Solidity - 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.


pragma solidity ^0.5.0;

contract Test {
   uint public x ;
   function() external { x = 1; }    
contract Sink {
   function() external payable { }
contract Caller {
   function callTest(Test test) public returns (bool) {
      (bool success,) = address(test).call(abi.encodeWithSignature("nonExistingFunction()"));
      // 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) public returns (bool) {
      address payable sinkPayable = address(sink);
      return (sinkPayable.send(2 ether));
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