Invariants and CHECKKs in C++

Defensive vs offensive coding

Defensive programming is the practice of writing software to enable continuous operation after and while experiencing unplanned issues. However, while defensive coding will result in code that is more resilient to crashes, it can leave the system running in an undefined state. Devices that are in an undefined state are broken and need to be reset. By using defensive programming, you are continuing the device's state to handle an undefined situation, which leaves the system vulnerable and masks the error.

Offensive programming, although seemingly opposite word choice, actually expands upon defensive programming and takes it one step further: instead of gracefully handling a failure point, offensive programming asserts that invariants always hold, and reveals to developers any information that will be helpful to debug.

See "Defensive Programming: Friend or Foe" for more information.

CHECK(), DCHECK(), NOTREACHED_NORETURN() and NOTIMPLEMENTED()

Assertions are an important tool of offensive programming to enforce assumptions and code paths that we expect to be a certain way. See this README for more information on this topic. Below are commonly used assertions:

• CHECK(): Use CHECK() to document and verify invariants. CHECK() will immediately kill the process if its condition is false for all builds, release and otherwise.

  Another use case for CHECK() is to track down known crashes when debugging or developing a feature. If a crash is happening and you cannot figure out why, add CHECK()s in the code that run before the crash to check your assumptions. This may move the crash further up the call stack thus helping you find the erroneous condition leading to the crash.

• DCHECK(): DCHECK() is like CHECK(), but is only included in builds where DCHECK_IS_ON is true (debug builds not end-user builds). Prefer CHECK() to DCHECK() for stronger assertive coding. Use DCHECK() only if the invariant is known to be too expensive to verify in production.

• NOTREACHED_NORETURN(): Use NOTREACHED_NORETURN() to enforce a particular branch of code should never be run. If run, it will cause a crash if DCHECK_IS_ON is enabled. Prefer to unconditionally CHECK() instead of conditionally hitting a NOTREACHED_NORETURN(). Prefer NOTREACHED_NOTRETURN() instead of NOTREACHED(). For example, if you are writing a switch statement that requires all enums, but only a subset of the enums are ever expected to be reached, use a NOTREACHED_NORETURN() on the others to declare they should never be run. For example,

  void FastPairPresenterImpl::OnPairingFailedDismissed(
     PairingFailedCallback callback,
     FastPairNotificationDismissReason dismiss_reason) {
   switch (dismiss_reason) {
     case FastPairNotificationDismissReason::kDismissedByUser:
       callback.Run(PairingFailedAction::kDismissedByUser);
       break;
     case FastPairNotificationDismissReason::kDismissedByOs:
       callback.Run(PairingFailedAction::kDismissed);
       break;
     case FastPairNotificationDismissReason::kDismissedByTimeout:
       // Fast Pair Error Notifications do not have a timeout, so this is never
       // expected to be hit.
       NOTREACHED();
       break;
   }
  }
  

• NOTIMPLEMENTED(): Use NOTIMPLEMENTED() to note that a path of code that is reached is not implemented via logs. For example, NOTIMPLEMENTED() can be used to ensure that a method in a base class is either overridden or never called.

See here for a shorthand reference.