Update and extension of enumeration section

CppCoreGuidelines.md

@@ -1,6 +1,6 @@
 # <a name="main"></a>C++ Core Guidelines
 
-july 16, 2016
+July 16, 2016
 
 Editors:
 
@@ -7018,15 +7018,16 @@ Enumerations are used to define sets of integer values and for defining types fo
 
 Enumeration rule summary:
 
-* [Enum.1: Prefer enums over macros](#Renum-macro)
-* [Enum.2: Use enumerations to represent sets of named constants](#Renum-set)
-* [Enum.3: Prefer class enums over "plain" enums](#Renum-class)
+* [Enum.1: Prefer enumerations over macros](#Renum-macro)
+* [Enum.2: Use enumerations to represent sets of related named constants](#Renum-set)
+* [Enum.3: Prefer `enum class`es over "plain" `enum`s](#Renum-class)
 * [Enum.4: Define operations on enumerations for safe and simple use](#Renum-oper)
 * [Enum.5: Don't use `ALL_CAPS` for enumerators](#Renum-caps)
-* [Enum.6: Use unnamed enumerations for ???](#Renum-unnamed)
-* ???
+* [Enum.6: Avoid unnamed enumerations](#Renum-unnamed)
+* [Enum.7: Specify the underlying type of an enumeration only when necessary](#Renum-underlying)
+* [Enum.8: Specify enumerator values only when necessary](#Renum-value)
 
-### <a name="Renum-macro"></a>Enum.1: Prefer enums over macros
+### <a name="Renum-macro"></a>Enum.1: Prefer enumerations over macros
 
 ##### Reason
 
@@ -7057,23 +7058,47 @@ Instead use an `enum`:
     int webby = blue;   // error: be specific
     Webcolor webby = Webcolor::blue;
 
+We used an `enum class` to avoid name clashes.
+
 ##### Enforcement
 
-Flag macros that define integer values
+Flag macros that define integer values.
 
-### <a name="Renum-set"></a>Enum.2: Use enumerations to represent sets of named constants
+
+### <a name="Renum-set"></a>Enum.2: Use enumerations to represent sets of related named constants
 
 ##### Reason
 
-An enumeration shows the enumerators to be related and can be a named type
+An enumeration shows the enumerators to be related and can be a named type.
+
+
 
 ##### Example
 
     enum class Webcolor { red = 0xFF0000, green = 0x00FF00, blue = 0x0000FF };
 
+
+##### Note
+
+Switching on an enumeration is common and the compiler can warn against unusual patterns of case labels. For example:
+
+    enum class Productinfo { red = 0, purple = 1, blue = 2 };
+
+    void print(Productinfo inf)
+    {
+        switch (inf) {
+        case Productinfo::red: cout << "red"; break;
+        case Productinfo::purple: cout << "purple"; break;
+        }
+    }
+
+Such off-by-one switch`statements are often the results of an added enumerator and insufficient testing.
+
 ##### Enforcement
 
-???
+* Flag `switch`-statements where the `case`s cover most but not all enumerators of an enumeration.
+* Flag `switch`-statements where the `case`s cover a few enumerators of an enumeration, but has no `default`.
+
 
 ### <a name="Renum-class"></a>Enum.3: Prefer class enums over "plain" enums
 
@@ -7117,11 +7142,20 @@ Convenience of use and avoidance of errors.
 
 ##### Example
 
-    ???
+    enum Day { mon, tue, wed, thu, fri, sat, sun };
+
+    Day operator++(Day& d)
+    {
+        return d==sun?mon:Day{++d};
+    }
+
+    Day today = sat;
+    Day tomorrow = ++today;
 
 ##### Enforcement
 
-???
+Flag repeated experssions cast back into a an enumeration.
+
 
 ### <a name="Renum-caps"></a>Enum.5: Don't use `ALL_CAPS` for enumerators
 
@@ -7129,27 +7163,102 @@ Convenience of use and avoidance of errors.
 
 Avoid clashes with macros.
 
-##### Example
+##### Example, bad
 
-    ???
+     // webcolors.h (third party header)
+    #define RED   0xFF0000
+    #define GREEN 0x00FF00
+    #define BLUE  0x0000FF
+
+    // productinfo.h
+    // The following define product subtypes based on color
+    
+    enum class Productinfo { RED, PURPLE, BLUE };   // syntax error
 
 ##### Enforcement
 
-???
+Flag ALL_CAPS ennumerators.
 
-### <a name="Renum-unnamed"></a>Enum.6: Use unnamed enumerations for ???
+### <a name="Renum-unnamed"></a>Enum.6: Avoid unnamed enumerations
 
 ##### Reason
 
-???
+If you can't name an enumeration, the values are not related
 
-##### Example
+##### Example, bad
 
-    ???
+    enum { red = 0x,FF0000, scale = 4, signed = 1 };
+
+Such code is not uncommon in code written before there were convenient alternative ways of specifying integer constants.
+
+##### Alternative
+
+Use `constexpr` values instead. For example:
+
+    constexpr int red = 0x,FF0000;
+    constexpr short scale = 4;
+    constexpr bool signed = true;
 
 ##### Enforcement
 
-???
+Flag unnamed enumerations.
+
+
+### <a name="Renum-underlying"></a>Enum.7: Specify the underlying type of an enumeration only when necessary
+
+##### Reason
+
+The default is the easiest to read and write.
+`int` is the default integer type.
+`int` is compatible with C `enum`s.
+
+##### Example
+
+    enum class Direction : char { n,s,e,w, ne, nw, se, sw };    // underlying type saves space
+    
+    enum class Webcolor : int { red = 0xFF0000, green = 0x00FF00, blue = 0x0000FF };  // underlying type is redundant
+
+##### Note
+
+Specifying the underlying type is necessary in forward declations of enumerations:
+
+    enum Flags : char;
+
+    void f(Flags);
+    
+    // ....
+
+    enum flags : char { /* ... */ };
+
+
+##### Enforcement
+
+????
+
+
+### <a name="Renum-value"></a>Enum.8: Specify enumerator values only when necessary
+
+##### Reason
+
+It's the simplest.
+It avoids duplicate enumerator values.
+The default gives a consequtive set of values that is good for `switch`-statement implementations.
+
+##### Example
+
+    enum class Col1 { red, yellow, blue };
+    enum class Col2 { red=1, red=2, blue=2 }; // typo
+    enum class Month { jan=1, feb, mar, apr, mar, jun, jul, august, sep, oct, nov, dec }; // starting with 1 is conventional
+    enum class Base_flag { dec=1, oct=dec<<1, hex=dec<<2 }; // set of bits
+
+Specifying values are neccessary to match conventional values (e.g., `Month`)
+and where consecutive values are undesirable (e.g., to get separate bits as in `Base_flag`).
+
+##### Enforcement
+
+* Flag duplicate enumerator values
+* flag explicitly specified all-consequtive enumerator values
+
 
 # <a name="S-resource"></a>R: Resource management