enable to show err msg for 2 different location

toml/region.hpp

@@ -6,7 +6,7 @@
 #include <algorithm>
 #include <initializer_list>
 #include <iterator>
-#include <iostream>
+#include <iomanip>
 
 namespace toml
 {
@@ -241,6 +241,64 @@ inline std::string format_underline(const std::string& message,
     return retval;
 }
 
+// to show a better error message.
+inline std::string format_underline(const std::string& message,
+        const region_base& reg1, const std::string& comment_for_underline1,
+        const region_base& reg2, const std::string& comment_for_underline2,
+        std::vector<std::string> helps = {})
+{
+#ifdef _WIN32
+    const auto newline = "\r\n";
+#else
+    const char newline = '\n';
+#endif
+    const auto line1        = reg1.line();
+    const auto line_number1 = reg1.line_num();
+    const auto line2        = reg2.line();
+    const auto line_number2 = reg2.line_num();
+    const auto line_num_width =
+        std::max(line_number1.size(), line_number2.size());
+
+    std::ostringstream retval;
+    retval << message;
+    retval << newline;
+    retval << " --> ";
+    retval << reg1.name() << newline;;
+//  ---------------------------------------
+    retval << ' ' << std::setw(line_num_width) << line_number1;
+    retval << " | " << line1 << newline;
+    retval << make_string(line_num_width + 1, ' ');
+    retval << " | ";
+    retval << make_string(reg1.before(), ' ');
+    retval << make_string(reg1.size(),   '~');
+    retval << ' ';
+    retval << comment_for_underline1 << newline;
+//  ---------------------------------------
+    retval << " ..." << newline;
+    retval << ' ' << std::setw(line_num_width) << line_number2;
+    retval << " | " << line2 << newline;
+    retval << make_string(line_num_width + 1, ' ');
+    retval << " | ";
+    retval << make_string(reg2.before(), ' ');
+    retval << make_string(reg2.size(),   '~');
+    retval << ' ';
+    retval << comment_for_underline2;
+    if(helps.size() != 0)
+    {
+        retval << newline;
+        retval << make_string(line_num_width + 1, ' ');
+        retval << " | ";
+        for(const auto help : helps)
+        {
+            retval << newline;
+            retval << "Hint: ";
+            retval << help;
+        }
+    }
+    return retval.str();
+}
+
+
 // to show a better error message.
 template<typename Container>
 std::string