[glom] Added Utils::const_list() and use it.

[Murray Cumming <murrayc src gnome org>]

commit 6f9315de11ad9e276b093c3e80de7632653254d2
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Mar 18 14:56:46 2016 +0100

    Added Utils::const_list() and use it.
    
    To make it clearer where this is necessary at the moment,
    to make it easier to fix it later.

 glom/libglom/algorithms_utils.h                   |   13 +++++++++++++
 glom/libglom/data_structure/layout/layoutgroup.cc |    4 +---
 glom/libglom/sql_utils.cc                         |    6 ++----
 3 files changed, 16 insertions(+), 7 deletions(-)
---
diff --git a/glom/libglom/algorithms_utils.h b/glom/libglom/algorithms_utils.h
index 0a79dbc..87ac0b5 100644
--- a/glom/libglom/algorithms_utils.h
+++ b/glom/libglom/algorithms_utils.h
@@ -22,6 +22,9 @@
 #define GLOM_ALGORITHMS_UTILS_H
 
 #include <algorithm>
+#include <vector>
+#include <memory>
+
 
 namespace Glom
 {
@@ -92,6 +95,16 @@ copy(T_container_input input, T_container_output& output)
     std::back_inserter(output));
 }
 
+template<typename T_element>
+decltype(auto)
+const_list(const std::vector<std::shared_ptr<T_element>>& input)
+{
+  std::vector<std::shared_ptr<const T_element>> result; //This should use RVO.
+  Utils::copy(input, result);
+  return result;
+}
+
+
 } //namespace Utils
 
 } //namespace Glom
diff --git a/glom/libglom/data_structure/layout/layoutgroup.cc 
b/glom/libglom/data_structure/layout/layoutgroup.cc
index 117e029..183d1e7 100644
--- a/glom/libglom/data_structure/layout/layoutgroup.cc
+++ b/glom/libglom/data_structure/layout/layoutgroup.cc
@@ -163,9 +163,7 @@ LayoutGroup::type_list_const_items LayoutGroup::get_items() const
 {
   //Get a const map from the non-const map:
   //TODO_Performance: Surely we should not need to copy the structure just to constize it?
-  type_list_const_items result;
-  Utils::copy(m_list_items, result);
-  return result;
+  return Utils::const_list(m_list_items);
 }
 
 LayoutGroup::type_list_const_items LayoutGroup::get_items_recursive() const
diff --git a/glom/libglom/sql_utils.cc b/glom/libglom/sql_utils.cc
index 56adb5a..013c26e 100644
--- a/glom/libglom/sql_utils.cc
+++ b/glom/libglom/sql_utils.cc
@@ -54,8 +54,7 @@ auto find_if_uses_relationship_has_relationship(T_Container& container, const st
 Glib::RefPtr<Gnome::Gda::SqlBuilder> build_sql_select_with_where_clause(const Glib::ustring& table_name, 
const type_vecLayoutFields& fieldsToGet, const Gnome::Gda::SqlExpr& where_clause, const std::shared_ptr<const 
Relationship>& extra_join, const type_sort_clause& sort_clause, guint limit)
 {
   //TODO_Performance:
-  type_vecConstLayoutFields constFieldsToGet;
-  Utils::copy(fieldsToGet, constFieldsToGet);
+  const auto constFieldsToGet = Utils::const_list(fieldsToGet);
 
   return build_sql_select_with_where_clause(table_name, constFieldsToGet, where_clause, extra_join, 
sort_clause, limit);
 }
@@ -309,8 +308,7 @@ Glib::RefPtr<Gnome::Gda::SqlBuilder> build_sql_select_with_where_clause(const Gl
 Glib::RefPtr<Gnome::Gda::SqlBuilder> build_sql_select_with_key(const Glib::ustring& table_name, const 
type_vecLayoutFields& fieldsToGet, const std::shared_ptr<const Field>& key_field, const Gnome::Gda::Value& 
key_value, const type_sort_clause& sort_clause, guint limit)
 {
   //TODO_Performance:
-  type_vecConstLayoutFields constFieldsToGet;
-  Utils::copy(fieldsToGet, constFieldsToGet);
+  const auto constFieldsToGet = Utils::const_list(fieldsToGet);
 
   return build_sql_select_with_key(table_name, constFieldsToGet, key_field, key_value, sort_clause, limit);
 }