diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ef5eed5..903272d 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -33,13 +33,16 @@ set(TEST_NAMES
     test_extended_conversions
 )
 
-CHECK_CXX_COMPILER_FLAG("-Wall" COMPILER_SUPPORTS_WALL)
+CHECK_CXX_COMPILER_FLAG("-Wall"      COMPILER_SUPPORTS_WALL)
+CHECK_CXX_COMPILER_FLAG("-Wextra"    COMPILER_SUPPORTS_WEXTRA)
 CHECK_CXX_COMPILER_FLAG("-Wpedantic" COMPILER_SUPPORTS_WPEDANTIC)
 
 if(COMPILER_SUPPORTS_WALL)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall")
 endif()
-
+if(COMPILER_SUPPORTS_WEXTRA)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wextra")
+endif()
 if(COMPILER_SUPPORTS_WPEDANTIC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wpedantic")
 endif()
diff --git a/tests/test_parse_file.cpp b/tests/test_parse_file.cpp
index e2d499b..518a98c 100644
--- a/tests/test_parse_file.cpp
+++ b/tests/test_parse_file.cpp
@@ -453,7 +453,7 @@ BOOST_AUTO_TEST_CASE(test_file_with_BOM)
             // value will be "\r\r\n". To avoid the additional "\r", use binary
             // mode.
             std::ofstream ofs("tmp.toml", std::ios_base::binary);
-            ofs.write(table.data(), table.size());
+            ofs.write(table.data(), static_cast<std::streamsize>(table.size()));
         }
         const auto data = toml::parse("tmp.toml");
 
diff --git a/toml/datetime.hpp b/toml/datetime.hpp
index f0ae405..dc52396 100644
--- a/toml/datetime.hpp
+++ b/toml/datetime.hpp
@@ -46,7 +46,7 @@ inline std::tm localtime_s(const std::time_t* src)
 #endif
 } // detail
 
-enum class month_t : std::int8_t
+enum class month_t : std::uint8_t
 {
     Jan =  0,
     Feb =  1,
@@ -98,9 +98,9 @@ struct local_date
         t.tm_sec   = 0;
         t.tm_min   = 0;
         t.tm_hour  = 0;
-        t.tm_mday  = this->day;
-        t.tm_mon   = this->month;
-        t.tm_year  = this->year - 1900;
+        t.tm_mday  = static_cast<int>(this->day);
+        t.tm_mon   = static_cast<int>(this->month);
+        t.tm_year  = static_cast<int>(this->year) - 1900;
         t.tm_wday  = 0; // the value will be ignored
         t.tm_yday  = 0; // the value will be ignored
         t.tm_isdst = -1;
@@ -188,22 +188,22 @@ struct local_time
     explicit local_time(const std::chrono::duration<Rep, Period>& t)
     {
         const auto h = std::chrono::duration_cast<std::chrono::hours>(t);
-        this->hour = h.count();
+        this->hour = static_cast<std::uint8_t>(h.count());
         const auto t2 = t - h;
         const auto m = std::chrono::duration_cast<std::chrono::minutes>(t2);
-        this->minute = m.count();
+        this->minute = static_cast<std::uint8_t>(m.count());
         const auto t3 = t2 - m;
         const auto s = std::chrono::duration_cast<std::chrono::seconds>(t3);
-        this->second = s.count();
+        this->second = static_cast<std::uint8_t>(s.count());
         const auto t4 = t3 - s;
         const auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(t4);
-        this->millisecond = ms.count();
+        this->millisecond = static_cast<std::uint16_t>(ms.count());
         const auto t5 = t4 - ms;
         const auto us = std::chrono::duration_cast<std::chrono::microseconds>(t5);
-        this->microsecond = us.count();
+        this->microsecond = static_cast<std::uint16_t>(us.count());
         const auto t6 = t5 - us;
         const auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(t6);
-        this->nanosecond = ns.count();
+        this->nanosecond = static_cast<std::uint16_t>(ns.count());
     }
 
     operator std::chrono::nanoseconds() const
diff --git a/toml/parser.hpp b/toml/parser.hpp
index 2d1d283..a7cf57e 100644
--- a/toml/parser.hpp
+++ b/toml/parser.hpp
@@ -253,6 +253,11 @@ std::string read_utf8_codepoint(const region<Container>& reg,
     std::istringstream iss(str);
     iss >> std::hex >> codepoint;
 
