From 5b2ce267213cbcd900aed6fde1fda87b9d706da3 Mon Sep 17 00:00:00 2001
From: ToruNiina <niina.toru.68u@gmail.com>
Date: Thu, 14 Mar 2019 00:56:35 +0900
Subject: [PATCH 1/4] refactor: remove redundant function

since N region format_underline() has been implemented, overloads for 1
and 2 region(s) are not needed.
---
 toml/region.hpp | 149 +-----------------------------------------------
 1 file changed, 3 insertions(+), 146 deletions(-)

diff --git a/toml/region.hpp b/toml/region.hpp
index 62a28e1..0601bce 100644
--- a/toml/region.hpp
+++ b/toml/region.hpp
@@ -237,149 +237,6 @@ struct region final : public region_base
     const_iterator first_, last_;
 };
 
-// to show a better error message.
-inline std::string format_underline(const std::string& message,
-        const region_base& reg, const std::string& comment_for_underline,
-        std::vector<std::string> helps = {})
-{
-#ifdef _WIN32
-    const auto newline = "\r\n";
-#else
-    const char newline = '\n';
-#endif
-    const auto line        = reg.line();
-    const auto line_number = reg.line_num();
-
-    std::string retval;
-    retval += message;
-    retval += newline;
-    retval += " --> ";
-    retval += reg.name();
-    retval += newline;
-    retval += ' ';
-    retval += line_number;
-    retval += " | ";
-    retval += line;
-    retval += newline;
-    retval += make_string(line_number.size() + 1, ' ');
-    retval += " | ";
-    retval += make_string(reg.before(), ' ');
-    if(reg.size() == 1)
-    {
-        // invalid
-        // ^------
-        retval += '^';
-        retval += make_string(reg.after(), '-');
-    }
-    else
-    {
-        // invalid
-        // ~~~~~~~
-        retval += make_string(reg.size(), '~');
-    }
-    retval += ' ';
-    retval += comment_for_underline;
-    if(helps.size() != 0)
-    {
-        retval += newline;
-        retval += make_string(line_number.size() + 1, ' ');
-        retval += " | ";
-        for(const auto help : helps)
-        {
-            retval += newline;
-            retval += "Hint: ";
-            retval += help;
-        }
-    }
-    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 << newline;
-    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(), ' ');
-    if(reg1.size() == 1)
-    {
-        // invalid
-        // ^------
-        retval << '^';
-        retval << make_string(reg1.after(), '-');
-    }
-    else
-    {
-        // invalid
-        // ~~~~~~~
-        retval << make_string(reg1.size(), '~');
-    }
-    retval << ' ';
-    retval << comment_for_underline1 << newline;
-//  ---------------------------------------
-    if(reg2.name() != reg1.name())
-    {
-        retval << " --> " << reg2.name() << newline;
-    }
-    else
-    {
-        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(), ' ');
-    if(reg2.size() == 1)
-    {
-        // invalid
-        // ^------
-        retval << '^';
-        retval << make_string(reg2.after(), '-');
-    }
-    else
-    {
-        // invalid
-        // ~~~~~~~
-        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.
 inline std::string format_underline(const std::string& message,
         std::vector<std::pair<region_base const*, std::string>> reg_com,
@@ -408,10 +265,11 @@ inline std::string format_underline(const std::string& message,
     {
         if(i!=0 && reg_com.at(i-1).first->name() == reg_com.at(i).first->name())
         {
-            retval << " ..." << newline;
+            retval << newline << " ..." << newline;
         }
         else
         {
+            if(i != 0) {retval << newline;}
             retval << " --> " << reg_com.at(i).first->name() << newline;
         }
 
@@ -439,7 +297,7 @@ inline std::string format_underline(const std::string& message,
         }
 
         retval << ' ';
-        retval << comment << newline;
+        retval << comment;
     }
 
     if(helps.size() != 0)
@@ -457,7 +315,6 @@ inline std::string format_underline(const std::string& message,
     return retval.str();
 }
 
-
 } // detail
 } // toml
 #endif// TOML11_REGION_H

From 0babe8d589a8a527b39eb23b99b3741bfa97fe1b Mon Sep 17 00:00:00 2001
From: ToruNiina <niina.toru.68u@gmail.com>
Date: Thu, 14 Mar 2019 00:59:10 +0900
Subject: [PATCH 2/4] fix: use format_underline for N regions everywhere

---
 toml/get.hpp    |  28 ++--
 toml/parser.hpp | 352 +++++++++++++++++++++++++++---------------------
 toml/value.hpp  |  36 +++--
 3 files changed, 236 insertions(+), 180 deletions(-)

diff --git a/toml/get.hpp b/toml/get.hpp
index fe51622..80336ce 100644
--- a/toml/get.hpp
+++ b/toml/get.hpp
@@ -210,8 +210,9 @@ T get(const value& v)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
             "[erorr] toml::get specified container size is ", container.size(),
-            " but there are ", ar.size(), " elements in toml array."),
-            detail::get_region(v), "here"));
+            " but there are ", ar.size(), " elements in toml array."), {
+                {std::addressof(detail::get_region(v)), "here"}
+            }));
     }
     std::transform(ar.cbegin(), ar.cend(), container.begin(),
                    [](const value& x){return ::toml::get<value_type>(x);});
@@ -233,7 +234,9 @@ T get(const value& v)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
             "[erorr] toml::get specified std::pair but there are ", ar.size(),
-            " elements in toml array."), detail::get_region(v), "here"));
+            " elements in toml array."), {
+                {std::addressof(detail::get_region(v)), "here"}
+            }));
     }
     return std::make_pair(::toml::get<first_type >(ar.at(0)),
                           ::toml::get<second_type>(ar.at(1)));
@@ -264,7 +267,9 @@ T get(const value& v)
         throw std::out_of_range(detail::format_underline(concat_to_string(
             "[erorr] toml::get specified std::tuple with ",
             std::tuple_size<T>::value, "elements, but there are ", ar.size(),
-            " elements in toml array."), detail::get_region(v), "here"));
+            " elements in toml array."), {
+                {std::addressof(detail::get_region(v)), "here"}
+            }));
     }
     return detail::get_tuple_impl<T>(ar,
             detail::make_index_sequence<std::tuple_size<T>::value>{});
@@ -340,8 +345,9 @@ find(const toml::value& v, const toml::key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), detail::get_region(v),
-            "in this table"));
+            "[error] key \"", ky, "\" not found"), {
+                {std::addressof(detail::get_region(v)), "in this table"}
+            }));
     }
     return ::toml::get<T>(tab.at(ky));
 }
@@ -353,8 +359,9 @@ find(toml::value& v, const toml::key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), detail::get_region(v),
-            "in this table"));
+            "[error] key \"", ky, "\" not found"), {
+                {std::addressof(detail::get_region(v)), "in this table"}
+            }));
     }
     return ::toml::get<T>(tab.at(ky));
 }
@@ -366,8 +373,9 @@ find(toml::value&& v, const toml::key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), detail::get_region(v),
-            "in this table"));
+            "[error] key \"", ky, "\" not found"), {
+                {std::addressof(detail::get_region(v)), "in this table"}
+            }));
     }
     return ::toml::get<T>(std::move(tab[ky]));
 }
diff --git a/toml/parser.hpp b/toml/parser.hpp
index 26b13b1..9c1d562 100644
--- a/toml/parser.hpp
+++ b/toml/parser.hpp
@@ -29,13 +29,13 @@ parse_boolean(location<Container>& loc)
         else // internal error.
         {
             throw toml::internal_error(format_underline(
-                "[error] toml::parse_boolean: internal error", reg,
-                "invalid token"));
+                "[error] toml::parse_boolean: internal error",
+                {{std::addressof(reg), "invalid token"}}));
         }
     }
     loc.iter() = first; //rollback
-    return err(format_underline("[error] toml::parse_boolean: ", loc,
-                           "the next token is not a boolean"));
+    return err(format_underline("[error] toml::parse_boolean: ",
+               {{std::addressof(loc), "the next token is not a boolean"}}));
 }
 
