CHECK/DCHECK: Explicitly state whether the condition can have side effects

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24889004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7394 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/base/checks.h

@@ -35,6 +35,9 @@
 //   she recognizes that if she is wrong, abrupt and unpleasant process
 //   termination is still better than carrying on with the assumption violated.
 //
+//   CHECK always evaluates its argument, so it's OK for x to have side
+//   effects.
+//
 // - DCHECK(x) is the same as CHECK(x)---an assertion that x is always
 //   true---except that x will only be evaluated in debug builds; in production
 //   builds, x is simply assumed to be true. This is useful if evaluating x is
@@ -46,6 +49,10 @@
 //   at night knowing that the process will suicide instead of carrying on in
 //   case you were wrong, use CHECK instead of DCHECK.
 //
+//   DCHECK only evaluates its argument in debug builds, so if x has visible
+//   side effects, you need to write e.g.
+//     bool w = x; DCHECK(w);
+//
 // - CHECK_EQ, _NE, _GT, ..., and DCHECK_EQ, _NE, _GT, ... are specialized
 //   variants of CHECK and DCHECK that print prettier messages if the condition
 //   doesn't hold. Prefer them to raw CHECK and DCHECK.