ActionChoices就是一个自定义的整型，当枚举数不够多时，它默认的类型为uint8，当枚举数足够多时，它会自动变成uint16。

pragma solidity ^0.4.4;

contract test {
    enum ActionChoices { GoLeft, GoRight, GoStraight, SitStill }
    ActionChoices _choice;
    ActionChoices constant defaultChoice = ActionChoices.GoStraight;

    function setGoStraight(ActionChoices choice) public {
        _choice = choice;
    }

    function getChoice() constant public returns (ActionChoices) {
        return _choice;
    }

    function getDefaultChoice() pure public returns (uint) {
        return uint(defaultChoice);
    }
}

上面的GoLeft == 0,GoRight == 1, GoStraight == 2, SitStill == 3。在setGoStraight方法中，我们传入的参数的值可以是0 - 3当传入的值超出这个范围时，就会中断报错。

invalid opcode  
    The execution might have thrown.
    Debug the transaction to get more information