 template<typename Container>
@@ -57,14 +57,14 @@ parse_binary_integer(location<Container>& loc)
             {
                 throw toml::internal_error(format_underline(
                     "[error] toml::parse_integer: internal error",
-                    token.unwrap(), "invalid token"));
+                    {{std::addressof(token.unwrap()), "invalid token"}}));
             }
         }
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.iter() = first;
-    return err(format_underline("[error] toml::parse_binary_integer:", loc,
-                           "the next token is not an integer"));
+    return err(format_underline("[error] toml::parse_binary_integer:",
+               {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
 template<typename Container>
@@ -84,8 +84,8 @@ parse_octal_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.iter() = first;
-    return err(format_underline("[error] toml::parse_octal_integer:", loc,
-                           "the next token is not an integer"));
+    return err(format_underline("[error] toml::parse_octal_integer:",
+               {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
 template<typename Container>
@@ -105,8 +105,8 @@ parse_hexadecimal_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.iter() = first;
-    return err(format_underline("[error] toml::parse_hexadecimal_integer", loc,
-                           "the next token is not an integer"));
+    return err(format_underline("[error] toml::parse_hexadecimal_integer",
+               {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
 template<typename Container>
@@ -133,8 +133,8 @@ parse_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.iter() = first;
-    return err(format_underline("[error] toml::parse_integer: ", loc,
-                           "the next token is not an integer"));
+    return err(format_underline("[error] toml::parse_integer: ",
+               {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
 template<typename Container>
@@ -222,8 +222,8 @@ parse_floating(location<Container>& loc)
         return ok(std::make_pair(v, token.unwrap()));
     }
     loc.iter() = first;
-    return err(format_underline("[error] toml::parse_floating: ", loc,
-                           "the next token is not a float"));
+    return err(format_underline("[error] toml::parse_floating: ",
+               {{std::addressof(loc), "the next token is not a float"}}));
 }
 
 template<typename Container, typename Container2>
@@ -252,8 +252,9 @@ std::string read_utf8_codepoint(const region<Container>& reg,
         {
             std::cerr << format_underline("[warning] "
                 "toml::read_utf8_codepoint: codepoints in the range "
-                "[0xD800, 0xDFFF] are not valid UTF-8.",
-                loc, "not a valid UTF-8 codepoint") << std::endl;
+                "[0xD800, 0xDFFF] are not valid UTF-8.", {{
+                    std::addressof(loc), "not a valid UTF-8 codepoint"
+                }}) << std::endl;
         }
         assert(codepoint < 0xD800 || 0xDFFF < codepoint);
         // 1110yyyy 10yxxxxx 10xxxxxx
@@ -267,8 +268,8 @@ std::string read_utf8_codepoint(const region<Container>& reg,
         {
             std::cerr << format_underline("[error] "
                 "toml::read_utf8_codepoint: input codepoint is too large to "
-                "decode as a unicode character.", loc,
-                "should be in [0x00..0x10FFFF]") << std::endl;
+                "decode as a unicode character.", {{std::addressof(loc),
+                "should be in [0x00..0x10FFFF]"}}) << std::endl;
         }
         // 11110yyy 10yyxxxx 10xxxxxx 10xxxxxx
         character += static_cast<unsigned char>(0xF0| codepoint >> 18);
@@ -280,7 +281,7 @@ std::string read_utf8_codepoint(const region<Container>& reg,
     {
         throw std::range_error(format_underline(concat_to_string("[error] "
                 "input codepoint (", str, ") is too large to encode as utf-8."),
-                reg, "should be in [0x00..0x10FFFF]"));
+                {{std::addressof(reg), "should be in [0x00..0x10FFFF]"}}));
     }
     return character;
 }
@@ -291,8 +292,8 @@ result<std::string, std::string> parse_escape_sequence(location<Container>& loc)
     const auto first = loc.iter();
     if(first == loc.end() || *first != '\\')
     {
-        return err(format_underline("[error]: toml::parse_escape_sequence: ", loc,
-                    "the next token is not an escape sequence \"\\\""));
+        return err(format_underline("[error]: toml::parse_escape_sequence: ", {{
+            std::addressof(loc), "the next token is not a backslash \"\\\""}}));
     }
     ++loc.iter();
     switch(*loc.iter())
@@ -314,7 +315,7 @@ result<std::string, std::string> parse_escape_sequence(location<Container>& loc)
             {
                 return err(format_underline("[error] parse_escape_sequence: "
                            "invalid token found in UTF-8 codepoint uXXXX.",
-                           loc, token.unwrap_err()));
+                           {{std::addressof(loc), token.unwrap_err()}}));
             }
         }
         case 'U':
@@ -327,16 +328,16 @@ result<std::string, std::string> parse_escape_sequence(location<Container>& loc)
             {
                 return err(format_underline("[error] parse_escape_sequence: "
                            "invalid token found in UTF-8 codepoint Uxxxxxxxx",
-                           loc, token.unwrap_err()));
+                           {{std::addressof(loc), token.unwrap_err()}}));
             }
         }
     }
 
