Mastering Blockchain Programming with Solidity

Using external function types

External function types require an address on which the function can be called. Also required is the function signature, this represents which function is to be called on the target address.

The following code shows an example that uses external function types:

contract OraclizeService {    address authorized = 0xefd8eD39D00D98bf43787ad0cef9afee2B5DB34F;    modifier onlyAuthorized() {        require(msg.sender == authorized);        _;    }    QueryData[] queries;    struct QueryData {        bytes currency;        function(uint, bytes memory)            external            returns (bool) callbackFunction;    }    event NewRequestEvent(uint requestID);    function query(        bytes _currency,        function(uint, bytes memory) external returns(bool) _callbackFn    ) public {        //Registering callback ...