Creating Strings

The simplest way to create a string object in source code is to use the Objective-C  @"..." construct:

NSString *temp = @"Contrafibularity";

Note, when creating a string constant in this fashion, you should use UTF-8 characters. You can put the actual Unicode data in the string for localized text, as in NSString *hello = @"こんにちは";. You can also insert \u or \U in the string for non-graphic characters.

Objective-C string constant is created at compile time and exists throughout your program’s execution. The compiler makes such object constants unique on a per-module basis, and they’re never deallocated. You can also send messages directly to a string constant as you do any other string:

BOOL same = [@"comparison" isEqualToString:myString];

char *utf8String = /* Assume this exists. */ ;

NSString *stringFromUTFString = [[NSString alloc] initWithUTF8String:utf8String];

char *macOSRomanEncodedString = /* assume this exists */ ;

NSString *stringFromMORString =

            [[NSString alloc] initWithCString:macOSRomanEncodedString

                              encoding:NSMacOSRomanStringEncoding];

NSData *shiftJISData =  /* assume this exists */ ;

NSString *stringFromShiftJISData =

            [[NSString alloc] initWithData:shiftJISData

                              encoding:NSShiftJISStringEncoding];

unichar ellipsis = 0x2026;

NSString *theString = [NSString stringWithFormat:@"To be continued%C", ellipsis];

NSData *asciiData = [theString dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];

NSString *asciiString = [[NSString alloc] initWithData:asciiData encoding:NSASCIIStringEncoding];

NSLog(@"Original: %@ (length %d)", theString, [theString length]);

NSLog(@"Converted: %@ (length %d)", asciiString, [asciiString length]);

// output:

// Original: To be continued… (length 16)

// Converted: To be continued... (length 18)

NSString *hString = @"Hello";

NSString *hwString = [hString stringByAppendingString:@", world!"];

NSString *greeting = NSLocalizedStringFromTable

    (@"Hello", @"greeting to present in first launch panel", @"greetings");

Combining and Extracting Strings

You can combine and extract strings in various ways. The simplest way to combine two strings is to append one to the other. The stringByAppendingString: method returns a string object formed from the receiver and the given argument.

NSString *beginning = @"beginning";

NSString *alphaAndOmega = [beginning stringByAppendingString:@" and end"];

// alphaAndOmega is @"beginning and end"

You can also combine several strings according to a template with the initWithFormat:, stringWithFormat:, and stringByAppendingFormat: methods; these are described in more detail in Formatting String Objects.

You can extract substrings from the beginning or end of a string to a particular index, or from a specific range, with the substringToIndex:, substringFromIndex:, and substringWithRange: methods. You can also split a string into substrings (based on a separator string) with the componentsSeparatedByString: method. These methods are illustrated in the following examples—notice that the index of the index-based methods starts at 0:

NSString *source = @"0123456789";

NSString *firstFour = [source substringToIndex:4];

// firstFour is @"0123"

NSString *allButFirstThree = [source substringFromIndex:3];

// allButFirstThree is @"3456789"

NSRange twoToSixRange = NSMakeRange(2, 4);

NSString *twoToSix = [source substringWithRange:twoToSixRange];

// twoToSix is @"2345"

NSArray *split = [source componentsSeparatedByString:@"45"];

// split contains { @"0123", @"6789" }

If you need to extract strings using pattern-matching rather than an index, you should use a scanner—see Scanners.

Getting C Strings

To get a C string from a string object, you are recommended to use UTF8String. This returns a const char * using UTF8 string encoding.

const char *cString = [@"Hello, world" UTF8String];

The C string you receive is owned by a temporary object, and will become invalid when automatic deallocation takes place. If you want to get a permanent C string, you must create a buffer and copy the contents of the const char * returned by the method.

Similar methods allow you to create string objects from characters in the Unicode encoding or an arbitrary encoding, and to extract data in these encodings. initWithData:encoding: and dataUsingEncoding: perform these conversions from and to NSData objects.

Conversion Summary

This table summarizes the most common means of creating and converting string objects: