https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81327
Bug ID: 81327
Summary: cast to void* does not suppress -Wclass-memaccess
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
In cases where the -Wclass-memaccess warning (new in GCC 8) points out benign
code the warning can be suppressed by casting a pointer to the class object to
char*. In Clang that implements a similar warning
(-Wdynamic-class-memaccess), the warning can also be suppressed by casting the
pointer to void* but the same doesn't work in GCC. It's been suggested that it
might be nice if GCC also recognized a void* cast as a suppression mechanism.
(Unfortunately, where the warning is implemented in the GCC C++ front end an
explicit void* cast is indistinguishable from the implicit conversion to the
void* argument to the raw memory function.)
$ cat t.C && gcc -S -Wall t.C
struct S { virtual ~S (); };
void f (struct S *s)
{
__builtin_memset (s, 0, sizeof *s);
}
void g (struct S *s)
{
__builtin_memset ((void*)s, 0, sizeof *s);
}
void h (struct S *s)
{
__builtin_memset ((char*)s, 0, sizeof *s);
}
t.C: In function ‘void f(S*)’:
t.C:5:36: warning: ‘void* __builtin_memset(void*, int, long unsigned int)’
clearing an object of type ‘struct S’ with no trivial copy-assignment; use
assignment or value-initialization instead [-Wclass-memaccess]
__builtin_memset (s, 0, sizeof *s);
^
t.C:1:8: note: ‘struct S’ declared here
struct S { virtual ~S (); };
^
t.C: In function ‘void g(S*)’:
t.C:10:43: warning: ‘void* __builtin_memset(void*, int, long unsigned int)’
clearing an object of type ‘struct S’ with no trivial copy-assignment; use
assignment or value-initialization instead [-Wclass-memaccess]
__builtin_memset ((void*)s, 0, sizeof *s);
^
t.C:1:8: note: ‘struct S’ declared here
struct S { virtual ~S (); };
^
Clang output for the same test case:
$ clang -S -Wall t.C
t.C:5:21: warning: destination for this '__builtin_memset' call is a pointer to
dynamic class 'S'; vtable pointer will be overwritten
[-Wdynamic-class-memaccess]
__builtin_memset (s, 0, sizeof *s);
~~~~~~~~~~~~~~~~ ^
t.C:5:21: note: explicitly cast the pointer to silence this warning
__builtin_memset (s, 0, sizeof *s);
^
(void*)
1 warning generated.