+    const auto to_char = [](const int i) noexcept -> char {
+        const auto uc = static_cast<unsigned char>(i);
+        return *reinterpret_cast<const char*>(std::addressof(uc));
+    };
+
     std::string character;
     if(codepoint < 0x80) // U+0000 ... U+0079 ; just an ASCII.
     {
@@ -261,8 +266,8 @@ std::string read_utf8_codepoint(const region<Container>& reg,
     else if(codepoint < 0x800) //U+0080 ... U+07FF
     {
         // 110yyyyx 10xxxxxx; 0x3f == 0b0011'1111
-        character += static_cast<unsigned char>(0xC0| codepoint >> 6);
-        character += static_cast<unsigned char>(0x80|(codepoint & 0x3F));
+        character += to_char(0xC0| codepoint >> 6);
+        character += to_char(0x80|(codepoint & 0x3F));
     }
     else if(codepoint < 0x10000) // U+0800...U+FFFF
     {
@@ -276,17 +281,17 @@ std::string read_utf8_codepoint(const region<Container>& reg,
         }
         assert(codepoint < 0xD800 || 0xDFFF < codepoint);
         // 1110yyyy 10yxxxxx 10xxxxxx
-        character += static_cast<unsigned char>(0xE0| codepoint >> 12);
-        character += static_cast<unsigned char>(0x80|(codepoint >> 6 & 0x3F));
-        character += static_cast<unsigned char>(0x80|(codepoint      & 0x3F));
+        character += to_char(0xE0| codepoint >> 12);
+        character += to_char(0x80|(codepoint >> 6 & 0x3F));
+        character += to_char(0x80|(codepoint      & 0x3F));
     }
     else if(codepoint < 0x110000) // U+010000 ... U+10FFFF
     {
         // 11110yyy 10yyxxxx 10xxxxxx 10xxxxxx
-        character += static_cast<unsigned char>(0xF0| codepoint >> 18);
-        character += static_cast<unsigned char>(0x80|(codepoint >> 12 & 0x3F));
-        character += static_cast<unsigned char>(0x80|(codepoint >> 6  & 0x3F));
-        character += static_cast<unsigned char>(0x80|(codepoint       & 0x3F));
+        character += to_char(0xF0| codepoint >> 18);
+        character += to_char(0x80|(codepoint >> 12 & 0x3F));
+        character += to_char(0x80|(codepoint >> 6  & 0x3F));
+        character += to_char(0x80|(codepoint       & 0x3F));
     }
     else // out of UTF-8 region
     {
@@ -655,9 +660,9 @@ parse_local_time(location<Container>& loc)
                 {{std::addressof(inner_loc), "here"}}));
         }
         local_time time(
-            static_cast<std::int8_t>(from_string<int>(h.unwrap().str(), 0)),
-            static_cast<std::int8_t>(from_string<int>(m.unwrap().str(), 0)),
-            static_cast<std::int8_t>(from_string<int>(s.unwrap().str(), 0)), 0, 0);
+            from_string<int>(h.unwrap().str(), 0),
+            from_string<int>(m.unwrap().str(), 0),
+            from_string<int>(s.unwrap().str(), 0), 0, 0);
 
         const auto before_secfrac = inner_loc.iter();
         if(const auto secfrac = lex_time_secfrac::invoke(inner_loc))
@@ -673,13 +678,13 @@ parse_local_time(location<Container>& loc)
             }
             if(sf.size() >= 6)
             {
-                time.millisecond = from_string<std::int16_t>(sf.substr(0, 3), 0);
-                time.microsecond = from_string<std::int16_t>(sf.substr(3, 3), 0);
+                time.millisecond = from_string<std::uint16_t>(sf.substr(0, 3), 0u);
+                time.microsecond = from_string<std::uint16_t>(sf.substr(3, 3), 0u);
             }
             else if(sf.size() >= 3)
             {
-                time.millisecond = from_string<std::int16_t>(sf, 0);
-                time.microsecond = 0;
+                time.millisecond = from_string<std::uint16_t>(sf, 0u);
+                time.microsecond = 0u;
             }
         }
         else
@@ -1934,7 +1939,8 @@ parse(std::istream& is, const std::string& fname = "unknown file")
     is.seekg(beg);
 
