payable. If not, the contract will throw an exception if it receives ether without any data.
It is limited to 2300 gas if invoked by other functions.

Example

Let's see an example of calling a function in Solidity in the code snippet below:

pragma solidity ^0.5.12;
// contract with fallback function
contract A {
  uint n;
  function set(uint value) external {
    n = value;
  }
  function() external payable {
    n = 0;
  }
}
// contract to interact with contract A
contract example {
  function callA(A a) public returns (bool) {
     // calling a non-existing function
      (bool success,) = address(a).call(abi.encodeWithSignature("setter()"));
      require(success);
      // sending ether to A
      address payable payableA = address(uint160(address(a)));
      return(payableA.send(2 ether));
   }
}

Explanation

Line 1: We import the pragma package.
Line 4–13: We create a contract A that has a state variable n, a function set() that accepts a value and stores it in n, and a fallback function.
Line 16–25: We create one more contract example to interact with the contract A. We call a non-existing function of contract A (line 19) and then try to send ether to the contract (line 24).

Output

The function call in line 19 does not exist in the contract A. So, its fallback function will be invoked.
Line 24 will return false, as sending of the ether failed.

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