Fallback function is a special function available to a contract. It has following features −
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It is called when a non-existent function is called on the contract.
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It is required to be marked external.
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It has no name.
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It has no arguments
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It can not return any thing.
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It can be defined one per contract.
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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 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()")); 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) public returns (bool) { address payable sinkPayable = address(sink); return (sinkPayable.send(2 ether)); } }