fallback

video-scripts/fallback.md

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+```
+pragma solidity ^0.5.11;
+
+/*
+Fallback function
+- unamed function
+- cannot take any inputs, cannot return any outputs
+- must be external
+- executed when
+    - calling a function that does not exist (call video)
+    - receiving Ether (NOTE: not executed when calling another payable function) (must be payable)
+        - except coinbase (TODO: explain coinbase)
+- sent via
+    - account (wallet), call (no gas limit)
+    - send / transfer (forwards 2300 gas)
+        - 2300 gas is not enough to
+            - Write to storage
+            - Create a contract
+            - Call an external function which consumes a large amount of gas
+            - Sending Ether
+        - TODO: briefly explain re-entrancy and how 2300 gas prevents it
+*/
+contract Fallback {
+    event Log(uint gas);
+
+    // NOTE: unnamed
+    // NOTE: no input no output
+    // NOTE: external
+    // NOTE: payable
+    function () external payable {
+        // TODO: gasleft
+        emit Log(gasleft());
+    }
+
+    function getBalance() public view returns (uint) {
+        return address(this).balance;
+    }
+}
+
+contract SendToFallback {
+    function callFallback(address payable _to) public payable {
+        // NOTE: recommended
+        // NOTE: forwards all gas
+        // NOTE: briefly explain re-entrancy
+        (bool sent,) = _to.call.value(msg.value)("");
+        require(sent, "Failed to send Ether");
+    }
+
+    function transferToFallback(address payable _to) public payable {
+        // NOTE: no longer recommended
+        // NOTE: forwards 2300 gas
+        _to.transfer(msg.value);
+    }
+}
+
+/*
+DEMO
+- callFallback (check gas)
+- transferToFallback (check gas)
+*/
+```