     const auto msg = format_underline("[error] parse_escape_sequence: "
-           "unknown escape sequence appeared.", loc, "escape sequence is one of"
-           " \\, \", b, t, n, f, r, uxxxx, Uxxxxxxxx", {"if you want to write "
-           "backslash as just one backslash, use literal string like:",
-           "regex    = '<\\i\\c*\\s*>'"});
+           "unknown escape sequence appeared.", {{std::addressof(loc),
+           "escape sequence is one of \\, \", b, t, n, f, r, uxxxx, Uxxxxxxxx"}},
+           /* Hints = */{"if you want to write backslash as just one backslash, "
+           "use literal string like: regex    = '<\\i\\c*\\s*>'"});
     loc.iter() = first;
     return err(msg);
 }
@@ -359,7 +360,7 @@ parse_ml_basic_string(location<Container>& loc)
         {
             throw internal_error(format_underline("[error] "
                 "parse_ml_basic_string: invalid token",
-                inner_loc, "should be \"\"\""));
+                {{std::addressof(inner_loc), "should be \"\"\""}}));
         }
         // immediate newline is ignored (if exists)
         /* discard return value */ lex_newline::invoke(inner_loc);
@@ -385,7 +386,7 @@ parse_ml_basic_string(location<Container>& loc)
             {
                 throw internal_error(format_underline("[error] "
                     "parse_ml_basic_string: unexpected end of region",
-                    inner_loc, "not sufficient token"));
+                    {{std::addressof(inner_loc), "not sufficient token"}}));
             }
             delim = lex_ml_basic_string_delim::invoke(inner_loc);
         }
@@ -412,7 +413,7 @@ parse_basic_string(location<Container>& loc)
         if(!quot)
         {
             throw internal_error(format_underline("[error] parse_basic_string: "
-                "invalid token", inner_loc, "should be \""));
+                "invalid token", {{std::addressof(inner_loc), "should be \""}}));
         }
 
         std::string retval;
@@ -434,7 +435,7 @@ parse_basic_string(location<Container>& loc)
             {
                 throw internal_error(format_underline("[error] "
                     "parse_ml_basic_string: unexpected end of region",
-                    inner_loc, "not sufficient token"));
+                    {{std::addressof(inner_loc), "not sufficient token"}}));
             }
             quot = lex_quotation_mark::invoke(inner_loc);
         }
@@ -461,7 +462,7 @@ parse_ml_literal_string(location<Container>& loc)
         {
             throw internal_error(format_underline("[error] "
                 "parse_ml_literal_string: invalid token",
-                inner_loc, "should be '''"));
+                {{std::addressof(inner_loc), "should be '''"}}));
         }
         // immediate newline is ignored (if exists)
         /* discard return value */ lex_newline::invoke(inner_loc);
@@ -473,7 +474,7 @@ parse_ml_literal_string(location<Container>& loc)
         {
             throw internal_error(format_underline("[error] "
                 "parse_ml_literal_string: invalid token",
-                inner_loc, "should be '''"));
+                {{std::addressof(inner_loc), "should be '''"}}));
         }
         return ok(std::make_pair(
                   toml::string(body.unwrap().str(), toml::string_t::literal),
@@ -500,7 +501,7 @@ parse_literal_string(location<Container>& loc)
         {
             throw internal_error(format_underline("[error] "
                 "parse_literal_string: invalid token",
-                inner_loc, "should be '"));
+                {{std::addressof(inner_loc), "should be '"}}));
         }
 
         const auto body = repeat<lex_literal_char, unlimited>::invoke(inner_loc);
@@ -510,7 +511,7 @@ parse_literal_string(location<Container>& loc)
         {
             throw internal_error(format_underline("[error] "
                 "parse_literal_string: invalid token",
-                inner_loc, "should be '"));
+                {{std::addressof(inner_loc), "should be '"}}));
         }
         return ok(std::make_pair(
                   toml::string(body.unwrap().str(), toml::string_t::literal),
@@ -531,8 +532,8 @@ parse_string(location<Container>& loc)
     if(const auto rslt = parse_ml_literal_string(loc)) {return rslt;}
     if(const auto rslt = parse_basic_string(loc))      {return rslt;}
     if(const auto rslt = parse_literal_string(loc))    {return rslt;}
-    return err(format_underline("[error] toml::parse_string: ", loc,
-                "the next token is not a string"));
+    return err(format_underline("[error] toml::parse_string: ",
+                {{std::addressof(loc), "the next token is not a string"}}));
 }
 
 template<typename Container>
@@ -547,21 +548,23 @@ parse_local_date(location<Container>& loc)
         const auto y = lex_date_fullyear::invoke(inner_loc);
         if(!y || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
         {
+            const std::string msg = y.map_err_or_else(
+                [](const std::string& msg) {return msg;}, "should be `-`");
+
             throw internal_error(format_underline("[error]: "
                 "toml::parse_inner_local_date: invalid year format",
-                inner_loc, y.map_err_or_else([](const std::string& msg) {
-                        return msg;
-                    }, "should be `-`")));
+                {{std::addressof(inner_loc), msg}}));
         }
         ++inner_loc.iter();
         const auto m = lex_date_month::invoke(inner_loc);
         if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
         {
+            const std::string msg = m.map_err_or_else(
+                [](const std::string& msg) {return msg;}, "should be `-`");
+
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_date: invalid month format",
-                inner_loc, m.map_err_or_else([](const std::string& msg) {
-                        return msg;
-                    }, "should be `-`")));
+                {{std::addressof(inner_loc), msg}}));
         }
         ++inner_loc.iter();
         const auto d = lex_date_mday::invoke(inner_loc);
@@ -569,7 +572,7 @@ parse_local_date(location<Container>& loc)
         {
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_date: invalid day format",
-                inner_loc, d.unwrap_err()));
+                {{std::addressof(inner_loc), d.unwrap_err()}}));
         }
         return ok(std::make_pair(local_date(
             static_cast<std::int16_t>(from_string<int>(y.unwrap().str(), 0)),
@@ -581,8 +584,8 @@ parse_local_date(location<Container>& loc)
     else
     {
         loc.iter() = first;
-        return err(format_underline("[error]: toml::parse_local_date: ", loc,
-                    "the next token is not a local_date"));
+        return err(format_underline("[error]: toml::parse_local_date: ",
+            {{std::addressof(loc), "the next token is not a local_date"}}));
     }
 }
 
