common: add cmd_getval_cast_or()

This slight variation of cmd_getval_or() can be used where
the object type is different from the configuration item
type (as when the object is a wrapper around an integer).
It allows specifying the 'default' value in the object type.

Signed-off-by: Ronen Friedman <[email protected]>

Author: ronen-fr

src/common/cmdparse.h

@@ -102,11 +102,34 @@ T cmd_getval_or(const cmdmap_t& cmdmap, std::string_view k,
 		const V& defval)
 {
   auto found = cmdmap.find(k);
-  if (found == cmdmap.end()) {
+  if (found == cmdmap.cend()) {
     return T(defval);
   }
   try {
-    return boost::get<T>(cmdmap.find(k)->second);
+    return boost::get<T>(found->second);
+  } catch (boost::bad_get&) {
+    throw bad_cmd_get(k, cmdmap);
+  }
+}
+
+/**
+ * with default, to be used when the parameter type (which matches the type of the
+ * 'default' object) is not one of the limited set of parameters type.
+ * Added to support "safe types" - typed wrappers "around" simple types (e.g.
+ * shard_id_t which is a structure holding one int8_t), without requiring the
+ * user to specify the default in the 'internal type'.
+ *
+ * Throws if the key is found, but the type is not the expected one.
+ */
+template <typename CMD_TYPE, typename T>
+T cmd_getval_cast_or(const cmdmap_t& cmdmap, std::string_view k, T defval)
+{
+  auto found = cmdmap.find(k);
+  if (found == cmdmap.cend()) {
+    return defval;
+  }
+  try {
+    return T(boost::get<CMD_TYPE>(found->second));
   } catch (boost::bad_get&) {
     throw bad_cmd_get(k, cmdmap);
   }