     // read whole file as a sequence of char
-    std::vector<char> letters(fsize);
+    assert(fsize >= 0);
+    std::vector<char> letters(static_cast<std::size_t>(fsize));
     is.read(letters.data(), fsize);
 
     detail::location<std::vector<char>>
diff --git a/toml/region.hpp b/toml/region.hpp
index b6ac6e8..e5ed659 100644
--- a/toml/region.hpp
+++ b/toml/region.hpp
@@ -69,8 +69,9 @@ struct region_base
 template<typename Container>
 struct location final : public region_base
 {
-    using const_iterator = typename Container::const_iterator;
-    using source_ptr     = std::shared_ptr<const Container>;
+    using const_iterator  = typename Container::const_iterator;
+    using difference_type = typename const_iterator::difference_type;
+    using source_ptr      = std::shared_ptr<const Container>;
 
     static_assert(std::is_same<char, typename Container::value_type>::value,"");
     static_assert(std::is_same<std::random_access_iterator_tag,
@@ -103,15 +104,17 @@ struct location final : public region_base
     // to the location changes the point to look. So an overload of `iter()`
     // which returns mutable reference is removed and `advance()`, `retrace()`
     // and `reset()` is added.
-    void advance(std::size_t n = 1) noexcept
+    void advance(difference_type n = 1) noexcept
     {
-        this->line_number_ += std::count(this->iter_, this->iter_ + n, '\n');
+        this->line_number_ += static_cast<std::size_t>(
+                std::count(this->iter_, std::next(this->iter_, n), '\n'));
         this->iter_ += n;
         return;
     }
-    void retrace(std::size_t n = 1) noexcept
+    void retrace(difference_type n = 1) noexcept
     {
-        this->line_number_ -= std::count(this->iter_ - n, this->iter_, '\n');
+        this->line_number_ -= static_cast<std::size_t>(
+                std::count(std::prev(this->iter_, n), this->iter_, '\n'));
         this->iter_ -= n;
         return;
     }
@@ -121,11 +124,13 @@ struct location final : public region_base
         // iterators and returns a negative value if `first > last`.
         if(0 <= std::distance(rollback, this->iter_)) // rollback < iter
         {
-            this->line_number_ -= std::count(rollback, this->iter_, '\n');
+            this->line_number_ -= static_cast<std::size_t>(
+                    std::count(rollback, this->iter_, '\n'));
         }
         else // iter < rollback [[unlikely]]
         {
-            this->line_number_ += std::count(this->iter_, rollback, '\n');
+            this->line_number_ += static_cast<std::size_t>(
+                    std::count(this->iter_, rollback, '\n'));
         }
         this->iter_ = rollback;
         return;
@@ -162,11 +167,15 @@ struct location final : public region_base
     }
     std::size_t before() const noexcept override
     {
-        return std::distance(this->line_begin(), this->iter());
+        const auto sz = std::distance(this->line_begin(), this->iter());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
     }
     std::size_t after() const noexcept override
     {
-        return std::distance(this->iter(), this->line_end());
+        const auto sz = std::distance(this->iter(), this->line_end());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
     }
 
     source_ptr const& source() const& noexcept {return source_;}
@@ -250,15 +259,21 @@ struct region final : public region_base
 
     std::size_t size() const noexcept override
     {
-        return std::distance(first_, last_);
+        const auto sz = std::distance(first_, last_);
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
     }
     std::size_t before() const noexcept override
     {
-        return std::distance(this->line_begin(), this->first());
+        const auto sz = std::distance(this->line_begin(), this->first());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
     }
     std::size_t after() const noexcept override
     {
-        return std::distance(this->last(), this->line_end());
+        const auto sz = std::distance(this->last(), this->line_end());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
     }
 
     bool contain_newline() const noexcept
@@ -409,13 +424,14 @@ inline std::string format_underline(const std::string& message,
 {
     assert(!reg_com.empty());
 
