Objective - C只支持类之间的单一继承，而UML支持多重继承。

Objective-C only supports single inheritance between classes, while UML supports multiple inheritances.

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对象模型支持单一继承，能让用户扩展（系列化）一个存在的类，而不用从头再写或复制。

The object model supported single inheritance, which let users extend (specialize) an existing class without having to write it from scratch or copy it.

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单独定义类，以便从某框架继承类，而该框架受到单一的类属性限制。

Classes are defined solely to inherit from some framework, where that framework is narrowed by a single class attribute.

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