isItPythonOrJavaScript.m

#import <Foundation/Foundation.h>
#include <stdio.h>

@interface NSString (MyPrintable)
- (const char *)toCString;
@end

@implementation NSString (MyPrintable)
// NSString is a library provided class that doesn't have
// a toCString method. We're dynamically extending the class
// below adding the method to every class instance.
- (const char *)toCString {
  return [self cStringUsingEncoding: NSASCIIStringEncoding];
}
@end

@interface NSNumber (MyPrintable)
- (const char *)toCString;
@end

@implementation NSNumber (MyPrintable)
// stringValue is a library provided method of NSNumber
// that returns NSString. We're dynamically extending
// NSNumber here with a new toCString method making
// use of the toCString we just added to NSString.
- (const char *)toCString {
  return [[self stringValue] toCString];
}
@end

int main() {
  // The array below contains instances of classes
  // NSString, NSNumber, NSString, and NSNumber in order.
  NSArray *list = @[@"3.14", @4.5, @"5", @6];

  // NSString type annotation below is only used as a request
  // to the compiler to check if the str variable is used
  // as an instance of NSString, for instance, that only
  // methods present in the NSString class are called.
  // The compiler doesn't check that str actually is an
  // NSString, and in fact, it's not as half of the time
  // it's NSNumber. There will be neither warning nor error
  // from the compiler here.
  for (NSString *str in list)
    printf("%s\n", [str toCString]);

  int sum = 0;
  for (NSNumber *num in list)
    sum += [num intValue];
  printf("%d\n", sum);

  // The program successfully outputs the following:
  //
  // 3.14
  // 4.5
  // 5
  // 6
  // 18
  //
  // It's perfectly correct, there is no undefined
  // behavior, no memory leaks or any other issues.
  return 0;
}