Question

As a part of my transition process from C++ to Objective-C, I intensively read book Cocoa and Objective C Up and Running.

In one of the book code examples, there is a line that does not make sense to me with my current level of knowledge:

It is a declaration of class method + (Photo*) photo;.

Could anybody explain me the reason, please, why the author had decided for the method (Photo*) photo; to declare it as a class method instead of instance method?

I have studiet the theory, that the instane method is something like a class member function and that class method is something like static function in C++. But this still does not answer my question.

Here is the declaration code:

#import <Foundation/Foundation.h>


@interface Photo : NSObject{

    NSString* caption;
    NSString* photographer;    
}

+ (Photo*) photo;

- (NSString*) caption;
- (NSString*) photographer;

- (void) setCaption: (NSString*)input;
- (void) setPhotographer: (NSString*)input;

@end

The implementation code follows:

#import "Photo.h"


@implementation Photo

- (id)init
{
    self = [super init];
    if (self) {
        [self setCaption:@"Default Caption"];
        [self setPhotographer:@"Default Photographer"];
    }

    return self;
}


+ (Photo*) photo {
    Photo* newPhoto = [[Photo alloc] init];
    return [newPhoto autorelease];
}


- (NSString*) caption {
    return caption;
}


- (NSString*) photographer {
    return photographer;
}


- (void) setCaption:(NSString *)input {
    ;
    caption = [input retain];
}


- (void) setPhotographer: (NSString *)input {
    [photographer autorelease];
    photographer = [input retain];
}


- (void)dealloc
{
    [self setCaption:nil];
    [self setPhotographer:nil];

    [super dealloc];
}

@end

Answer 1

Could anybody explain me the reason, please, why the author had decided for the method (Photo*) photo; to declare it as a class method instead of instance method?

It’s basically a wrapper of the constructor litany. Note the source:

+ (Photo*) photo {
    Photo* newPhoto = [[Photo alloc] init];
    return [newPhoto autorelease];
}

Allocate a new Photo, initialize it, mark it autorelease, and return it. Since it creates the object, there is no object yet to operate upon, ergo this needs to be a class method.