Re: [Boost-users] Ugly warnings from BOOST_STATIC_ASSERT

From: John Maddock (john_at_[hidden])
Date: 2008-03-18 13:31:55

Jean-Francois Bastien wrote:
>> We have this really neat and simple macro in our project:
>> #define compile_assert(x) typedef bool
>> COMPILE_ASSERT[(x) ? 1 : -1]
>>
>> Although it does what we need and serves us well, I
>> tried to replace
>> it with BOOST_STATIC_ASSERT. Unfortunately this resulted
>> in a lot of
>> ugly warnings throughout the compile:
>>   warning: use of old-style cast
>>
>
> There's the following comment in the static assert header:
>
> // Note that the argument to the assert is explicitly cast to bool
> using old-
> // style casts: too many compilers currently have problems with
> static_cast
> // when used inside integral constant expressions.
>
> The solution would be to add even more defines to use old style or
> static casts depending on which compiler is used.

Right: historically this is why the header doesn't use static_cast here.

Unfortunately GCC appears to be one of the compilers that chokes if you use 
static_cast in an integral constant expression: testing the diff below with 
gcc-3.4 (cygwin) results in the tests failing (where as enabling the same 
static_cast useage with msvc or intel does work OK).

If you anyone can test this with gcc-4.x and let me know which versions 
might accept this patch OK that would be very useful, I'm attaching the 
updated static_assert.hpp as well as the diff.

John.

Here's the diff:

Index: C:/data/boost/boost/trunk/boost/static_assert.hpp
===================================================================
--- C:/data/boost/boost/trunk/boost/static_assert.hpp (revision 43667)
+++ C:/data/boost/boost/trunk/boost/static_assert.hpp (working copy)
@@ -28,6 +28,17 @@
 #  define BOOST_SA_GCC_WORKAROUND
 #endif

+//
+// If the compiler supports the use of static_cast inside an integral
+// constant expression, and it issues warnings about old C style casts,
+// then enable this:
+//
+#if defined(__GNUC__) && ((__GNUC__ > 3) || ((__GNUC__ == 3) && 
(__GNUC_MINOR__ >= 4)))
+#define BOOST_STATIC_ASSERT_BOOL_CAST static_cast<bool>
+#else
+#define BOOST_STATIC_ASSERT_BOOL_CAST (bool)
+#endif
+
 #ifdef BOOST_HAS_STATIC_ASSERT
 #  define BOOST_STATIC_ASSERT( B ) static_assert(B, #B)
 #else
@@ -78,14 +89,14 @@
 #elif defined(BOOST_MSVC)
 #define BOOST_STATIC_ASSERT( B ) \
    typedef ::boost::static_assert_test<\
-      sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >)>\
+      sizeof(::boost::STATIC_ASSERTION_FAILURE< 
BOOST_STATIC_ASSERT_BOOL_CAST ( B ) >)>\
          BOOST_JOIN(boost_static_assert_typedef_, __COUNTER__)
 #elif defined(BOOST_INTEL_CXX_VERSION) || defined(BOOST_SA_GCC_WORKAROUND)
 // agurt 15/sep/02: a special care is needed to force Intel C++ issue an 
error
 // instead of warning in case of failure
 # define BOOST_STATIC_ASSERT( B ) \
     typedef char BOOST_JOIN(boost_static_assert_typedef_, __LINE__) \
-        [ ::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >::value ]
+        [ ::boost::STATIC_ASSERTION_FAILURE< 
BOOST_STATIC_ASSERT_BOOL_CAST( B ) >::value ]
 #elif defined(__sgi)
 // special version for SGI MIPSpro compiler
 #define BOOST_STATIC_ASSERT( B ) \
@@ -100,12 +111,12 @@
 #define BOOST_STATIC_ASSERT( B ) \
    BOOST_STATIC_CONSTANT(int, \
      BOOST_JOIN(boost_static_assert_test_, __LINE__) = \
-       sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >) )
+       sizeof(::boost::STATIC_ASSERTION_FAILURE< 
BOOST_STATIC_ASSERT_BOOL_CAST( B ) >) )
 #else
 // generic version
 #define BOOST_STATIC_ASSERT( B ) \
    typedef ::boost::static_assert_test<\
-      sizeof(::boost::STATIC_ASSERTION_FAILURE< (bool)( B ) >)>\
+      sizeof(::boost::STATIC_ASSERTION_FAILURE< 
BOOST_STATIC_ASSERT_BOOL_CAST( B ) >)>\
          BOOST_JOIN(boost_static_assert_typedef_, __LINE__)
 #endif