ssign,copy,retain,readonly,writeonly,strong,weak
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//对于基本属性的简单理解
assign 为简单赋值,一般用于修饰 常量.
Copy建立一个索计数为一的对象,然后释放旧的对象.适用于NSString 对 象.copy 是内容拷贝.(为什么要用copy就是不想让他们占用同一块内存)
Retain释放旧的对象,将旧对象的值输入对象,再提高输入对象的索引计数为一.对应NSObject及其子类.retain 是指针拷贝.
Readonly只读属性
Writeonly只写属性.
在arc 模式下,默认属性是strong,牵扯到循环引用就用weak.strong相当于retain,weak相当于assign.声明为weak的指针,指针指向的地址一旦被释放.这些指针都将被赋值为nil.这样的好处能有效防止野指针.
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