Diff of NSString

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[[NSObject]]

- [[NSMutableString]]

- Objective-c iPhone percent encode a string?
--  http://stackoverflow.com/questions/3423545/objective-c-iphone-percent-encode-a-string
The iOS 7 SDK now has a better alternative tostringByAddingPercentEscapesUsingEncoding that does let you specify that you want all characters escaped except certain allowed ones. It works well if you are building up the URL in parts:
 NSString * unescapedQuery = [[NSString alloc] initWithFormat:@"?myparam=%d", numericParamValue];
 NSString * escapedQuery = [unescapedQuery stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet URLQueryAllowedCharacterSet]];
 NSString * urlString = [[NSString alloc] initWithFormat:@"http://ExampleOnly.com/path.ext%@", escapedQuery];

- C言語の文字const char *とObjectiveCの文字列NSStringの変換
--  http://d.hatena.ne.jp/eidolon/20121125
 NSString *str = @"abc";
 const char *p = [str UTF8string];

 char str_c[100];
 strcpy(str_c,"abc");
 NSString *str = [ [ NSString alloc ] initWithUTF8String:str ];