@@ -598,21 +601,23 @@ parse_local_time(location<Container>& loc)
         const auto h = lex_time_hour::invoke(inner_loc);
         if(!h || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
         {
+            const std::string msg = h.map_err_or_else(
+                [](const std::string& msg) {return msg;}, "should be `:`");
+
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_time: invalid year format",
-                inner_loc, h.map_err_or_else([](const std::string& msg) {
-                        return msg;
-                    }, "should be `:`")));
+                {{std::addressof(inner_loc), msg}}));
         }
         ++inner_loc.iter();
         const auto m = lex_time_minute::invoke(inner_loc);
         if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
         {
+            const std::string msg = m.map_err_or_else(
+                [](const std::string& msg) {return msg;}, "should be `:`");
+
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_time: invalid month format",
-                inner_loc, m.map_err_or_else([](const std::string& msg) {
-                        return msg;
-                    }, "should be `:`")));
+                {{std::addressof(inner_loc), msg}}));
         }
         ++inner_loc.iter();
         const auto s = lex_time_second::invoke(inner_loc);
@@ -620,7 +625,7 @@ parse_local_time(location<Container>& loc)
         {
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_time: invalid second format",
-                inner_loc, s.unwrap_err()));
+                {{std::addressof(inner_loc), s.unwrap_err()}}));
         }
         local_time time(
             static_cast<std::int8_t>(from_string<int>(h.unwrap().str(), 0)),
@@ -656,7 +661,7 @@ parse_local_time(location<Container>& loc)
             {
                 throw internal_error(format_underline("[error]: "
                     "toml::parse_local_time: invalid subsecond format",
-                    inner_loc, secfrac.unwrap_err()));
+                    {{std::addressof(inner_loc), secfrac.unwrap_err()}}));
             }
         }
         return ok(std::make_pair(time, token.unwrap()));
@@ -664,8 +669,8 @@ parse_local_time(location<Container>& loc)
     else
     {
         loc.iter() = first;
-        return err(format_underline("[error]: toml::parse_local_time: ", loc,
-                    "the next token is not a local_time"));
+        return err(format_underline("[error]: toml::parse_local_time: ",
+            {{std::addressof(loc), "the next token is not a local_time"}}));
     }
 }
 
@@ -680,25 +685,26 @@ parse_local_datetime(location<Container>& loc)
         const auto date = parse_local_date(inner_loc);
         if(!date || inner_loc.iter() == inner_loc.end())
         {
+            const std::string msg = date.map_err_or_else(
+                [](const std::string& msg) {return msg;}, "date, not datetime");
+
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_datetime: invalid datetime format",
-                inner_loc, date.map_err_or_else([](const std::string& msg){
-                        return msg;
-                    }, "date, not datetime")));
+                {{std::addressof(inner_loc), msg}}));
         }
         const char delim = *(inner_loc.iter()++);
         if(delim != 'T' && delim != 't' && delim != ' ')
         {
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_datetime: invalid datetime format",
-                inner_loc, "should be `T` or ` ` (space)"));
+                {{std::addressof(inner_loc), "should be `T` or ` ` (space)"}}));
         }
         const auto time = parse_local_time(inner_loc);
         if(!time)
         {
             throw internal_error(format_underline("[error]: "
                 "toml::parse_local_datetime: invalid datetime format",
-                inner_loc, "invalid time fomrat"));
+                {{std::addressof(inner_loc), "invalid time fomrat"}}));
         }
         return ok(std::make_pair(
             local_datetime(date.unwrap().first, time.unwrap().first),
@@ -707,8 +713,8 @@ parse_local_datetime(location<Container>& loc)
     else
     {
         loc.iter() = first;
-        return err(format_underline("[error]: toml::parse_local_datetime: ", loc,
-                    "the next token is not a local_datetime"));
+        return err(format_underline("[error]: toml::parse_local_datetime: ",
+            {{std::addressof(loc), "the next token is not a local_datetime"}}));
     }
 }
 
@@ -723,11 +729,12 @@ parse_offset_datetime(location<Container>& loc)
         const auto datetime = parse_local_datetime(inner_loc);
         if(!datetime || inner_loc.iter() == inner_loc.end())
         {
+            const std::string msg = datetime.map_err_or_else(
+                [](const std::string& msg){return msg;}, "date, not datetime");
+
             throw internal_error(format_underline("[error]: "
                 "toml::parse_offset_datetime: invalid datetime format",
-                inner_loc, datetime.map_err_or_else([](const std::string& msg){
-                        return msg;
-                    }, "date, not datetime")));
+                {{std::addressof(inner_loc), msg}}));
         }
         time_offset offset(0, 0);
         if(const auto ofs = lex_time_numoffset::invoke(inner_loc))
@@ -748,7 +755,7 @@ parse_offset_datetime(location<Container>& loc)
         {
             throw internal_error(format_underline("[error]: "
                 "toml::parse_offset_datetime: invalid datetime format",
-                inner_loc, "should be `Z` or `+HH:MM`"));
+                {{std::addressof(inner_loc), "should be `Z` or `+HH:MM`"}}));
         }
         return ok(std::make_pair(offset_datetime(datetime.unwrap().first, offset),
                                  token.unwrap()));
@@ -756,8 +763,8 @@ parse_offset_datetime(location<Container>& loc)
     else
     {
         loc.iter() = first;
-        return err(format_underline("[error]: toml::parse_offset_datetime: ", loc,
-                    "the next token is not a local_datetime"));
+        return err(format_underline("[error]: toml::parse_offset_datetime: ",
+            {{std::addressof(loc), "the next token is not a local_datetime"}}));
     }
 }
 
@@ -778,8 +785,8 @@ parse_simple_key(location<Container>& loc)
         const auto reg = bare.unwrap();
         return ok(std::make_pair(reg.str(), reg));
     }
-    return err(format_underline("[error] toml::parse_simple_key: ", loc,
-        "the next token is not a simple key"));
+    return err(format_underline("[error] toml::parse_simple_key: ",
+            {{std::addressof(loc), "the next token is not a simple key"}}));
 }
 
 // dotted key become vector of keys
@@ -806,7 +813,7 @@ parse_key(location<Container>& loc)
             {
                 throw internal_error(format_underline("[error] "
                     "toml::detail::parse_key: dotted key contains invalid key",
-                    inner_loc, k.unwrap_err()));
+                    {{std::addressof(inner_loc), k.unwrap_err()}}));
             }
 
             lex_ws::invoke(inner_loc);
@@ -821,8 +828,8 @@ parse_key(location<Container>& loc)
             else
             {
                 throw internal_error(format_underline("[error] toml::parse_key: "
-                    "dotted key contains invalid key ", inner_loc,
-                    "should be `.`"));
+                    "dotted key contains invalid key ",
+                    {{std::addressof(inner_loc), "should be `.`"}}));
             }
         }
         return ok(std::make_pair(keys, reg));
@@ -835,7 +842,8 @@ parse_key(location<Container>& loc)
         return ok(std::make_pair(std::vector<key>(1, smpl.unwrap().first),
                                  smpl.unwrap().second));
     }
-    return err(format_underline("[error] toml::parse_key: ", loc, "is not a valid key"));
+    return err(format_underline("[error] toml::parse_key: ",
+                {{std::addressof(loc), "is not a valid key"}}));
 }
 
