updated advice on setting constants in Objective-C?

Question

I read through the advice here Constants in Objective-C, but I find two errors in the accepted answer:

1. I keep getting linker errors when I implement the approach of using .m and .h

2. In my constants.m file, I get the error "unknown type" for NSString

   I am also getting an error following the advice from @VictorHanHee as a further suggestion of how to link the constants to a .pch file:

3. I don't have a .pch file by default, and when I create a new one it doesn't have the formatting expected from the answer.

All I have done is create constants.h and constants.m and put the constant declarations in them as provided in the accepted answer. I also created a .pch file, but as I said it doesn't at all match VictorHanHee's description.

Is this older posting outdated, or if not what am I missing? I don't really know how to go forward with a linker error. I simply want to have a file constants.h where I declare all constants and can include this file in as many classes as I want to make life easier. Can someone tell me how to do this nowadays or point to a more recent discussion?

Also here is the linker error (or part of log I can copy and paste anyway, not full message)

    -[AccountInfoViewController viewDidLoad] in AccountInfoViewController.o
ld: symbol(s) not found for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)

As you can see, I am trying to access the constant from viewDidLoad. I am simply trying to print it out with NSLog.

Here are the .h and .m files I was originally using:

.h

extern NSString * const PREFS_MY_CONSTANT;

.m

NSString * const PREFS_MY_CONSTANT = @"prefs_my_constant";

Ok here is the template that actually seems to work:

.h

#ifndef Project_prefs_h
#define Project_prefs_h


#endif


extern NSString * const PREFS_MY_CONSTANT;

.m

#import <Foundation/Foundation.h>
NSString * const PREFS_MY_CONSTANT = @"prefs_my_constant";

So the older post is perhaps misleading or I misunderstood them when they said remove everything apart from the constants - but if you don't actually want to remove everything, shouldn't you say so? That's a fair amount of text in there not to mention...

Answer 1

In recent version of Xcode there is no ProjectName-Prefix.pch created automatically file so you should import the file where it is required then try using the constant.

Easiest way:

// Prefs.h
#import <Foundation/Foundation.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";

There are a few different ways to create constants in Objective-C, but the easiest way is to use #define statements.

To start, create a new header file. This is where we’ll store all the constants. Name it something original like “Constants.h”.

Here’s a sample file with a few constants:

//
//  Constants.h
//  App Name


#import <Foundation/Foundation.h>

#define NUM_SECTIONS        7

#define NUM_SECTION_1_ITEMS 2
#define NUM_SECTION_2_ITEMS 14
#define NUM_SECTION_3_ITEMS 5

#define APP_TITLE           @"App Name"
#define APP_AUTHOR          @"Miscellanea"

To use these constants in your project, you need to import your header file in each implementation file (*.m) where you’ll be referencing them.

#import "Constants.h"
– (void)viewDidLoad
{
    self.title = APP_TITLE;

    for (int i = 0; i < NUM_SECTIONS; ++ i)
    {
        // etc.
    }
}

Answer 2

Here is the template that does seem to work. It's not different from the old answer except that you don't delete everything apart from the constants.

.h

#ifndef Project_prefs_h
#define Project_prefs_h  
#endif
extern NSString * const PREFS_MY_CONSTANT;

.m

#import <Foundation/Foundation.h>
NSString * const PREFS_MY_CONSTANT = @"prefs_my_constant";

Apart from this it appears no longer possible to use .pch file to automatically include the constants across all classes.