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[PATCH 2/3] Support C2X and C++17 static_assert
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From: |
Paul Eggert |
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Subject: |
[PATCH 2/3] Support C2X and C++17 static_assert |
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Date: |
Thu, 9 May 2019 09:10:33 -0700 |
C2X and C++17 finally added support for a simple, single-argument
‘static_assert’ that implements what the Gnulib ‘verify’ macro was
doing back in 2005. Implement static_assert on older platforms.
The only remaining advantage of ‘verify’ is a shorter name.
* doc/posix-headers/assert.texi (assert.h):
* doc/verify.texi (Compile-time Assertions):
Modernize for C2X and C++17.
* lib/verify.h (_GL_HAVE__STATIC_ASSERT1, _GL_HAVE_STATIC_ASSERT1):
New macros.
(_GL_HAVE__STATIC_ASSERT): Remove.
(_GL_HAVE__STATIC_ASSERT): Rely more heavily on __STDC_VERSION__.
(_GL_VERIFY_TRUE, _GL_VERIFY_TYPE): Remove 2nd arg, the diagnostic
string. All callers changed.
(_GL_VERIFY): Require 3 or more args, of which only the first 2
are used. All callers changed.
(_Static_assert): Allow either 1 or 2 args, and define if
!_GL_HAVE__STATIC_ASSERT1 instead of defining if
!_GL_HAVE__STATIC_ASSERT.
(static_assert): Define if !_GL_HAVE_STATIC_ASSERT1 instead
of defining if !_GL_HAVE_STATIC_ASSERT.
(verify_expr, verify): Don’t bother trying to copy the expression
into the diagnostic, since 1-argument static_assert doesn’t.
(verify): Prefer 1-argument _Static_assert if it works.
* m4/assert_h.m4 (gl_ASSERT_H): Check for 1-argument static_assert.
---
ChangeLog | 28 ++++++++++
doc/posix-headers/assert.texi | 21 +++++---
doc/verify.texi | 15 +++---
lib/verify.h | 97 +++++++++++++++++++----------------
m4/assert_h.m4 | 2 +
5 files changed, 105 insertions(+), 58 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index 12231d97b..760cde3c6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,31 @@
+2019-05-09 Paul Eggert <address@hidden>
+
+ Support C2X and C++17 static_assert
+ C2X and C++17 finally added support for a simple, single-argument
+ ‘static_assert’ that implements what the Gnulib ‘verify’ macro was
+ doing back in 2005. Implement static_assert on older platforms.
+ The only remaining advantage of ‘verify’ is a shorter name.
+ * doc/posix-headers/assert.texi (assert.h):
+ * doc/verify.texi (Compile-time Assertions):
+ Modernize for C2X and C++17.
+ * lib/verify.h (_GL_HAVE__STATIC_ASSERT1, _GL_HAVE_STATIC_ASSERT1):
+ New macros.
+ (_GL_HAVE__STATIC_ASSERT): Remove.
+ (_GL_HAVE__STATIC_ASSERT): Rely more heavily on __STDC_VERSION__.
+ (_GL_VERIFY_TRUE, _GL_VERIFY_TYPE): Remove 2nd arg, the diagnostic
+ string. All callers changed.
+ (_GL_VERIFY): Require 3 or more args, of which only the first 2
+ are used. All callers changed.
+ (_Static_assert): Allow either 1 or 2 args, and define if
+ !_GL_HAVE__STATIC_ASSERT1 instead of defining if
+ !_GL_HAVE__STATIC_ASSERT.
+ (static_assert): Define if !_GL_HAVE_STATIC_ASSERT1 instead
+ of defining if !_GL_HAVE_STATIC_ASSERT.
+ (verify_expr, verify): Don’t bother trying to copy the expression
+ into the diagnostic, since 1-argument static_assert doesn’t.
+ (verify): Prefer 1-argument _Static_assert if it works.
+ * m4/assert_h.m4 (gl_ASSERT_H): Check for 1-argument static_assert.
+
2019-05-08 Paul Eggert <address@hidden>
Fix _GL_HAVE__STATIC_ASSERT typo
diff --git a/doc/posix-headers/assert.texi b/doc/posix-headers/assert.texi
index 785a07aa9..fa99d3b88 100644
--- a/doc/posix-headers/assert.texi
+++ b/doc/posix-headers/assert.texi
@@ -5,29 +5,34 @@ POSIX specification:@*
@url{http://www.opengroup.org/onlinepubs/9699919799/based
Gnulib module: assert-h
-See also the Gnulib module @code{assert}.
+See also the Gnulib modules @code{assert} and @code{verify}.
Portability problems fixed by Gnulib:
@itemize
@item
-The C11 and C++11 @code{static_assert}, and the C11
address@hidden, are not supported by many platforms.
