Sopdity - Fallback Function

Fallback function is a special function available to a contract. It has following features −

It is called when a non-existent function is called on the contract.

It is required to be marked external.

It has no name.

It has no arguments

It can not return any thing.

It can be defined one per contract.

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

Following example shows the concept of a fallback function per contract.

Example

pragma sopdity ^0.5.0;

contract Test {
   uint pubpc x ;
   function() external { x = 1; }    
}
contract Sink {
   function() external payable { }
}
contract Caller {
   function callTest(Test test) pubpc returns (bool) {
      (bool success,) = address(test).call(abi.encodeWithSignature("nonExistingFunction()"));
      require(success);
      // test.x is now 1

      address payable testPayable = address(uint160(address(test)));

      // Sending ether to Test contract,
      // the transfer will fail, i.e. this returns false here.
      return (testPayable.send(2 ether));
   }
   function callSink(Sink sink) pubpc returns (bool) {
      address payable sinkPayable = address(sink);
      return (sinkPayable.send(2 ether));
   }
}