Issue 090303.1: Support C++0x strongly typed enumerations
Background
----------
For a detailed description of the feature, please refer to:
http://en.wikipedia.org/wiki/C%2B%2B0x#Strongly_typed_enumerations
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2347.pdf
Overview
--------
Current C++ enums are not type-safe. This is because an enumeration type is
promoted to an integer by integral promotion.
enum Color { ClrRed, ClrOrange, ClrYellow, ClrGreen, ClrBlue, ClrViolet };
enum Alert { CndGreen, CndYellow, CndRed };
Color c = ClrRed;
Alert a = CndGreen;
bool armWeapons = ( a >= ClrYellow ); // Compiles okay, but not desirable
A new strongly-type enum is therefore created: enum class. There is no implicit
conversion between enum class and int. And the underlying type is always well-specified.
enum class A { A1=1 }; // underlying type is int by default
enum class B: unsigned long { B1=1 }; // underlying type is unsigned long
enum class introduces its own scope. The names of enumerators are in the enum’s
scope, and are not injected into the enclosing scope.
enum class E { E1, E2, E3 = 100, E4 /* = 101 */ };
E e1 = E1; // error
E e2 = E::E2; // ok
Proposed change to DWARF
------------------------
New DWARF attribute
DW_AT_enum_class 0x70 flag A strongly typed enumeration type
5.8 Enumeration Type Entries
If an enumeration type has type-safe semantics such that:
* enumerators are contained in the scope of the enumeration type
* enumerators do not implicitly convert to or from another type
then the enumeration type may have a DW_AT_enum_class attribute
specifying those semantics. The value of this attribute is a flag.
*In C++, a type declared using enum class is a 'strongly typed
enumeration type', and would be represented with this attribute.*
Example
enum class E { E1, E2=100 };
E e1;
11$: DW_TAG_enumeration_type
DW_AT_name("E")
DW_AT_type(reference to base type "int")
DW_AT_enum_class(yes)
12$: DW_TAG_enumerator
DW_AT_name("E1")
DW_AT_const_value(0)
13$: DW_TAG_enumerator
DW_AT_name("E2")
DW_AT_const_value(100)
14$: DW_TAG_variable
DW_AT_name("e1")
DW_AT_type(reference to 11$)
Change History
May 7, 2009.
* Remove DW_TAG_strong_enumueration_type, in favor of DW_AT_enum_class
* Update example to use new attribute.
* Update 5.8 to describe strongly typed enumeration with DW_AT_enum_class
June 3, 2009.
* Reword 5.8 to remove C++0x from normative text
June 8, 2009.
* John Bishop suggests alternate wording for 5.8. (With normative text in front)
July 2, 2009.
* David Gross suggests replacing "enumeration elements" with "enumerators"
* David Gross pointed out correct indentation for "DW_TAG_enumerator".
--
August 11, 2009 -- Accepted, with non-normative text changed to "when there is
only one semantic for enumerations in the language, this attribute is not needed".
