01-Foundation introduction, NSObject, copy, NSString

Function, first of all to comply with the Protocol, must be implemented

And the initialization method.

Message. You can create class objects.

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The unchangeable string can be used repeatedly. Generally, only one object is created for all string objects with the same value.

Method To create a string, even if the content is the same, the address is not the same

Note that the sequence set problem exists. By default, ASC (excluding Chinese) specifies the corresponding Chinese sequence set (GBK, GBK2313, BIG5 traditional Chinese, and UTF8)

Encoding: error:];

Method To compare whether the current string is the same as the corresponding string,

The method can compare two strings and compare the ASCII values of the characters in the string. the return value is defined by NSComparisonResult.

This method is used in the verification code function.