 // forward-decl to implement parse_array and parse_table
@@ -880,22 +888,16 @@ parse_array(location<Container>& loc)
                 array_start_loc.iter() = first;
 
                 throw syntax_error(format_underline("[error] toml::parse_array: "
-                    "type of elements should be the same each other.",
-                    std::vector<std::pair<region_base const*, std::string>>{
-                        std::make_pair(
-                            std::addressof(array_start_loc),
-                            std::string("array starts here")
-                        ),
-                        std::make_pair(
+                    "type of elements should be the same each other.", {
+                        {std::addressof(array_start_loc), "array starts here"},
+                        {
                             std::addressof(get_region(retval.front())),
-                            std::string("value has type ") +
-                                stringize(retval.front().type())
-                        ),
-                        std::make_pair(
+                            "value has type " + stringize(retval.front().type())
+                        },
+                        {
                             std::addressof(get_region(val.unwrap())),
-                            std::string("value has different type, ") +
-                                stringize(val.unwrap().type())
-                        )
+                            "value has different type, " + stringize(val.unwrap().type())
+                        }
                     }));
             }
             retval.push_back(std::move(val.unwrap()));
@@ -906,9 +908,10 @@ parse_array(location<Container>& loc)
             array_start_loc.iter() = first;
 
             throw syntax_error(format_underline("[error] toml::parse_array: "
-                "value having invalid format appeared in an array",
-                array_start_loc, "array starts here",
-                loc, "it is not a valid value."));
+                "value having invalid format appeared in an array", {
+                    {std::addressof(array_start_loc), "array starts here"},
+                    {std::addressof(loc), "it is not a valid value."}
+                }));
         }
 
         using lex_array_separator = sequence<maybe<lex_ws>, character<','>>;
@@ -928,14 +931,17 @@ parse_array(location<Container>& loc)
                 array_start_loc.iter() = first;
 
                 throw syntax_error(format_underline("[error] toml::parse_array:"
-                    " missing array separator `,` after a value",
-                    array_start_loc, "array starts here", loc, "should be `,`"));
+                    " missing array separator `,` after a value", {
+                        {std::addressof(array_start_loc), "array starts here"},
+                        {std::addressof(loc),             "should be `,`"}
+                    }));
             }
         }
     }
     loc.iter() = first;
     throw syntax_error(format_underline("[error] toml::parse_array: "
-            "array did not closed by `]`", loc, "should be closed"));
+            "array did not closed by `]`",
+            {{std::addressof(loc), "should be closed"}}));
 }
 
