iOS A very simple question about #import

[mikezang]

I remember all Objective-C guides and samples show me to use @class CLASS_NAME in header file, then use #import CLASS_HEADER.

I just want to know why not use #import CLASS_HEADER in header file directly, so that you don't need to modify two files.

// xxx.h
@class DetailTableViewController;

@interface MasterTableViewController : UITableViewController {
    DetailTableViewController *detailTableViewController;
}

// xxx.m
#import "DetailTableViewController.h"

@implementation MasterTableViewController

@synthesize detailTableViewController;

// xxx.h
#import "DetailTableViewController.h"

@interface MasterTableViewController : UITableViewController {
    DetailTableViewController *detailTableViewController;
}

// xxx.m
@implementation MasterTableViewController

@synthesize detailTableViewController;

 

[lloyddean]

Reduces coupling, thus compile times.

 

[mikezang]

Reduces coupling, thus compile times.

That is very easy to understand. I will follow it.

Thanks.

 

[lloyddean]

Actually looking at your first post now that I'm awake I'll revise that following a query.

Your first fragment above - is that from a header or an implementation file?

 

[mikezang]

Actually looking at your first post now that I'm awake I'll revise that following a query.

Your first fragment above - is that from a header or an implementation file?

Why do you ask this? I think I had point which one is header and source

 

[lloyddean]

Why do you ask this? I think I had point which one is header and source

You're right, my first answer was correct and I still need more sleep.

 

[PhoneyDeveloper]

Also reduces likelihood of circular imports.

Not everyone agrees with this reasoning. Some prefer using the imports in the headers because it results in fewer lines of code.

I prefer to use the forward declarations wherever possible and imports wherever required.

 

[bredell]

Also reduces likelihood of circular imports.

Shouldn't circular imports be impossible? I thought that the #import directive only included the file if it hadn't been included before.

 

[lloyddean]

Shouldn't circular imports be impossible? I thought that the #import directive only included the file if it hadn't been included before.

You are correct!

 

[yaniv92648]

Yes but it's irrelevant to circular imports

Because if Class A tries to import Class B and Class B tries to import Class A
so assuming Class A tries to import first then the compiler tries to fully compile Class B but that means that it has to return to Class A and fully compile it in order for Class B to be fully compiled and so on...
this is Circular Import.

 

[lloyddean]

From Apples title "The Objective-C Programming Language, Tools & Languages: Objective-C" Copyright 2009, released 2009-10-19.

Concerning '#import": "This directive is identical to #include, except that it makes sure that the same file is never included more than once."

 

[yaniv92648]

Yes it is correct..

But it is irrelevant to circular imports, circular imports can still occur!