lib: add macro that performs explicit static casts when using a C++ compiler

C++ doesn't support implicit casts from void pointers like C
does. And the libfrr headers have some bits of code that rely on
implicit casts in order to work. To solve this problem, add a new
"static_cast" macro that performs explicit static casts when a C++
compiler is being used, or do nothing otherwise.

NOTE: since macros are only evaluated when they are used, there
might be other macros from libfrr that will need to use "static_cast"
as well. If a header is successfully compiled using a C++ compiler,
there's no guarantee that its macros are compatible with C++. We'll
only know about such macros when they are used by C++ code, then
we'll need to adapt them one by one in the future.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/lib/zebra.h b/lib/zebra.h
index 43ab4309c2..b96fb5a206 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -232,6 +232,15 @@ typedef unsigned char uint8_t;
 
 #include "zassert.h"
 
+/*
+ * Add explicit static cast only when using a C++ compiler.
+ */
+#ifdef __cplusplus
+#define static_cast(l, r) static_cast<decltype(l)>((r))
+#else
+#define static_cast(l, r) (r)
+#endif
+
 #ifndef HAVE_STRLCAT
 size_t strlcat(char *__restrict dest,
 	       const char *__restrict src, size_t destsize);