 template<typename Container>
@@ -953,7 +959,8 @@ parse_key_value_pair(location<Container>& loc)
         {
             loc.iter() = first;
             msg = format_underline("[error] toml::parse_key_value_pair: "
-                "empty key is not allowed.", loc, "key expected before '='");
+                "empty key is not allowed.",
+                {{std::addressof(loc), "key expected before '='"}});
         }
         return err(std::move(msg));
     }
@@ -968,14 +975,16 @@ parse_key_value_pair(location<Container>& loc)
         if(std::find(loc.iter(), line_end, '=') != line_end)
         {
             msg = format_underline("[error] toml::parse_key_value_pair: "
-                "invalid format for key", loc, "invalid character in key", {
-                "Did you forget '.' to separate dotted-key?",
+                "invalid format for key",
+                {{std::addressof(loc), "invalid character in key"}},
+                {"Did you forget '.' to separate dotted-key?",
                 "Allowed characters for bare key are [0-9a-zA-Z_-]."});
         }
         else // if not, the error is lack of key-value separator.
         {
             msg = format_underline("[error] toml::parse_key_value_pair: "
-                "missing key-value separator `=`", loc, "should be `=`");
+                "missing key-value separator `=`",
+                {{std::addressof(loc), "should be `=`"}});
         }
         loc.iter() = first;
         return err(std::move(msg));
@@ -992,8 +1001,8 @@ parse_key_value_pair(location<Container>& loc)
         {
             loc.iter() = after_kvsp;
             msg = format_underline("[error] toml::parse_key_value_pair: "
-                    "missing value after key-value separator '='", loc,
-                    "expected value, but got nothing");
+                    "missing value after key-value separator '='",
+                    {{std::addressof(loc), "expected value, but got nothing"}});
         }
         else // there is something not a comment/whitespace, so invalid format.
         {
@@ -1122,19 +1131,26 @@ insert_nested_key(table& root, const toml::value& v,
                         // show special err msg for conflicting table
                         throw syntax_error(format_underline(concat_to_string(
                             "[error] toml::insert_value: array of table (\"",
-                            format_dotted_keys(first, last), "\") cannot insert"
-                            "ed"), get_region(tab->at(k)), "table already defined",
-                            get_region(v), "this conflicts with the previous table"));
+                            format_dotted_keys(first, last),
+                            "\") cannot be defined"), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 "table already defined"},
+                                {std::addressof(get_region(v)),
+                                 "this conflicts with the previous table"}
+                            }));
                     }
                     else if(!(tab->at(k).is(value_t::Array)))
                     {
                         throw syntax_error(format_underline(concat_to_string(
                             "[error] toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last), "\") collides with"
-                            " existing value"), get_region(tab->at(k)),
-                            concat_to_string("this ", tab->at(k).type(),
-                            " value already exists"), get_region(v),
-                            "while inserting this array-of-tables"));
+                            " existing value"), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 concat_to_string("this ", tab->at(k).type(),
+                                                  " value already exists")},
+                                {std::addressof(get_region(v)),
+                                 "while inserting this array-of-tables"}
+                            }));
                     }
                     array& a = tab->at(k).template cast<toml::value_t::Array>();
                     if(!(a.front().is(value_t::Table)))
@@ -1142,10 +1158,13 @@ insert_nested_key(table& root, const toml::value& v,
                         throw syntax_error(format_underline(concat_to_string(
                             "[error] toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last), "\") collides with"
-                            " existing value"), get_region(tab->at(k)),
-                            concat_to_string("this ", tab->at(k).type(),
-                            " value already exists"), get_region(v),
-                            "while inserting this array-of-tables"));
+                            " existing value"), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 concat_to_string("this ", tab->at(k).type(),
+                                                  " value already exists")},
+                                {std::addressof(get_region(v)),
+                                 "while inserting this array-of-tables"}
+                            }));
                     }
                     // avoid conflicting array of table like the following.
                     // ```toml
@@ -1167,10 +1186,13 @@ insert_nested_key(table& root, const toml::value& v,
                         throw syntax_error(format_underline(concat_to_string(
                             "[error] toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last), "\") collides with"
-                            " existing array-of-tables"), get_region(tab->at(k)),
-                            concat_to_string("this ", tab->at(k).type(),
-                            " value has static size"), get_region(v),
-                            "appending this to the statically sized array"));
+                            " existing array-of-tables"), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 concat_to_string("this ", tab->at(k).type(),
+                                                  " value has static size")},
+                                {std::addressof(get_region(v)),
+                                 "appending it to the statically sized array"}
+                            }));
                     }
                     a.push_back(v);
                     return ok(true);
@@ -1191,11 +1213,14 @@ insert_nested_key(table& root, const toml::value& v,
                                 tab->at(k), first, iter, last))
                     {
                         throw syntax_error(format_underline(concat_to_string(
-                                "[error] toml::insert_value: table (\"",
-                                format_dotted_keys(first, last),
-                                "\") already exists."),
-                            get_region(tab->at(k)), "table already exists here",
-                            get_region(v), "table defined twice"));
+                            "[error] toml::insert_value: table (\"",
+                            format_dotted_keys(first, last),
+                            "\") already exists."), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 "table already exists here"},
+                                {std::addressof(get_region(v)),
+                                 "table defined twice"}
+                            }));
                     }
                     // to allow the following toml file.
                     // [a.b.c]
@@ -1217,18 +1242,23 @@ insert_nested_key(table& root, const toml::value& v,
                 {
                     throw syntax_error(format_underline(concat_to_string(
                         "[error] toml::insert_value: array of tables (\"",
-                        format_dotted_keys(first, last), "\") already exists."),
-                        get_region(tab->at(k)), "array of tables defined here",
-                        get_region(v), "table conflicts with the previous array"
-                        " of table"));
+                        format_dotted_keys(first, last), "\") already exists."), {
+                            {std::addressof(get_region(tab->at(k))),
+                             "array of tables defined here"},
+                            {std::addressof(get_region(v)),
+                            "table conflicts with the previous array of table"}
+                        }));
                 }
                 else
                 {
                     throw syntax_error(format_underline(concat_to_string(
                         "[error] toml::insert_value: value (\"",
-                        format_dotted_keys(first, last), "\") already exists."),
-                        get_region(tab->at(k)), "value already exists here",
-                        get_region(v), "value defined twice"));
+                        format_dotted_keys(first, last), "\") already exists."), {
+                            {std::addressof(get_region(tab->at(k))),
+                             "value already exists here"},
+                            {std::addressof(get_region(v)),
+                             "value defined twice"}
+                        }));
                 }
             }
             tab->insert(std::make_pair(k, v));
@@ -1260,9 +1290,11 @@ insert_nested_key(table& root, const toml::value& v,
                     throw syntax_error(format_underline(concat_to_string(
                         "[error] toml::insert_value: target (",
                         format_dotted_keys(first, std::next(iter)),
-                        ") is neither table nor an array of tables"),
-                        get_region(a.back()), concat_to_string("actual type is ",
-                        a.back().type()), get_region(v), "inserting this"));
+                        ") is neither table nor an array of tables"), {
+                            {std::addressof(get_region(a.back())),
+                             concat_to_string("actual type is ", a.back().type())},
+                            {std::addressof(get_region(v)), "inserting this"}
+                        }));
                 }
                 tab = std::addressof(a.back().template cast<value_t::Table>());
             }
@@ -1271,9 +1303,11 @@ insert_nested_key(table& root, const toml::value& v,
                 throw syntax_error(format_underline(concat_to_string(
                     "[error] toml::insert_value: target (",
                     format_dotted_keys(first, std::next(iter)),
-                    ") is neither table nor an array of tables"),
-                    get_region(tab->at(k)), concat_to_string("actual type is ",
-                    tab->at(k).type()), get_region(v), "inserting this"));
+                    ") is neither table nor an array of tables"), {
+                        {std::addressof(get_region(tab->at(k))),
+                         concat_to_string("actual type is ", tab->at(k).type())},
+                        {std::addressof(get_region(v)), "inserting this"}
+                    }));
             }
         }
     }
@@ -1288,8 +1322,8 @@ parse_inline_table(location<Container>& loc)
     table retval;
     if(!(loc.iter() != loc.end() && *loc.iter() == '{'))
     {
-        return err(format_underline("[error] toml::parse_inline_table: ", loc,
-                               "the next token is not an inline table"));
+        return err(format_underline("[error] toml::parse_inline_table: ", 
+            {{std::addressof(loc), "the next token is not an inline table"}}));
     }
     ++loc.iter();
     // it starts from "{". it should be formatted as inline-table
@@ -1335,13 +1369,14 @@ parse_inline_table(location<Container>& loc)
             {
                 throw syntax_error(format_underline("[error] "
                     "toml:::parse_inline_table: missing table separator `,` ",
-                    loc, "should be `,`"));
+                    {{std::addressof(loc), "should be `,`"}}));
             }
         }
     }
     loc.iter() = first;
     throw syntax_error(format_underline("[error] toml::parse_inline_table: "
-            "inline table did not closed by `}`", loc, "should be closed"));
+            "inline table did not closed by `}`",
+            {{std::addressof(loc), "should be closed"}}));
 }
 
 template<typename Container>
@@ -1350,7 +1385,8 @@ result<value, std::string> parse_value(location<Container>& loc)
     const auto first = loc.iter();
     if(first == loc.end())
     {
-        return err(format_underline("[error] toml::parse_value: input is empty", loc, ""));
+        return err(format_underline("[error] toml::parse_value: input is empty",
+                   {{std::addressof(loc), ""}}));
     }
     if(auto r = parse_string         (loc))
     {return ok(value(std::move(r.unwrap().first), std::move(r.unwrap().second)));}
@@ -1374,7 +1410,7 @@ result<value, std::string> parse_value(location<Container>& loc)
     {return ok(value(std::move(r.unwrap().first), std::move(r.unwrap().second)));}
 
     const auto msg = format_underline("[error] toml::parse_value: "
-            "unknown token appeared", loc, "unknown");
+            "unknown token appeared", {{std::addressof(loc), "unknown"}});
     loc.iter() = first;
     return err(msg);
 }
@@ -1391,7 +1427,8 @@ parse_table_key(location<Container>& loc)
         if(!open || inner_loc.iter() == inner_loc.end())
         {
             throw internal_error(format_underline("[error] "
-                "toml::parse_table_key: no `[`", inner_loc, "should be `[`"));
+                "toml::parse_table_key: no `[`",
+                {{std::addressof(inner_loc), "should be `[`"}}));
         }
         // to skip [ a . b . c ]
         //          ^----------- this whitespace
@@ -1400,7 +1437,8 @@ parse_table_key(location<Container>& loc)
         if(!keys)
         {
             throw internal_error(format_underline("[error] "
-                "toml::parse_table_key: invalid key", inner_loc, "not key"));
+                "toml::parse_table_key: invalid key",
+                {{std::addressof(inner_loc), "not key"}}));
         }
         // to skip [ a . b . c ]
         //                    ^-- this whitespace
@@ -1409,7 +1447,8 @@ parse_table_key(location<Container>& loc)
         if(!close)
         {
             throw internal_error(format_underline("[error] "
-                "toml::parse_table_key: no `]`", inner_loc, "should be `]`"));
+                "toml::parse_table_key: no `]`",
+                {{std::addressof(inner_loc), "should be `]`"}}));
         }
 
