diff --git a/toml/region.hpp b/toml/region.hpp
new file mode 100644
index 0000000..e9ced53
--- /dev/null
+++ b/toml/region.hpp
@@ -0,0 +1,136 @@
+#ifndef TOML11_REGION_H
+#define TOML11_REGION_H
+#include <memory>
+#include <algorithm>
+#include <iterator>
+#include <string>
+
+namespace toml
+{
+namespace detail
+{
+
+// location in a container, normally in a file content.
+// shared_ptr points the resource that the iter points.
+// it can be used not only for resource handling, but also error message.
+template<typename Container>
+struct location
+{
+    using const_iterator = typename Container::const_iterator;
+
+    location(Container cont)
+        : source(std::make_shared<Container>(std::move(cont))),
+          iter(source->begin())
+    {}
+    location(const location&) = default;
+    location(location&&)      = default;
+    location& operator=(const location&) = default;
+    location& operator=(location&&)      = default;
+   ~location() = default;
+
+    std::shared_ptr<Container> source;
+    const_iterator iter;
+};
+
+// region in a container, normally in a file content.
+// shared_ptr points the resource that the iter points.
+// combinators returns this.
+// it can be used not only for resource handling, but also error message.
+template<typename Container>
+struct region
+{
+    using const_iterator = typename Container::const_iterator;
+
+    region(const location<Container>& loc)
+        : first(loc.iter), last(loc.iter), source(loc.source)
+    {}
+    region(location<Container>&& loc)
+        : first(loc.iter), last(loc.iter), source(std::move(loc.source))
+    {}
+
+    region(const region&) = default;
+    region(region&&)      = default;
+    region& operator=(const region&) = default;
+    region& operator=(region&&)      = default;
+    ~region() = default;
+
+    const_iterator first, last;
+    std::shared_ptr<Container> source;
+};
+
+// to show a better error message.
+template<typename Container>
+std::string
+format_underline(const region<Container>& reg, const std::string& msg)
+{
+    using const_iterator = typename region<Container>::const_iterator;
+    using reverse_iterator = std::reverse_iterator<const_iterator>;
+    const auto line_begin = std::find(
+            reverse_iterator(reg.first),
+            reverse_iterator(reg.source->cbegin()),
+            '\n').base();
+    const auto line_end = std::find(reg.last, reg.source->cend(), '\n');
+
+    const auto line_number = std::to_string(
+            1 + std::count(reg.source->cbegin(), reg.first, '\n'));
+
+    std::string retval;
+    retval += ' ';
+    retval += line_number;
+    retval += " | ";
+    retval += std::string(line_begin, line_end);
+    retval += '\n';
+    retval += std::string(line_number.size() + 1, ' ');
+    retval += " | ";
+    retval += std::string(std::distance(line_begin, reg.first), ' ');
+    retval += std::string(std::distance(reg.first, reg.last), '~');
+    retval += ' ';
+    retval += msg;
+    return retval;
+}
+
+// to show a better error message.
+template<typename Container>
+std::string format_underline(const region<Container>& reg,
+                             typename Container::const_iterator pos,
+                             const std::string& msg)
+{
+    using const_iterator = typename region<Container>::const_iterator;
+    using reverse_iterator = std::reverse_iterator<const_iterator>;
+    const auto line_begin = std::find(
+            reverse_iterator(reg.first),
+            reverse_iterator(reg.source->cbegin()),
+            '\n').base();
+    const auto line_end = std::find(reg.last, reg.source->cend(), '\n');
+
+    const auto line_number = std::to_string(
+            1 + std::count(reg.source->cbegin(), reg.first, '\n'));
+
+    std::string retval;
+    retval += ' ';
+    retval += line_number;
+    retval += " | ";
+    retval += std::string(line_begin, line_end);
+    retval += '\n';
+    retval += std::string(line_number.size() + 1, ' ');
+    retval += " | ";
+    retval += std::string(std::distance(line_begin, reg.first), ' ');
+
+    if(std::distance(reg.first, std::prev(pos)) > 0)
+    {
+        retval += std::string(std::distance(reg.first, std::prev(pos)), '-');
+    }
+    retval += '^';
+    if(std::distance(pos, reg.last) > 0)
+    {
+        retval += std::string(std::distance(std::next(pos), reg.last), '-');
+    }
+
+    retval += ' ';
+    retval += msg;
+    return retval;
+}
+
+} // detail
+} // toml
+#endif// TOML11_REGION_H