Constants in Objective-C – Dev

The best answers to the question “Constants in Objective-C” in the category Dev.

QUESTION:

I’m developing a Cocoa application, and I’m using constant NSStrings as ways to store key names for my preferences.

I understand this is a good idea because it allows easy changing of keys if necessary.
Plus, it’s the whole ‘separate your data from your logic’ notion.

Anyway, is there a good way to make these constants defined once for the whole application?

I’m sure that there’s an easy and intelligent way, but right now my classes just redefine the ones they use.

ANSWER:

Easiest way:

// Prefs.h
#define PREFS_MY_CONSTANT @"prefs_my_constant"

Better way:

// Prefs.h
extern NSString * const PREFS_MY_CONSTANT;

// Prefs.m
NSString * const PREFS_MY_CONSTANT = @"prefs_my_constant";

One benefit of the second is that changing the value of a constant does not cause a rebuild of your entire program.

ANSWER:

You should create a header file like

// Constants.h
FOUNDATION_EXPORT NSString *const MyFirstConstant;
FOUNDATION_EXPORT NSString *const MySecondConstant;
//etc.

(you can use extern instead of FOUNDATION_EXPORT if your code will not be used in mixed C/C++ environments or on other platforms)

You can include this file in each file that uses the constants or in the pre-compiled header for the project.

You define these constants in a .m file like

// Constants.m
NSString *const MyFirstConstant = @"FirstConstant";
NSString *const MySecondConstant = @"SecondConstant";

Constants.m should be added to your application/framework’s target so that it is linked in to the final product.

The advantage of using string constants instead of #define‘d constants is that you can test for equality using pointer comparison (stringInstance == MyFirstConstant) which is much faster than string comparison ([stringInstance isEqualToString:MyFirstConstant]) (and easier to read, IMO).

ANSWER:

The accepted (and correct) answer says that “you can include this [Constants.h] file… in the pre-compiled header for the project.”

As a novice, I had difficulty doing this without further explanation — here’s how: In your YourAppNameHere-Prefix.pch file (this is the default name for the precompiled header in Xcode), import your Constants.h inside the #ifdef __OBJC__ block.

#ifdef __OBJC__
  #import <UIKit/UIKit.h>
  #import <Foundation/Foundation.h>
  #import "Constants.h"
#endif

Also note that the Constants.h and Constants.m files should contain absolutely nothing else in them except what is described in the accepted answer. (No interface or implementation).

ANSWER:

There is also one thing to mention. If you need a non global constant, you should use static keyword.

Example

// In your *.m file
static NSString * const kNSStringConst = @"const value";

Because of the static keyword, this const is not visible outside of the file.


Minor correction by @QuinnTaylor: static variables are visible within a compilation unit. Usually, this is a single .m file (as in this example), but it can bite you if you declare it in a header which is included elsewhere, since you’ll get linker errors after compilation