assign: 簡(jiǎn)單賦值,不更改索引計(jì)數(shù)(Reference Counting)。copy: 建立一個(gè)索引計(jì)數(shù)為1的對(duì)象,然后釋放舊對(duì)象retain:釋放舊的對(duì)象,將舊對(duì)象的值賦予
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