-For example, GCC versions before 4.6.0 do not support @code{_Static_assert},
-and G++ versions through at least 4.6.0 do not support @code{static_assert}.
+On older platforms @code{static_assert} and @code{_Static_assert} do
+not allow the second string-literal argument to be omitted. For
+example, GCC versions before 9.1 do not support the single-argument
address@hidden that was standardized by C2X and C++17.
address@hidden
+Even-older platforms do not support @code{static_assert} or
address@hidden at all. For example, GCC versions before 4.6 do
+not support @code{_Static_assert}, and G++ versions before 4.3 do not
+support @code{static_assert}, which was standardized by C11 and C++11.
@end itemize
Portability problems not fixed by Gnulib:
@itemize
@item
-C11 @code{_Static_assert} and C++11 @code{static_assert}
+C @code{_Static_assert} and C++ @code{static_assert}
are keywords that can be used without including @code{<assert.h>}.
The Gnulib substitutes are macros that require including @code{<assert.h>}.
@item
-The C11 @code{static_assert} and @code{_Static_assert} can also
+The C @code{static_assert} and @code{_Static_assert} can also
be used within a @code{struct} or @code{union} specifier, in place of
an ordinary declaration of a member of the struct or union. The
Gnulib substitute can be used only as an ordinary declaration.
@item
-In C99, @code{assert} can be applied to any scalar expression.
+In C99 and later, @code{assert} can be applied to any scalar expression.
In C89, the argument to @code{assert} is of type @code{int}.
@end itemize
diff --git a/doc/verify.texi b/doc/verify.texi
index 3a92f2e05..65aede210 100644
--- a/doc/verify.texi
+++ b/doc/verify.texi
@@ -52,15 +52,18 @@ integer constant expression, then a compiler might reject a
usage like
@samp{verify (@var{V});} even when @var{V} is
nonzero.
-Although the standard @code{assert} macro is a runtime test, C11
-specifies a builtin @code{_Static_assert (@var{V},
address@hidden)}, its @file{assert.h} header has a similar macro
-named @code{static_assert}, and C++11 has a similar
+Although the standard @code{assert} macro is a runtime test, C2X
+specifies a builtin @code{_Static_assert (@var{V})},
+its @file{assert.h} header has a similar macro
+named @code{static_assert}, and C++17 has a similar
@code{static_assert} builtin. These builtins and macros differ
from @code{verify} in two major ways. First, they can also be used
within a @code{struct} or @code{union} specifier, in place of an
-ordinary member declaration. Second, they require the programmer to
-specify a compile-time diagnostic as a string literal.
+ordinary member declaration. Second, they allow the programmer to
+specify, as an optional second argument, a compile-time diagnostic as
+a string literal. If your program is not intended to be portable to
+compilers that lack C2X or C++17 @code{static_assert}, the only
+advantage of @code{verify} is that its name is a bit shorter.
The @file{verify.h} header defines one more macro, @code{assume
(@var{E})}, which expands to an expression of type @code{void}
diff --git a/lib/verify.h b/lib/verify.h
index eccd7e201..d7e15bc8a 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -21,23 +21,31 @@
#define _GL_VERIFY_H
-/* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per C11.
- This is supported by GCC 4.6.0 and later, in C mode, and its use
- here generates easier-to-read diagnostics when verify (R) fails.
-
- Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11.
- This is supported by GCC 6.1.0 and later, in C++ mode.
-
- Use this only with GCC. If we were willing to slow 'configure'
- down we could also use it with other compilers, but since this
- affects only the quality of diagnostics, why bother? */
-#if (4 < __GNUC__ + (6 <= __GNUC_MINOR__) \
- && (201112L <= __STDC_VERSION__ || !defined __STRICT_ANSI__) \
- && !defined __cplusplus)
-# define _GL_HAVE__STATIC_ASSERT 1
-#endif
-#if 6 <= __GNUC__ && defined __cplusplus
-# define _GL_HAVE_STATIC_ASSERT 1
+/* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert (R, DIAGNOSTIC)
+ works as per C11. This is supported by GCC 4.6.0 and later, in C
+ mode.
+
+ Define _GL_HAVE__STATIC_ASSERT1 to 1 if _Static_assert (R) works as
+ per C2X, and define _GL_HAVE_STATIC_ASSERT1 if static_assert (R)
+ works as per C++17. This is supported by GCC 9.1 and later.
