CODEWITHSUNDEEP
objective-cconstants
Max
Max

Reputation: 21

How to set a constant instance variable in Objective C?

I want to create an instance variable called "pi" of class "Circle" and set its value of 3.14 as a constant that once is set cant be changed and classes inheriting the Circle class will also get pi variable without needing to set its value each time;

My Code:

#import <Foundation/Foundation.h>

@interface Circle: NSObject

{

const double pi = 3.14;

}


@property float radius;

-(float) area;

-(float) perimeter;

@end

Upvotes: 1

Views: 395

Answers (1)

Rob
Rob

Reputation: 438082

If it's not really something that is not unique to any particular instance, I might use a class property (or class method):

@property (class, readonly) double pi;

And then I'd implement a custom getter to return the value in question:

+ (double)pi {
    return M_PI;
}

Or, if it involved complicated calculation, then you might use the dispatch_once pattern so it's only calculated once:

+ (double)pi {
    static dispatch_once_t once;
    static double _pi = 0;

    dispatch_once(&once, ^{
        // an unnecessary complicated calculation of pi, just to illustrate we'll do this only once

        int k = 1;
        double previous = 0;
        BOOL sign = TRUE;
        do {
            previous = _pi;
            _pi = previous + 4.0 * (sign ? 1.0 : -1.0) / k;
            k += 2;
            sign = !sign;
        } while (fabs(_pi - previous) > 0.0001);
    });

    return _pi;
}

And then you can reference this pi property:

- (float)area {
    return self.radius * self.radius * [Circle pi];
}

- (float)perimeter {
    return self.radius * [Circle pi] * 2.0;
}

Upvotes: 1

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