NSString 转换成NSData 对象

NSData *data = [@"testdata" dataUsingEncoding:NSUTF8StringEncoding];

NSData 转换成NSString对象

NSString *str = [[NSString alloc] initWithData:data  encoding:NSUTF8StringEncoding]; ```
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#NSData 转换成char* 
```NSData *data;
char *testData = [data bytes];```
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#char* 转换成NSData对象 
```byte *testData = malloc(sizeof(byte)*16);
NSData *content=[NSData dataWithBytes:testData length:16];```