Question

Got this code:

#import <Foundation/Foundation.h>
@interface CalculatorBrain : NSObject
- (void)pushOperand:(double)operand;
- (double)performOperation:(NSString *)op;
@property (nonatomic, readonly) id program;
+ (NSString *)descriptionOfProgram:(id)program;
+ (double)runProgram:(id)program;
@end

And this one:

#import "CalculatorBrain.h"
@interface CalculatorBrain()
@property (nonatomic, strong) NSMutableArray *programStack;
@end
@implementation CalculatorBrain
@synthesize programStack = _programStack;
- (NSMutableArray *)programStack
{
if (_programStack == nil) _programStack = [[NSMutableArray alloc] init];
return _programStack;
}
- (id)program
{
return [self.programStack copy];
}
+ (NSString *)descriptionOfProgram:(id)program
{
return @"blablabla";
}
    - (void)pushOperand:(double)operand
{
   [self.programStack addObject:[NSNumber numberWithDouble:operand]];
}

- (double)performOperation:(NSString *)operation
{
[self.programStack addObject:operation];
return [[self class] runProgram:self.program];
}

+ (double)popOperandOffProgramStack:(NSMutableArray *)stack
{
   double result = 0;
   return result;
}
+ (double)runProgram:(id)program
{
   NSMutableArray *stack;
   if ([program isKindOfClass:[NSArray class]]) {
      stack = [program mutableCopy];
   }
   return [self popOperandOffProgramStack:stack];
}
@end

The code is fine an it runs, so the question is, Where is declared popOperandOffProgramStack in the interface? why it compiles and it’s okay? it should crash but I can not find an explanation to this….

Thank you!

Answer 1

You only need to declare methods in the @interface in the .h file if you’re exposing them to the world. Otherwise, no declaration needed.

And nowadays, the order that they appear in the implementation doesn’t matter, either. Historically, if the method was implemented later in the @implementation than where it was invoked, you needed to have the method declared above (generally in the @interface). Now the compiler doesn’t care whether the implementation is earlier or later in the .m file.