// objective-c design by contract

the macros herein are based on a set of macros devised by nicolas roard quite a while back. i was mega-stoked when i came across them, but it turned out that they weren't ARC-compatible. so i rewrote them and added a lil' flair of my own.

how

there are four pseudo-keywords defined in Contracts.h:

• @invariants
• @preconditions
• @postconditions
• @freeze
• frozen (note that this one lacks that stylish ampersand... just couldn't get that going, unfortunately)

an example class implementation

a typical class implementation will look like this:

@implementation MyClass

@invariants (
  _yungWeezy != nil,
  _flavaFlav > 7,
  [_compton shouldntBeMessedWith] == YES
)

// ...

@end

note the @invariants ( ... ) block, which for stylistic purposes ought to go at the top of your @implementation section. you can simply provide a list of comma-separated expressions that evaluate to a boolean, and these will be checked a bunch automatically as you call methods on your class.

an example method implementation

your methods will need to look something like this:

- (NSUInteger) recalculateNumberOfProblems:(BOOL)aBitchIsOne
                               newProblems:(NSArray *)newProblems {
  @preconditions (
    _money > 0,
    _problems != nil,
    _problems.count > 0,
    (newProblems != nil
        ? newProblems.count > 0
        : YES /* pass-through */ )
  )

  @freeze(_money, _problems)
  @postconditions(
    (aBitchIsOne
        ? self.money == 0
        : self.money < [frozen(_money) unsignedIntegerValue]),

    (newProblems != nil
        ? self.problems.count > [frozen(_problems) count]
        : self.problems.count == [frozen(_problems) count])
  )

  /** begin method body **/
  if (newProblems != nil) {
    [self.problems addObjectsFromArray:newProblems];
    self.money = [self recalculateMoney];
  }

  if (aBitchIsOne) {
    self.money = 0;
  }

  return (self.money * kMoneyProblemsMultiplier) + (aBitchIsOne ? 1 : 0);
}

a few things to take note of here:

• even if you have no pre- or postconditions, still include both blocks. they help make sure that the invariants are checked.
• you can omit @freeze if you don't need it.
• @freeze stores the value of a property/ivar on the object so that you can write postconditions about how it should've changed during the execution of the method body.
• @freeze's internal machinery uses key/value stuff to store these values, so any scalar value is going to get encoded as an Objective-C object. that's why you see [frozen(_money) unsignedIntegerValue] instead of just frozen(_money).
• i'm pretty sure it's a good idea to use self.x in your @postconditions block rather than simply _x. why? because @postconditions defines an Objective-C block that gets executed upon leaving the method's scope. i can't recall the specifics of how blocks copy the values of properties/ivars referenced in their block bodies, but just in case they copy the actual ivars instead of taking an automatic, silent reference to self, it'd be a good idea to reference the ivar via self. otherwise, the copy of the ivar/property that the @postconditions block has access to will be from the very beginning of the method's execution, where @postconditions is defined, rather than the value upon exiting the method.

authors & contributors

• bryn austin bellomy < bryn.bellomy@gmail.com >
• nicolas roard < nicolas@roard.com >
• justin spahr-summers, whose libextobjc (jspahrsummers/libextobjc) is full of incredible shit

license (MIT)

Copyright (c) 2012 bryn austin bellomy, robot bubble bath LLC (robotbubblebath.com)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.