Re: [PATCH] Type-safe wrapper for enum flags

[Simon Marchi <simon dot marchi at ericsson dot com>]

On 15-10-29 02:45 PM, Pedro Alves wrote:
> This patch fixes C++ build errors like this:
> 
> /home/pedro/gdb/mygit/cxx-convertion/src/gdb/linux-tdep.c:1126:35: error: invalid conversion from âintâ to âfilterflagsâ [-fpermissive]
>        | COREFILTER_HUGETLB_PRIVATE);
>                                    ^
> 
> This is a case of enums used as bit flags.  Unlike "regular" enums,
> these values are supposed to be or'ed together.  However, in C++, the
> type of "(ENUM1 | ENUM2)" is int, and you then can't assign an int to
> an enum variable without a cast.  That means that this:
> 
>   enum foo_flags flags = 0;
> 
>   if (...)
>     flags |= FOO_FLAG1;
>   if (...)
>     flags |= FOO_FLAG2;
> 
> ... would have to be written as:
> 
>   enum foo_flags flags = (enum foo_flags) 0;
> 
>   if (...)
>     flags = (enum foo_flags) (flags | FOO_FLAG1);
>   if (...)
>     flags = (enum foo_flags) (flags | FOO_FLAG2);
> 
> which is ... ugly.  Alternatively, we'd have to use an int for the
> variable's type, which isn't ideal either.
> 
> This patch instead adds an "enum flags" class.  "enum flags" are
> exactly the enums where the values are bits that are meant to be ORed
> together.
> 
> This allows writing code like the below, while with raw enums this
> would fail to compile without casts to enum type at the assignments to
> 'f':
> 
>   enum some_flag
>   {
>      flag_val1 = 1 << 1,
>      flag_val2 = 1 << 2,
>      flag_val3 = 1 << 3,
>      flag_val4 = 1 << 4,
>   };
>   DEF_ENUM_FLAGS_TYPE(enum some_flag, some_flags)
> 
>   some_flags f = flag_val1 | flag_val2;
>   f |= flag_val3;
> 
> It's also possible to assign literal zero to an enum flags variable
> (meaning, no flags), dispensing either adding an awkward explicit "no
> value" value to the enumeration or the cast to assignments from 0.
> For example:
> 
>   some_flags f = 0;
>   f |= flag_val3 | flag_val4;
> 
> Note that literal integers other than zero do fail to compile:
> 
>   some_flags f = 1; // error
> 
> C is still supported -- DEF_ENUM_FLAGS_TYPE is just a typedef in that
> case.

Could we also make c_string_type an enum flags type?  Like so?  There are
some ugly casts remaining in the branch, like

  str_type = (enum c_string_type) (str_type & ~C_CHAR);


From 2b1ee0d61aafa948243e809614522353b9e88e3c Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@polymtl.ca>
Date: Thu, 29 Oct 2015 21:54:35 -0400
Subject: [PATCH] Convert c_string_type to an enum flags type

c_string_type contains values meant to be OR'ed together (even though
some bits are mutually exclusive), so it makes sense to make it an
enum flags type.

gdb/ChangeLog:

	* c-exp.y (exp): Adjust, change enum c_string_type to
	c_string_type.
	(parse_string_or_char): Likewise.
	* c-lang.c (charset_for_string_type): Likewise.
	(classify_type): Likewise.
	(c_printchar): Likewise.
	(c_printstr): Likewise.
	(evaluate_subexp_c): Likewise.  And change cast to enum
	c_string_type_values.
	* c-lang.h (enum c_string_type): Rename to...
	(enum c_string_type_values): ...this.
	(c_string_type): Define new enum flags type.
---
 gdb/c-exp.y  |  6 +++---
 gdb/c-lang.c | 17 ++++++++---------
 gdb/c-lang.h |  4 +++-
 3 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/gdb/c-exp.y b/gdb/c-exp.y
index f2de0ae..9127756 100644
--- a/gdb/c-exp.y
+++ b/gdb/c-exp.y
@@ -864,7 +864,7 @@ string_exp:
 exp	:	string_exp
 			{
 			  int i;
-			  enum c_string_type type = C_STRING;
+			  c_string_type type = C_STRING;

