Why are alloc and init called separately in Object

问题:

Note: I'm relatively new to Objective-C and am coming from Java and PHP.

Could someone explain to me why I always have to first allocate and then initialize an instance?

Couldn't this be done in the init methods like this:

+ (MyClass*)init {
    MyClass *instance = [MyClass alloc];
    [instance setFoo:@"bla"];

    return instance;
}

+ (MyClass*)initWithString:(NSString*)text {
    MyClass *instance = [MyClass init];
    [instance setFoo:text];

    return instance;
}
...

Is this just a relict from the old C days or is there something that I'm not seeing?

I know this isn't a problem as I could as well always call alloc and init, but since it's a bit tedious I'd like to at least know why I'm doing it.

I'm liking the expressiveness of the language so far, but this is something that I want to fully understand in order to think the Objective-C way.

Thank you!

回答1:

+new ends up sending an +alloc message to the class and an -init message to whatever comes back from +alloc.

The reason that NeXT departed from Stepstone's convention of using the +new message (which was a Smalltalk idea) is that early on, they encountered situations where they wanted to be able to initialize the same object more than once.

回答2:

Because creating an instance and initializing an instance are two separate jobs.

You send an alloc message to the class to get an uninitialized instance. You must then initialize the instance, and you often have several ways to do that. For example:

myStr = [[NSString alloc] init]; //Empty string
myStr = [[NSString alloc] initWithFormat:@"%@.%@", parentKeyPath, key];
myStr = [[NSString alloc] initWithData:utf16data encoding:NSUnicodeStringEncoding error:&error];
myStr = [[NSString alloc] initWithContentsOfURL:URL encoding:NSUTF8StringEncoding error:&error];

Each of these initializes the string in a completely different way. How you initialize the string depends on what you want to initialize it from.

Of course, nobody likes writing alloc and then init and then autorelease every time, so you usually have convenience methods (e.g., stringWithFormat:) that do all three steps for you.

Edit: For more on this topic, including essential insights from commenters, see my blog post “Reunification”.

回答3:

See NSZone.

+alloc is a shortcut cut for +allocWithZone:, which is a mechanism Cocoa provides for optimizing memory allocation.

So you have the option to do something like this:

foo = [[NSString allocWithZone:MyZone] initWithString:@"Foo"];
foo2 = [foo copyWithZone:MyZone];

The idea behind memory zones is that if you have a large number of similar objects that are frequently allocated and deallocated it may more efficient to use a separate memory zone for those objects.

In order for zoning to be effective you'd want to have +allocWithZone: available to every NSObject subclass, hence you need to separate allocation and initialization. You can create and use all the shortcuts you want, like +new, but underneath it all you need an -init method that initializes an object that has already been allocated.

回答4:

"Separating the allocation and initialization stages of instance creation provides many benefits. It’s possible to use any variation of the +alloc class method to allocate an instance and then use any available initializer with the new instance.This makes it possible to create your own initialization methods without needing to provide alternate implementations of all allocation methods. New allocation methods are seldom created because the existing methods meet almost every need. However, one or more new initializers are created for almost every class. Due to the separation of allocation and initialization stages, initializer implementations only have to deal with the variables of new instances and can completely ignore the issues sur- rounding allocation.The separation simplifies the process of writing initializers. Furthermore, Cocoa standard initializers like -initWithCoder: work with instances regardless of the way memory for the instance was allocated. One negative consequence of the separation of allocation and initialization is the need to be aware of conventions such as the designated initializer.You must know which methods are designated initializers and how to create and document new initializers in sub- classes. In the long run, using designated initializers simplifies software development, but there is an argument to be made that theTwo-Stage Creation pattern adds to the early learning curve for Cocoa developers."

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(c) Cocoa Design Patterns by Erik M. Buck and Donald A. Yacktman

回答5:

You don't have to. You can use [MyClass new]. This is similar to your hypothetical init method.

Basically, Objective-C, which didn't have garbage collection initially, separates the concept of memory allocation and class initialization. That's why there are two distinct methods. When you call alloc, you are explicitly allocating memory.

回答6:

Most classes have what you are asking for. You have gotten answers before on why this is like it is and why you wouldn't always want to use this all the time but if you read the documentation to classes you will see many class methods that act this way and they are often used.

For NSString you have, for example:

+ (id)string  // (Empty string)
+ (id)stringWithFormat:...  // Formatted string (like you use)
+ (id)stringWithContentsOfURL:... // String populated with contents of URL

And so on. And you would then use this like: NSString *myString = [NSString stringWithFormat:@"Hello %@\n", userName];

Most other classes have this, like NSArray:

+ (id)array
+ (id)arrayWithContentsOfFile:...
+ (id)arrayWithContentsOfURL:...
+ (id)arrayWithObjects:...

You just need to read the documentation. :) And read the other replies on why you don't want to use this too much.

回答7:

alloc : Memory is allocated/given to the object-reference. Now reference has the possession of the memory but has not done anything yet. This memory be empty(rarest case) or with some anonymous data.

alloc and init : Allocated memory is cleaned/emptied. Memory is initiated by zero bit.

alloc and initwithdata... : Allocated memory is initiated with desired data respected to properties of the class.

For example when you purchase a plot you get the possession. This plot is given to you as it is, ruined bricks or old house may be there. This is alloc.

When you clean your plot and remove all dirt and litter. This is alloc with init.

When you build that into some valuable house it becomes more meaningful to you. And it is alloc initwith...