NSString

1. 創建常量字符串
   NSString *string = @”This is a String”;//不足要釋放內粗
2. 創建空字符串，然後賦值
   NSString * string = [[NSString alloc] init];
   string = @”This is a String”;
3. 通過字符串創建字符串
   [[NSString alloc] initWithString:@”This is a String”];
   [NSString stringWithString:@”This is a Strng”]; //不需要管理內存
   或者
   [[NSString alloc] initWithFormat:@”My age is %i”, 17];

4. 用C標準創建字符串
   char *s = “This is a String”;
   [[NSString alloc] initWithCString:cString encoding:NSUTF8StringEncoding];
   或者：
   [[NSString alloc] initWithUTF8String:cstring];
   對應：
   [NSString stringWithUTF8String:”This is a Sting”];
   區別在於下面不需要管理內存

5. 從文件中讀取字符串
   NSString *path = /Users/apple/a.txt;
   NSString *str1 = [NSString stringWithContentsOfFile:path]; //不能解析中文，過期方法
   或：

NSSerror *error = nil; //記錄錯誤信息
NSString *path = @"/User/apple/a.txt";
NSString *string = [[NSString alloc]initWithContentsOfFile:path encoding:NSUTF8StringEncoding error:&error];
if (error) {
    NSString *desc = [error localizedDescription];
    NSSLog(@"文件讀取失敗：%@", desc);
} else {
    NSLog(@"文件讀取成功：%@"， string);
}
// 將中文字符編碼轉換爲NSStringEncoding格式的編碼
NSStringEncoding enc = CFStringConvertEncodingToNSStringEncoding（kCFStringEncodingGB_18030_2000）；

1. URL讀取本地文件

NSURL *url = [NSURL URLWithString:@"file:///User/apple/a.txt"];
NSString *str2 = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncodeing error:nil];

1. URL讀取遠程文本

NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
NSString *str3 = [NSString stringWithContensOfURL:url encoding:NSUTF8StringEncodeing error:nil];