 			  for (i = 0; i < $1.len; ++i)
 			    {
@@ -878,7 +878,7 @@ exp	:	string_exp
 				  if (type != C_STRING
 				      && type != $1.tokens[i].type)
 				    error (_("Undefined string concatenation."));
-				  type = (enum c_string_type) $1.tokens[i].type;
+				  type = (enum c_string_type_values) $1.tokens[i].type;
 				  break;
 				default:
 				  /* internal error */
@@ -2164,7 +2164,7 @@ parse_string_or_char (const char *tokptr, const char **outptr,
 		      struct typed_stoken *value, int *host_chars)
 {
   int quote;
-  enum c_string_type type;
+  c_string_type type;
   int is_objc = 0;

   /* Build the gdb internal form of the input string in tempbuf.  Note
diff --git a/gdb/c-lang.c b/gdb/c-lang.c
index 384783c..2646b05 100644
--- a/gdb/c-lang.c
+++ b/gdb/c-lang.c
@@ -41,8 +41,7 @@ extern void _initialize_c_language (void);
    character set name.  */

 static const char *
-charset_for_string_type (enum c_string_type str_type,
-			 struct gdbarch *gdbarch)
+charset_for_string_type (c_string_type str_type, struct gdbarch *gdbarch)
 {
   switch (str_type & ~C_CHAR)
     {
@@ -72,11 +71,11 @@ charset_for_string_type (enum c_string_type str_type,
    characters of this type in target BYTE_ORDER to the host character
    set.  */

-static enum c_string_type
+static c_string_type
 classify_type (struct type *elttype, struct gdbarch *gdbarch,
 	       const char **encoding)
 {
-  enum c_string_type result;
+  c_string_type result;

   /* We loop because ELTTYPE may be a typedef, and we want to
      successively peel each typedef until we reach a type we
@@ -155,7 +154,7 @@ c_emit_char (int c, struct type *type,
 void
 c_printchar (int c, struct type *type, struct ui_file *stream)
 {
-  enum c_string_type str_type;
+  c_string_type str_type;

   str_type = classify_type (type, get_type_arch (type), NULL);
   switch (str_type)
@@ -191,7 +190,7 @@ c_printstr (struct ui_file *stream, struct type *type,
 	    const char *user_encoding, int force_ellipses,
 	    const struct value_print_options *options)
 {
-  enum c_string_type str_type;
+  c_string_type str_type;
   const char *type_encoding;
   const char *encoding;

@@ -577,7 +576,7 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,
 	struct obstack output;
 	struct cleanup *cleanup;
 	struct value *result;
-	enum c_string_type dest_type;
+	c_string_type dest_type;
 	const char *dest_charset;
 	int satisfy_expected = 0;

@@ -589,8 +588,8 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,

 	++*pos;
 	limit = *pos + BYTES_TO_EXP_ELEM (oplen + 1);
-	dest_type
-	  = (enum c_string_type) longest_to_int (exp->elts[*pos].longconst);
+	dest_type = ((enum c_string_type_values)
+		     longest_to_int (exp->elts[*pos].longconst));
 	switch (dest_type & ~C_CHAR)
 	  {
 	  case C_STRING:
diff --git a/gdb/c-lang.h b/gdb/c-lang.h
index 5e30481..b1823ce 100644
--- a/gdb/c-lang.h
+++ b/gdb/c-lang.h
@@ -34,7 +34,7 @@ struct parser_state;
 /* The various kinds of C string and character.  Note that these
    values are chosen so that they may be or'd together in certain
    ways.  */
-enum c_string_type
+enum c_string_type_values
   {
     /* An ordinary string: "value".  */
     C_STRING = 0,
@@ -56,6 +56,8 @@ enum c_string_type
     C_CHAR_32 = 7
   };

+DEF_ENUM_FLAGS_TYPE (enum c_string_type_values, c_string_type);
+
 /* Defined in c-exp.y.  */

 extern int c_parse (struct parser_state *);
-- 
2.5.1