gdb: pass/return setting setter/getter scalar values by value

The getter and setter in struct setting always receive and return values
by const reference.  This is not necessary for scalar values (like bool
and int), but more importantly it makes it a bit annoying to write a
getter, you have to use a scratch static variable or something similar
that you can refer to:

  const bool &
  my_getter ()
  {
    static bool value;
    value = function_returning_bool ();
    return value;
  }

Change the getter and setter function signatures to receive and return
value by value instead of by reference, when the underlying data type is
scalar.  This means that string-based settings will still use
references, but all others will be by value.  The getter above would
then be re-written as:

  bool
  my_getter ()
  {
    return function_returning_bool ();
  }

This is useful for a patch later in this series that defines a boolean
setting with a getter and a setter.

Change-Id: Ieca3a2419fcdb75a6f75948b2c920b548a0af0fd

1 files changed, 50 insertions, 34 deletions

diff --git a/gdb/command.h b/gdb/command.h
index d6e94b97eb1..bcd3882f811 100644
--- a/gdb/command.h
+++ b/gdb/command.h
@@ -180,15 +180,31 @@ inline bool var_type_uses<const char *> (var_types t)
   return t == var_enum;
 }
 
-/* Function signature for a callback used to get a value from a setting.  */
+template<bool is_scalar, typename T> struct setting_func_types_1;
 
 template<typename T>
-using setting_getter_ftype = const T &(*) ();
+struct setting_func_types_1<true, T>
+{
+  using type = T;
+  using set = void (*) (type);
+  using get = type (*) ();
+};
 
-/* Function signature for a callback used to set a value to a setting.  */
+template<typename T>
+struct setting_func_types_1<false, T>
+{
+  using type = const T &;
+  using set = void (*) (type);
+  using get = type (*) ();
+};
 
 template<typename T>
-using setting_setter_ftype = void (*) (const T &);
+struct setting_func_types
+{
+  using type = typename setting_func_types_1<std::is_scalar<T>::value, T>::type;
+  using set = typename setting_func_types_1<std::is_scalar<T>::value, T>::set;
+  using get = typename setting_func_types_1<std::is_scalar<T>::value, T>::get;
+};
 
 /* Generic/type-erased function pointer.  */
 
@@ -224,8 +240,8 @@ struct setting
   template<typename T>
   static erased_args erase_args (var_types var_type,
 				 T *var,
-				 setting_setter_ftype<T> set_setting_func,
-				 setting_getter_ftype<T> get_setting_func)
+				 typename setting_func_types<T>::set set_setting_func,
+				 typename setting_func_types<T>::get get_setting_func)
   {
     gdb_assert (var_type_uses<T> (var_type));
   /* The getter and the setter must be both provided or both omitted.  */
@@ -259,8 +275,8 @@ struct setting
      Type T must match the var type VAR_TYPE (see VAR_TYPE_USES).  */
   template<typename T>
   setting (var_types var_type,
-	   setting_setter_ftype<T> setter,
-	   setting_getter_ftype<T> getter)
+	   typename setting_func_types<T>::set setter,
+	   typename setting_func_types<T>::get getter)
     : m_var_type (var_type)
   {
     gdb_assert (var_type_uses<T> (var_type));
@@ -284,14 +300,14 @@ struct setting
      The template parameter T is the type of the variable used to store the
      setting.  */
   template<typename T>
-  const T &get () const
+  typename setting_func_types<T>::type get () const
   {
     gdb_assert (var_type_uses<T> (m_var_type));
 
