Triangle example for http://stackoverflow.com/questions/37261266/objects-in-objective-c-how-to-identify

Triangle.m

@interface Triangle : NSObject

@property (nonatomic) double sideALength;
@property (nonatomic) double sideBLength;
@property (nonatomic) double height;
@property (nonatomic, readonly) double area;

@end

@implementation Triangle

- (double)area {
    // do your calculation here

    return calculatedValue;
}

@end

Note, there is no asterisk next to the double properties, as double is a fundamental data type, not an object. But if you then use the Triangle class, that instance is an object, so you'd use an asterisk there, e.g.:

Triangle *foo = [[Triangle alloc] init];
foo.sideALength = 5.0;
foo.sideBLength = 4.0;
foo.height = 3.0;

double result = foo.area;