...meArgument)一样，你可以在可选方法名称后加上?来检查该方法是否被实现。可选方法和可选属性都会返回一个可选值(optional value)，当其不可访问时，?之后语句不会执行，并返回nil。
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OPTIONAL DECLARED VALUE 价值
optional value
可选值
以上为机器翻译结果，长、整句建议使用 人工翻译 。
The scalar component is an optional value.
标量是一个可变值。
youdao
In both cases, the value of the whole expression is an optional value.
在这两种情况下，整个表达式的值是一个可选值。
The href attribute is an optional value that provides the URI of the resource to include.
hre f属性是一个可选值，它提供需要包含的资源的URI。
Owners of S corporations and partnerships using a fiscal year have worked hard to establish their right to use one of these optional tax years, and they value the deferral it provides.
FORBES: Business Owners Could Find Their Tax Deferral Backfiring
Gold is very difficult to value, making SPDR Gold Shares ( GLD) an optional diversification play, as opposed to a primary choice for asset allocation.
FORBES: Tough Year For Stocks, Gold And Treasury ETFs Did Better
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