How to change #define value

Question

I know #define must be constant but please give me any good tips.

In my case, I define a constant value by #define (e.g. #define kImageQuality 0.7). However, I would like to change the constant value from Settings.Bundle before opening an app. That means to change the constant value, doesn't it? Is that any way to implement as my aim?

It should change to instance variable instead of #define? Any tips given by you that would be really appreciated.

you can define a property and set its default value on viewWillAppear or viewDidLoad and change it later in the future.. — Eddwin Paz
– Eddwin Paz, Commented Feb 7, 2015 at 2:22

carloabelli · Accepted Answer · 2015-02-07 05:25:58Z

5

#define constants are replaced before compilation even begins by the preprocessor (e.g. kImageQuality gets replaced by 0.7 before compilation). Therefore loading it before the app starts is impossible as the app does not recompile every time. You need to use a variable:

float imageQuality = 0.7f;

edited Feb 7, 2015 at 5:25

answered Feb 7, 2015 at 2:19

carloabelli

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Linuxios · Accepted Answer · 2015-02-07 02:21:13Z

4

This is not possible because this:

#define constant 3
...
y = x + constant

Is completely equivalent to this:

y = x + 3

#defined constants are replaced by their value in the preprocessing stage before the code is even compiled. To change the value dynamically, you have to either use a global variable, or some other persistent mechanism like NSUserDefaults.

answered Feb 7, 2015 at 2:21

Linuxios

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1 Comment

user1574429 Over a year ago

As long as enabling to change a constant value within - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions, I would be happy. So I am going for "y = x + 3" program. Thank you very much!

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