     if (m_var == nullptr)
       {
 	gdb_assert (m_getter != nullptr);
-	auto getter = reinterpret_cast<setting_getter_ftype<T>> (m_getter);
+	auto getter = reinterpret_cast<typename setting_func_types<T>::get> (m_getter);
 	return getter ();
       }
     else
@@ -321,7 +337,7 @@ struct setting
     if (m_var == nullptr)
       {
 	gdb_assert (m_setter != nullptr);
-	auto setter = reinterpret_cast<setting_setter_ftype<T>> (m_setter);
+	auto setter = reinterpret_cast<typename setting_func_types<T>::set> (m_setter);
 	setter (v);
       }
     else
@@ -643,8 +659,8 @@ extern set_show_commands add_setshow_enum_cmd
 extern set_show_commands add_setshow_enum_cmd
   (const char *name, command_class theclass, const char *const *enumlist,
    const char *set_doc, const char *show_doc,
-   const char *help_doc, setting_setter_ftype<const char *> set_func,
-   setting_getter_ftype<const char *> get_func, show_value_ftype *show_func,
+   const char *help_doc, setting_func_types<const char *>::set set_func,
+   setting_func_types<const char *>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_auto_boolean_cmd
@@ -656,8 +672,8 @@ extern set_show_commands add_setshow_auto_boolean_cmd
 extern set_show_commands add_setshow_auto_boolean_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<enum auto_boolean> set_func,
-   setting_getter_ftype<enum auto_boolean> get_func,
+   setting_func_types<enum auto_boolean>::set set_func,
+   setting_func_types<enum auto_boolean>::get get_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
@@ -670,8 +686,8 @@ extern set_show_commands add_setshow_boolean_cmd
 extern set_show_commands add_setshow_boolean_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<bool> set_func,
-   setting_getter_ftype<bool> get_func, show_value_ftype *show_func,
+   setting_func_types<bool>::set set_func,
+   setting_func_types<bool>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_filename_cmd
@@ -683,8 +699,8 @@ extern set_show_commands add_setshow_filename_cmd
 extern set_show_commands add_setshow_filename_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<std::string> set_func,
-   setting_getter_ftype<std::string> get_func, show_value_ftype *show_func,
+   setting_func_types<std::string>::set set_func,
+   setting_func_types<std::string>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_string_cmd
@@ -696,8 +712,8 @@ extern set_show_commands add_setshow_string_cmd
 extern set_show_commands add_setshow_string_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<std::string> set_func,
-   setting_getter_ftype<std::string> get_func,
+   setting_func_types<std::string>::set set_func,
+   setting_func_types<std::string>::get get_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
@@ -710,8 +726,8 @@ extern set_show_commands add_setshow_string_noescape_cmd
 extern set_show_commands add_setshow_string_noescape_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<std::string> set_func,
-   setting_getter_ftype<std::string> get_func, show_value_ftype *show_func,
+   setting_func_types<std::string>::set set_func,
+   setting_func_types<std::string>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_optional_filename_cmd
@@ -723,8 +739,8 @@ extern set_show_commands add_setshow_optional_filename_cmd
 extern set_show_commands add_setshow_optional_filename_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<std::string> set_func,
-   setting_getter_ftype<std::string> get_func,
+   setting_func_types<std::string>::set set_func,
+   setting_func_types<std::string>::get get_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);
 
@@ -737,8 +753,8 @@ extern set_show_commands add_setshow_integer_cmd
 extern set_show_commands add_setshow_integer_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<int> set_func,
-   setting_getter_ftype<int> get_func, show_value_ftype *show_func,
+   setting_func_types<int>::set set_func,
+   setting_func_types<int>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_uinteger_cmd
@@ -750,8 +766,8 @@ extern set_show_commands add_setshow_uinteger_cmd
 extern set_show_commands add_setshow_uinteger_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<unsigned int> set_func,
-   setting_getter_ftype<unsigned int> get_func, show_value_ftype *show_func,
+   setting_func_types<unsigned int>::set set_func,
+   setting_func_types<unsigned int>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_zinteger_cmd
@@ -763,8 +779,8 @@ extern set_show_commands add_setshow_zinteger_cmd
 extern set_show_commands add_setshow_zinteger_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<int> set_func,
-   setting_getter_ftype<int> get_func, show_value_ftype *show_func,
+   setting_func_types<int>::set set_func,
+   setting_func_types<int>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_zuinteger_cmd
@@ -776,8 +792,8 @@ extern set_show_commands add_setshow_zuinteger_cmd
 extern set_show_commands add_setshow_zuinteger_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<unsigned int> set_func,
-   setting_getter_ftype<unsigned int> get_func, show_value_ftype *show_func,
+   setting_func_types<unsigned int>::set set_func,
+   setting_func_types<unsigned int>::get get_func, show_value_ftype *show_func,
    cmd_list_element **set_list, cmd_list_element **show_list);
 
 extern set_show_commands add_setshow_zuinteger_unlimited_cmd
@@ -789,7 +805,7 @@ extern set_show_commands add_setshow_zuinteger_unlimited_cmd
 extern set_show_commands add_setshow_zuinteger_unlimited_cmd
   (const char *name, command_class theclass, const char *set_doc,
    const char *show_doc, const char *help_doc,
-   setting_setter_ftype<int> set_func, setting_getter_ftype<int> get_func,
+   setting_func_types<int>::set set_func, setting_func_types<int>::get get_func,
    show_value_ftype *show_func, cmd_list_element **set_list,
    cmd_list_element **show_list);