         // after [table.key], newline or EOF(empty table) requried.
@@ -1422,7 +1461,7 @@ parse_table_key(location<Container>& loc)
             {
                 throw syntax_error(format_underline("[error] "
                     "toml::parse_table_key: newline required after [table.key]",
-                    loc, "expected newline"));
+                    {{std::addressof(loc), "expected newline"}}));
             }
         }
         return ok(std::make_pair(keys.unwrap().first, token.unwrap()));
@@ -1445,23 +1484,24 @@ parse_array_table_key(location<Container>& loc)
         if(!open || inner_loc.iter() == inner_loc.end())
         {
             throw internal_error(format_underline("[error] "
-                "toml::parse_array_table_key: no `[[`", inner_loc,
-                "should be `[[`"));
+                "toml::parse_array_table_key: no `[[`",
+                {{std::addressof(inner_loc), "should be `[[`"}}));
         }
         lex_ws::invoke(inner_loc);
         const auto keys = parse_key(inner_loc);
         if(!keys)
         {
             throw internal_error(format_underline("[error] "
-                "toml::parse_array_table_key: invalid key", inner_loc,
-                "not key"));
+                "toml::parse_array_table_key: invalid key",
+                {{std::addressof(inner_loc), "not a key"}}));
         }
         lex_ws::invoke(inner_loc);
         const auto close = lex_array_table_close::invoke(inner_loc);
         if(!close)
         {
             throw internal_error(format_underline("[error] "
-                "toml::parse_table_key: no `]]`", inner_loc, "should be `]]`"));
+                "toml::parse_table_key: no `]]`",
+                {{std::addressof(inner_loc), "should be `]]`"}}));
         }
 
         // after [[table.key]], newline or EOF(empty table) requried.
@@ -1472,9 +1512,9 @@ parse_array_table_key(location<Container>& loc)
             const auto nl = lex_newline_after_table_key::invoke(loc);
             if(!nl)
             {
-                throw syntax_error(format_underline("[error] "
-                    "toml::parse_array_table_key: newline required after "
-                    "[[table.key]]", loc, "expected newline"));
+                throw syntax_error(format_underline("[error] toml::"
+                    "parse_array_table_key: newline required after [[table.key]]",
+                    {{std::addressof(loc), "expected newline"}}));
             }
         }
         return ok(std::make_pair(keys.unwrap().first, token.unwrap()));
@@ -1550,8 +1590,8 @@ result<table, std::string> parse_ml_table(location<Container>& loc)
             const auto before = loc.iter();
             lex_ws::invoke(loc); // skip whitespace
             const auto msg = format_underline("[error] toml::parse_table: "
-                "invalid line format", loc, concat_to_string(
-                "expected newline, but got '", show_char(*loc.iter()), "'."));
+                "invalid line format", {{std::addressof(loc), concat_to_string(
+                "expected newline, but got '", show_char(*loc.iter()), "'.")}});
             loc.iter() = before;
             return err(msg);
         }
@@ -1621,7 +1661,7 @@ result<table, std::string> parse_toml_file(location<Container>& loc)
             continue;
         }
         return err(format_underline("[error]: toml::parse_toml_file: "
-                "unknown line appeared", loc, "unknown format"));
+            "unknown line appeared", {{std::addressof(loc), "unknown format"}}));
     }
     return ok(data);
 }
diff --git a/toml/value.hpp b/toml/value.hpp
index 03812c7..27c2d8a 100644
--- a/toml/value.hpp
+++ b/toml/value.hpp
@@ -681,9 +681,11 @@ typename detail::toml_default_type<T>::type& value::cast() &
 {
     if(T != this->type_)
     {
-        throw type_error(format_underline(concat_to_string(
-            "[error] toml::value bad_cast to ", T), *region_info_,
-            concat_to_string("the actual type is ", this->type_)));
+        throw type_error(detail::format_underline(concat_to_string(
+            "[error] toml::value bad_cast to ", T), {
+                {this->region_info_.get(),
+                 concat_to_string("the actual type is ", this->type_)}
+            }));
     }
     return switch_cast<T>::invoke(*this);
 }
@@ -692,9 +694,11 @@ typename detail::toml_default_type<T>::type const& value::cast() const&
 {
     if(T != this->type_)
     {
-        throw type_error(format_underline(concat_to_string(
-            "[error] toml::value bad_cast to ", T), *region_info_,
-            concat_to_string("the actual type is ", this->type_)));
+        throw type_error(detail::format_underline(concat_to_string(
+            "[error] toml::value bad_cast to ", T), {
+                {this->region_info_.get(),
+                 concat_to_string("the actual type is ", this->type_)}
+            }));
     }
     return switch_cast<T>::invoke(*this);
 }
@@ -703,9 +707,11 @@ typename detail::toml_default_type<T>::type&&      value::cast() &&
 {
     if(T != this->type_)
     {
-        throw type_error(format_underline(concat_to_string(
-            "[error] toml::value bad_cast to ", T), *region_info_,
-            concat_to_string("the actual type is ", this->type_)));
+        throw type_error(detail::format_underline(concat_to_string(
+            "[error] toml::value bad_cast to ", T), {
+                {this->region_info_.get(),
+                 concat_to_string("the actual type is ", this->type_)}
+             }));
     }
     return switch_cast<T>::invoke(std::move(*this));
 }
@@ -792,8 +798,9 @@ inline std::string format_error(const std::string& err_msg,
         const toml::value& v, const std::string& comment,
         std::vector<std::string> hints = {})
 {
-    return detail::format_underline(err_msg, detail::get_region(v), comment,
-                                    std::move(hints));
+    return detail::format_underline(err_msg, {
+            {std::addressof(detail::get_region(v)), comment}
+        }, std::move(hints));
 }
 
