c++: do not exhibit private macros

[Akim Demaille]

We should be clear in the doc: YY_ macros are private, do not
depend on them.

commit 602f8e2a77e008bbf4a90213b6abfbb52265ff48
Author: Akim Demaille <address@hidden>
Date:   Thu Oct 18 17:37:18 2018 +0200

    c++: do not exhibit private macros
    
    * examples/variant-11.yy: here.

diff --git a/examples/variant-11.yy b/examples/variant-11.yy
index 660b2009..ba01c6c4 100644
--- a/examples/variant-11.yy
+++ b/examples/variant-11.yy
@@ -125,7 +125,7 @@ namespace yy
   yylex ()
   {
     static auto count = 0u;
-    auto stage = count;
+    const auto stage = count;
     ++count;
     auto loc = parser::location_type{nullptr, stage + 1, stage + 1};
     if (stage == 0)
@@ -150,7 +150,7 @@ int
 main (int argc, const char *argv[])
 {
   if (2 <= argc && isdigit (*argv[1]))
-    max = strtol (argv[1], YY_NULLPTR, 10);
+    max = strtol (argv[1], nullptr, 10);
   auto&& p = yy::parser{};
   p.set_debug_level (!!getenv ("YYDEBUG"));
   return p.parse ();
diff --git a/examples/variant.yy b/examples/variant.yy
index b440fa6b..2938c50c 100644
--- a/examples/variant.yy
+++ b/examples/variant.yy
@@ -97,6 +97,11 @@ item:
 
 namespace yy
 {
+  // Use nullptr with pre-C++11.
+#if defined __cplusplus && __cplusplus < 201103L
+# define nullptr 0
+#endif
+
   // The yylex function providing subsequent tokens:
   // TEXT         "I have three numbers for you."
   // NUMBER       1
@@ -111,7 +116,7 @@ namespace yy
   {
     static int stage = -1;
     ++stage;
-    parser::location_type loc (YY_NULLPTR, stage + 1, stage + 1);
+    parser::location_type loc (nullptr, stage + 1, stage + 1);
     switch (stage)
       {
       case 0: