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--- blog:2023:08:26:2023-08-26_-_handling_enums_in_c [2023/08/26 15:43] – created basz
+++ blog:2023:08:26:2023-08-26_-_handling_enums_in_c [2023/08/26 15:50] (current) – tips and tricks on handlign enums in C++ basz
@@ Line 73: / Line 73: @@
 ===== handling some values =====
+another common case where ''enum''s are used is adding special functions, that check if value is in a given logical state / group. eg. determining if it's an error:
+<code cpp>
+enum class State
+{
+  Initializing,
+  Running,
+  InputError
+};
+bool isOk(State s)
+{
+  return s != State::InputError;
+}
+</code>
+looks good, all tests pass. however... you probably already know where it's going. ;) let's consider extending ''State'' with a new error state:
+<code cpp>
+enum class State
+{
+  Initializing,
+  Running,
+  InputError,
+  OutputError
+};
+</code>
+...and now ''isOk(State::OutputError) == true''. whoops...
+a better way of writing these kind of function is to list all ''enum'' values explicitly, and assign ''return''s accordingly, eg.:
+<code cpp>
+#include <cassert>
+// ...
+bool isOk(State s)
+{
+  switch(s)
+  {
+    case State::Initializing:
+    case State::Running:
+      return true;
+    case State::InputError:
+      return false;
+  }
+  assert(!"unknown value");
+}
+</code>
+now, if states change, we get a proper error:
+.cpp: In function ‘bool isOk(State)’:
+.cpp:13:9: error: enumeration value ‘OutputError’ not handled in switch [-Werror=switch]
+|   switch(s)
+        |         ^
+  cc1plus: all warnings being treated as errors
+so we know we need to fix it:
+<code cpp>
+#include <cassert>
+// ...
+bool isOk(State s)
+{
+  switch(s)
+  {
+    case State::Initializing:
+    case State::Running:
+      return true;
+    case State::InputError:
+    case State::OutputError:
+      return false;
+  }
+  assert(!"unknown value");
+}
+</code>
+while writing this kind of functions using ''switch''-''case'' may look a bit odd at first, it has a very nice property of ensuring code being future-proof. as a side effect, it also makes core far easier to read. for comparison just imagine ''isOk()'' where there are 7 "ok" states and 13 "error" states...

blog/2023/08/26/2023-08-26_-_handling_enums_in_c.txt · Last modified: 2023/08/26 15:50 by basz

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