Question

To my understand self refers to the current class and when i use a dot after self is to use one of its properties. In the code here there’s a use in self.popOperand that i don’t understand if popOpernad is not a property. Another thing i don’t understand is why
[self pushOperand:result]; works and [self.pushOperand:result]; doesn’t.

#import "Calcbrain.h"
@interface Calcbrain()

@property (nonatomic,strong) NSMutableArray *operandStack;


@end


@implementation Calcbrain
@synthesize operandStack  = _operandStack;

-(NSMutableArray *) operandStack
{
    if(_operandStack == nil) _operandStack = [[NSMutableArray alloc]init];
    return _operandStack;
}
-(double)popOperand
{
    NSNumber *objectNum = [self.operandStack lastObject];
    if (objectNum)[self.operandStack removeLastObject];
    return [objectNum doubleValue];

}
/*-(void) setOperandStack:(NSMutableArray *)operandStack
{
    _operandStack = operandStack;
}*/

-(void)pushOperand:(double)opernand
{
    [self.operandStack addObject:[NSNumber numberWithDouble:opernand]];

}
-(double)performOperation:(NSString *)operation
{
    double result=0;

    if([operation isEqualToString:@"+"])
    {
        result = self.popOperand + self.popOperand;
    }
    else if ([@"*" isEqualToString:operation])
    {
        result = self.popOperand * self.popOperand;
    }
    [self pushOperand:result];

    return result;
}


@end

Answer 1

Whilst the . notation is primarily used for properties, it can be used for paramaterless methods that return a value. Why? Because the synthesised getter for a property is in the same form.

-(double)calcValue {
    ....
    return value;
}

Is equivalent to the property declaration:

@property (nonatomic, readonly) double calcValue;

Whilst there may be no property declaration, it doesn’t mean the . notation cannot be used. The compiler will effectively change . notation to a method call when compiling, as . is a form of syntactic sugar. As so:

self.popOperand
// Translates to
[self popOperand];

This leads on to part 2, why does [self.pushOperand:result]; not work? The reason being is that . does not support the passing of parameters directly.

The only way to assign/push a parameter to a property is via self.pushOperand = result, but this wouldn’t work, because there isn’t a corresponding - (void)setPushOperand:(double)pushOperand; that the . notation assignment maps to.

[self pushOperand:result]; works because you’re being explicit in calling a particular method, called pushOperand:.

Overall, keep . notation for properties only, and if you’re using a method that isn’t designed to be a ‘property’, be explicit.

Update: self is a reserved keyword, that represents a pointer to the instance we’re working within at that time.

For example, I can create two instances of Calcbrain outside of Calcbrain, for example BrainViewController:

Calcbrain* instance1;
Calcbrain* instance2;

Now, Calcbrain has methods declared within it, let’s use -(double)performOperation:(NSString *)operation as an example. Now, if I wanted to call that from BrainViewController, I would do:

[instance1 performOperation:@"+"];
[instance2 performOperation:@"+"];

Because we are calling a method which is part of another class, I have to determine the correct instance I’ve created to refer to it (i.e. instance1 and instance2). But how would I call that from within the class itself, and make sure it applies to the correct instance? The instance I’ve created is unaware of the other instances I’ve created. Use self. self allows you to reference yourself within methods. So if I wanted to performOperation within Calcbrain itself, I would need to use self:

[self performOperation:@"+"];