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What is an import statement where the filename contains "+"?

I have seen in some source code (by other developers) something like this:

#import "SomeClass+SomeOtherClass.h"

What is +

for? What does it mean?

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2 answers


Let's say you want to add functionality to an existing class (exp:) NSString

. You can do this by creating a subclass or you can use a category. And usually called file, where the category is determined by using a template: MyClass+MyCategory.h

.

For example, we can add a method reverseString

to a class NSString

in a category:

// File NSString+reversable.h
- (NSString *)reverseString;

// File NSString+reversable.m
- (NSString *)reverseString
{
    // Implementation
}


Take a look at the documentation for more information on categories.

Then you can use this category in another class:

#import "NSString+reversable.h"
// ...

NSString *aString = @"Hello!";
NSString *reversedString = [aString reverseString];
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The "+" in header / source names is - by convention - used to describe implementations Category

.

Example:

Let's say you want to add some functionality to an existing class (like a class NSString

). ( NSString+Utilities.h

)

// NSString+Utilities.h

@interface NSString (Utilities)
-(NSString *) doSthWithThisString;
@end

      

        
        
        
      

    
// NSString+Utilities.m

@implementation NSString (Utilities)

-(NSString *) doSthWithThisString
{
  NSMutableString *transformedStr = [self copy];

  // Do sth

  return transformedStr;
}

@end


Using:

// in another file

#import "NSString+Utilities.h"

- (void)awakeFromNib
{
    NSString* myString = @"This is a string";

    // you may use our new NSString method as much as any already-existing one
    NSString* newString = [myString doSthWithThisString];
}

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