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contract Mapper { event AddressMapped(address primary, address secondary); event Error(uint code, address sender); mapping (address => address) public primaryToSecondary; mapping (address => bool) public secondaryInUse; modifier secondaryAddressMustBeUnique(address secondary) { if(secondaryInUse[secondary]) { Error(1, msg.sender); throw; } _; } function mapAddress(address secondary) secondaryAddressMustBeUnique(secondary) { // If there is no mapping, this does nothing secondaryInUse[primaryToSecondary[msg.sender]] = false; primaryToSecondary[msg.sender] = secondary; secondaryInUse[secondary] = true; AddressMapped(msg.sender, secondary); } } Although this contract is a bit more complex than we have seen so far, it remains fairly accessible. Let’s break it down: • There are two events: AddressMapped and Error. The AddressMapped event is generated any time a user’s primary Ethereum address is mapped to a secondary, login-only, address. The Error event is only generated in case of errors, such as when a mapping using an existing secondary address already exists. • Thentwovariablesaredeclared:primaryToSecondaryandsecondaryInUse.primaryToSecondary is a map of addresses: given the primary address, it can tell the secondary address mapped to it. secondaryInUse is a map of addresses to booleans, used to check whether a secondary address is already in use. • Next comes secondaryAddressMustBeUnique. This special function is a modifier. Mod- ifiers in Solidity are special functions that can be attached to contract methods. These run before the method code and can be used to modify their behavior. In this case, secondaryAddressMustBeUnique uses the secondaryInUse variable to confirm whether the secondary address passed as parameter is in use. If it is, this is flagged as an error and the Error event is emitted. If it is not in use, then execution continues. The _ placeholder is where the code from the modified function is logically inserted. • And lastly there is the mapAddress method. This method takes a secondary address and maps 55PDF Image | Intro to Ethereum and Smart Contracts
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