Interface Types

When a function takes a contract address as an argument, it is better to pass an interface or contract type rather than a raw address. If the function is called elsewhere within the source code, the compiler will provide additional type safety guarantees.

Here we see two alternatives:

contract Validator {
    function validate(uint) external returns(bool);

contract TypeSafeAuction {
    // good
    function validateBet(Validator _validator, uint _value) internal returns(bool) {
        bool valid = _validator.validate(_value);
        return valid;

contract TypeUnsafeAuction {
    // bad
    function validateBet(address _addr, uint _value) internal returns(bool) {
        Validator validator = Validator(_addr);
        bool valid = validator.validate(_value);
        return valid;

The benefits of using the TypeSafeAuction contract above can then be seen from the following example. If validateBet() is called with an address argument, or a contract type other than Validator, the compiler will throw this error:

contract NonValidator{}

contract Auction is TypeSafeAuction {
    NonValidator nonValidator;

    function bet(uint _value) {
        bool valid = validateBet(nonValidator, _value); // TypeError: Invalid type for argument in function call.
                                                        // Invalid implicit conversion from contract NonValidator
                                                        // to contract Validator requested.
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