Equivalent of public static final variables

Question

I understand that placing the word `extern` before a variable declaration in a header file declares the existence of a global static variable without initialising it. I also understand that if I import the file containing the `extern` variables, I can reference them without a class/file name. But where does one define them and their values? What I am trying to do is create a class of constants with global constants that I want to use throughout an iOS application's code. Does one put them inside the interface like this? Example.h ``` #import @interface Constraints : NSObject { extern NSString * const PREFS_NAME; } ``` Or does one put then outside of the interface like this Example.h ``` #import extern NSString * const PREFS_NAME; @interface Constraints : NSObject { } ``` Then in the implementation .m file how would one initialise the extern values? Inside the implementation area like this? Example.m ``` #import "Constraints.h" @imple…

Accepted Answer

You define it's value in the file inside which it's declared, which in your case is Example.m; You can still re-assign this variable, so the declaration in Example.h would look like this:

```
extern  NSString * PREFS_NAME;
```

This way every file that imports Example.h has access to this variable. The equivalent of public static final in Objective-C is const. If you also want it to be public you should make it be a class instance variable, but in this case you don't need it because it's already accessible everywhere. So in this case it would be:

```
// .m file
NSString* const PREFS_NAME = @"MyApp_Prefs";
// .h file
extern NSString* const PREFS_NAME;
```

Also notice that const NSString* is different from NSString* const. The latter is a const pointer to NSString. The former hasn't sense even if it's a correct syntax. In Objective-C the const qualifier doesn't affect objects, instead there are mutable and immutable classes. It would have sense in C++ meaning that you can use just const methods on the instance.

Equivalent of public static final variables

Solution

Problem

Related problems