+
+ Support compilers claiming conformance to the relevant standard,
+ and also support GCC when not pedantic. If we were willing to slow
+ 'configure' down we could also use it with other compilers, but
+ since this affects only the quality of diagnostics, why bother? */
+#ifndef __cplusplus
+# if (201112L <= __STDC_VERSION__ \
+ || (!defined __STRICT_ANSI__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)))
+# define _GL_HAVE__STATIC_ASSERT 1
+# endif
+# if (202000L <= __STDC_VERSION__ \
+ || (!defined __STRICT_ANSI__ && 9 <= __GNUC__))
+# define _GL_HAVE__STATIC_ASSERT1 1
+# endif
+#else
+# if 201703L <= __cplusplus || 9 <= __GNUC__
+# define _GL_HAVE_STATIC_ASSERT1 1
+# endif
#endif
/* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
@@ -167,11 +175,9 @@
#define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
/* Verify requirement R at compile-time, as an integer constant expression
- that returns 1. If R is false, fail at compile-time, preferably
- with a diagnostic that includes the string-literal DIAGNOSTIC. */
+ that returns 1. If R is false, fail at compile-time. */
-#define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
- (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
+#define _GL_VERIFY_TRUE(R) (!!sizeof (_GL_VERIFY_TYPE (R)))
#ifdef __cplusplus
# if !GNULIB_defined_struct__gl_verify_type
@@ -181,40 +187,43 @@ template <int w>
};
# define GNULIB_defined_struct__gl_verify_type 1
# endif
-# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
- _gl_verify_type<(R) ? 1 : -1>
-#elif defined _GL_HAVE__STATIC_ASSERT
-# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
+# define _GL_VERIFY_TYPE(R) _gl_verify_type<(R) ? 1 : -1>
+#elif defined _GL_HAVE__STATIC_ASSERT1
+# define _GL_VERIFY_TYPE(R) \
struct { \
- _Static_assert (R, DIAGNOSTIC); \
+ _Static_assert (R); \
int _gl_dummy; \
}
#else
-# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
+# define _GL_VERIFY_TYPE(R) \
struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
#endif
/* Verify requirement R at compile-time, as a declaration without a
- trailing ';'. If R is false, fail at compile-time, preferably
- with a diagnostic that includes the string-literal DIAGNOSTIC.
+ trailing ';'. If R is false, fail at compile-time.
+
+ This macro requires three or more arguments but uses at most the first
+ two, so that the _Static_assert macro optionally defined below supports
+ both the C11 two-argument syntax and the C2X one-argument syntax.
Unfortunately, unlike C11, this implementation must appear as an
ordinary declaration, and cannot appear inside struct { ... }. */
-#ifdef _GL_HAVE__STATIC_ASSERT
-# define _GL_VERIFY _Static_assert
+#if defined _GL_HAVE__STATIC_ASSERT
+# define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
#else
-# define _GL_VERIFY(R, DIAGNOSTIC) \
+# define _GL_VERIFY(R, DIAGNOSTIC, ...) \
extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
- [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
+ [_GL_VERIFY_TRUE (R)]
#endif
/* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */
#ifdef _GL_STATIC_ASSERT_H
-# if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
-# define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
+# if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
+# define _Static_assert(...) \
+ _GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
# endif
-# if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
+# if !defined _GL_HAVE_STATIC_ASSERT1 && !defined static_assert
# define static_assert _Static_assert /* C11 requires this #define. */
# endif
#endif
@@ -235,22 +244,22 @@ template <int w>
verify_true is obsolescent; please use verify_expr instead. */
-#define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
+#define verify_true(R) _GL_VERIFY_TRUE (R)
/* Verify requirement R at compile-time. Return the value of the
expression E. */
-#define verify_expr(R, E) \
- (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
+#define verify_expr(R, E) (_GL_VERIFY_TRUE (R) ? (E) : (E))
/* Verify requirement R at compile-time, as a declaration without a
- trailing ';'. */
+ trailing ';'. verify (R) acts like static_assert (R) except that
+ it is portable to C11/C++14 and earlier, and its name is shorter
+ and may be more convenient. */
-#ifdef __GNUC__
-# define verify(R) _GL_VERIFY (R, "verify (" #R ")")
+#ifdef _GL_HAVE__STATIC_ASSERT1
+# define verify(R) _Static_assert (R)
#else
-/* PGI barfs if R is long. Play it safe. */
-# define verify(R) _GL_VERIFY (R, "verify (...)")
+# define verify(R) _GL_VERIFY (R, "verify (...)", -)
#endif
#ifndef __has_builtin
diff --git a/m4/assert_h.m4 b/m4/assert_h.m4
index cf11bae6a..ee278b206 100644
--- a/m4/assert_h.m4
+++ b/m4/assert_h.m4
@@ -14,9 +14,11 @@ AC_DEFUN([gl_ASSERT_H],
[AC_LANG_PROGRAM(
[[#include <assert.h>
static_assert (2 + 2 == 4, "arithmetic doesn't work");
+ static_assert (2 + 2 == 4);
]],
[[
static_assert (sizeof (char) == 1, "sizeof doesn't work");
+ static_assert (sizeof (char) == 1);
]])],
[gl_cv_static_assert=yes],
[gl_cv_static_assert=no])])
--
2.21.0