[Mac] Allow NSExceptions in certain cases.

base/mac/scoped_nsexception_enabler.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_MAC_SCOPED_NSEXCEPTION_ENABLER_H_
+#define BASE_MAC_SCOPED_NSEXCEPTION_ENABLER_H_
+#pragma once
+
+#include "base/basictypes.h"
+
+namespace base {
+namespace mac {
+
+// BrowserCrApplication attempts to restrict throwing of NSExceptions
+// because they interact badly with C++ scoping rules.  Unfortunately,
+// there are some cases where exceptions must be supported, such as
+// when third-party printer drivers are used.  These helpers can be
+// used to enable exceptions for narrow windows.
+
+// Make it easy to safely allow NSException to be thrown in a limited
+// scope.  Note that if an exception is thrown, then this object will
+// not be appropriately destructed!  If the exception ends up in the
+// top-level event loop, things are cleared in -reportException:.  If
+// the exception is caught at a lower level, a higher level scoper
+// should eventually reset things.
+class ScopedNSExceptionEnabler {
+ public:
+  // Passing |true| enables exceptions within this scope, passing
+  // |false| leaves the current setting alone.  This allows optionally
+  // enabling exceptions without requiring contrived code (like using
+  // |scoped_ptr<ScopedNSExceptionEnabler>|).

[stuartmorgan, 2011/05/24 00:54:42]

I'm not at all convinced that scoped_ptr is more contrived than an argument that
causes the whole object not to do anything. I had to read the API, then the
implementation, then the API again to actually have the right understanding of
what passing false does, because I assumed I misunderstood the first time.

To see if this was just me I showed this header to another engineer after
explaining the basic goal of this class, and then asked what false did, and the
answer was "I have no idea".

I understand how you got here, but I think in practice the scoped_ptr version
would be more readable in the code than a "false" argument anyway, and using
that would make the API much less confusing. In other words, I think this has
edged passed clever and into "clever" ;)

[Scott Hess - ex-Googler, 2011/05/24 19:38:09]

But then it's going to leak memory every time someone throws!

Done.

+  explicit ScopedNSExceptionEnabler(bool enable);
+  ~ScopedNSExceptionEnabler();
+
+ private:
+  bool was_enabled_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedNSExceptionEnabler);
+};
+
+// Access the exception setting for the current thread.  This is for
+// the support code in BrowserCrApplication, other code should use
+// the scoper.
+bool GetNSExceptionsAllowed();
+void SetNSExceptionsAllowed(bool allowed);
+
+}  // namespace mac
+}  // namespace base
+
+#endif BASE_MAC_SCOPED_NSEXCEPTION_ENABLER_H_