 inline std::string format_error(const std::string& err_msg,
@@ -801,9 +808,10 @@ inline std::string format_error(const std::string& err_msg,
         const toml::value& v2, const std::string& comment2,
         std::vector<std::string> hints = {})
 {
-    return detail::format_underline(err_msg, detail::get_region(v1), comment1,
-                                             detail::get_region(v2), comment2,
-                                             std::move(hints));
+    return detail::format_underline(err_msg, {
+            {std::addressof(detail::get_region(v1)), comment1},
+            {std::addressof(detail::get_region(v2)), comment2}
+        }, std::move(hints));
 }
 
 template<typename Visitor>

From 61dfa4a2dce2f8a328f7ec3d5c3d1e735b8049d8 Mon Sep 17 00:00:00 2001
From: ToruNiina <niina.toru.68u@gmail.com>
Date: Fri, 15 Mar 2019 12:38:37 +0900
Subject: [PATCH 3/4] feat: format any number of values into an err msg

```cpp
toml::format_error("[error] message", v1, "v1", v2, "v2", ...);
```
---
 toml/value.hpp | 42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/toml/value.hpp b/toml/value.hpp
index 27c2d8a..a866b04 100644
--- a/toml/value.hpp
+++ b/toml/value.hpp
@@ -794,24 +794,38 @@ inline bool operator>=(const toml::value& lhs, const toml::value& rhs)
     return !(lhs < rhs);
 }
 
-inline std::string format_error(const std::string& err_msg,
-        const toml::value& v, const std::string& comment,
-        std::vector<std::string> hints = {})
+namespace detail
 {
-    return detail::format_underline(err_msg, {
-            {std::addressof(detail::get_region(v)), comment}
-        }, std::move(hints));
+inline std::string format_error_impl(const std::string& err_msg,
+        std::vector<std::pair<region_base const*, std::string>> val,
+        std::vector<std::string> hints)
+{
+    return format_underline(err_msg, std::move(val), std::move(hints));
+}
+inline std::string format_error_impl(const std::string& err_msg,
+        std::vector<std::pair<region_base const*, std::string>> val)
+{
+    return format_underline(err_msg, std::move(val));
 }
 
-inline std::string format_error(const std::string& err_msg,
-        const toml::value& v1, const std::string& comment1,
-        const toml::value& v2, const std::string& comment2,
-        std::vector<std::string> hints = {})
+template<typename ... Ts>
+std::string format_error_impl(const std::string& err_msg,
+        std::vector<std::pair<region_base const*, std::string>> val,
+        const toml::value& v, const std::string& comment,
+        Ts&& ... args)
 {
-    return detail::format_underline(err_msg, {
-            {std::addressof(detail::get_region(v1)), comment1},
-            {std::addressof(detail::get_region(v2)), comment2}
-        }, std::move(hints));
+    val.push_back(std::make_pair(std::addressof(get_region(v)), comment));
+    return format_error_impl(err_msg, std::move(val), std::forward<Ts>(args)...);
+}
+} // detail
+
+template<typename ... Ts>
+std::string format_error(const std::string& err_msg, Ts&& ... args)
+{
+    std::vector<std::pair<detail::region_base const*, std::string>> val;
+    val.reserve(sizeof...(args) / 2);
+    return detail::format_error_impl(err_msg, std::move(val),
+                                     std::forward<Ts>(args)...);
 }
 
 template<typename Visitor>

From 59aaaab4360bb6c03d2b3b1c8d6ed7b04dfa61d2 Mon Sep 17 00:00:00 2001
From: ToruNiina <niina.toru.68u@gmail.com>
Date: Fri, 15 Mar 2019 12:40:01 +0900
Subject: [PATCH 4/4] test: add test to check format_error compiles

---
 tests/CMakeLists.txt        |  1 +
 tests/test_format_error.cpp | 75 +++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)
 create mode 100644 tests/test_format_error.cpp

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 0664150..8db2420 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -28,6 +28,7 @@ set(TEST_NAMES
     test_serialize_file
     test_parse_unicode
     test_error_detection
+    test_format_error
 )
 
 CHECK_CXX_COMPILER_FLAG("-Wall" COMPILER_SUPPORTS_WALL)
diff --git a/tests/test_format_error.cpp b/tests/test_format_error.cpp
new file mode 100644
index 0000000..36cb171
--- /dev/null
+++ b/tests/test_format_error.cpp
@@ -0,0 +1,75 @@
+#define BOOST_TEST_MODULE "test_value"
+#ifdef UNITTEST_FRAMEWORK_LIBRARY_EXIST
+#include <boost/test/unit_test.hpp>
+#else
+#define BOOST_TEST_NO_LIB
+#include <boost/test/included/unit_test.hpp>
+#endif
+#include <toml.hpp>
+
+// to check it successfully compiles. it does not check the formatted string.
+
+BOOST_AUTO_TEST_CASE(test_1_value)
+{
+    toml::value val(42);
+
+    {
+        const std::string pretty_error =
+            toml::format_error("[error] test error", val, "this is a value");
+        std::cout << pretty_error << std::endl;
+    }
+
+    {
+        const std::string pretty_error =
+            toml::format_error("[error] test error", val, "this is a value",
+                               std::vector<std::string>{"this is a hint"});
+        std::cout << pretty_error << std::endl;
+    }
+}
+
+BOOST_AUTO_TEST_CASE(test_2_values)
+{
+    toml::value v1(42);
+    toml::value v2(3.14);
+    {
+        const std::string pretty_error =
+            toml::format_error("[error] test error with two values",
+                               v1, "this is the answer",
+                               v2, "this is the pi");
+        std::cout << pretty_error << std::endl;
+    }
+
+    {
+        const std::string pretty_error =
+            toml::format_error("[error] test error with two values",
+                               v1, "this is the answer",
+                               v2, "this is the pi",
+                               std::vector<std::string>{"hint"});
+        std::cout << pretty_error << std::endl;
+    }
+}
+
+BOOST_AUTO_TEST_CASE(test_3_values)
+{
+    toml::value v1(42);
+    toml::value v2(3.14);
+    toml::value v3("foo");
+    {
+        const std::string pretty_error =
+            toml::format_error("[error] test error with two values",
+                               v1, "this is the answer",
+                               v2, "this is the pi",
+                               v3, "this is a meta-syntactic variable");
+        std::cout << pretty_error << std::endl;
+    }
+
+    {
+        const std::string pretty_error =
+            toml::format_error("[error] test error with two values",
+                               v1, "this is the answer",
+                               v2, "this is the pi",
+                               v3, "this is a meta-syntactic variable",
+                               std::vector<std::string>{"hint 1", "hint 2"});
+        std::cout << pretty_error << std::endl;
+    }
+}