-    const auto line_num_width = std::max_element(reg_com.begin(), reg_com.end(),
+    const auto line_num_width = static_cast<int>(std::max_element(
+        reg_com.begin(), reg_com.end(),
         [](std::pair<region_base const*, std::string> const& lhs,
            std::pair<region_base const*, std::string> const& rhs)
         {
             return lhs.first->line_num().size() < rhs.first->line_num().size();
         }
-    )->first->line_num().size();
+    )->first->line_num().size());
 
     std::ostringstream retval;
     retval << message << '\n';
@@ -438,7 +454,7 @@ inline std::string format_underline(const std::string& message,
 
         retval << ' ' << std::setw(line_num_width) << reg->line_num();
         retval << " | " << reg->line() << '\n';
-        retval << make_string(line_num_width + 1, ' ');
+        retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ');
         retval << " | " << make_string(reg->before(), ' ');
 
         if(reg->size() == 1)
@@ -462,7 +478,7 @@ inline std::string format_underline(const std::string& message,
     if(!helps.empty())
     {
         retval << '\n';
-        retval << make_string(line_num_width + 1, ' ');
+        retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ');
         retval << " | ";
         for(const auto help : helps)
         {
diff --git a/toml/serializer.hpp b/toml/serializer.hpp
index cba2923..4711236 100644
--- a/toml/serializer.hpp
+++ b/toml/serializer.hpp
@@ -29,7 +29,7 @@ struct serializer
     using array_type           = typename value_type::array_type          ;
     using table_type           = typename value_type::table_type          ;
 
-    serializer(const std::size_t w              = 80,
+    serializer(const std::size_t w              = 80u,
                const int         float_prec     = std::numeric_limits<toml::floating>::max_digits10,
                const bool        can_be_inlined = false,
                std::vector<toml::key> ks        = {})
@@ -50,7 +50,8 @@ struct serializer
     {
         const auto fmt = "%.*g";
         const auto bsz = std::snprintf(nullptr, 0, fmt, this->float_prec_, f);
-        std::vector<char> buf(bsz + 1, '\0'); // +1 for null character(\0)
+        // +1 for null character(\0)
+        std::vector<char> buf(static_cast<std::size_t>(bsz + 1), '\0');
         std::snprintf(buf.data(), buf.size(), fmt, this->float_prec_, f);
 
         std::string token(buf.begin(), std::prev(buf.end()));
@@ -58,16 +59,27 @@ struct serializer
         {
             token += '0';
         }
-        const auto e = std::find_if(token.cbegin(), token.cend(),
-            [](const char c) -> bool {
-                return c == 'E' || c == 'e';
+
+        const auto e = std::find_if(
+            token.cbegin(), token.cend(), [](const char c) noexcept -> bool {
+                return c == 'e' || c == 'E';
             });
-        if(e == token.cend())
+        const auto has_exponent = (token.cend() != e);
+        const auto has_fraction = (token.cend() != std::find(
+            token.cbegin(), token.cend(), '.'));
+
+        if(!has_exponent && !has_fraction)
+        {
+            // the resulting value does not have any float specific part!
+            token += ".0";
+            return token;
+        }
+        if(!has_exponent)
         {
             return token; // there is no exponent part. just return it.
         }
 
-        // zero-prefix in an exponent is not allowed in TOML.
+        // zero-prefix in an exponent is NOT allowed in TOML.
         // remove it if it exists.
         bool        sign_exists = false;
         std::size_t zero_prefix = 0;
@@ -81,7 +93,8 @@ struct serializer
         {
             const auto offset = std::distance(token.cbegin(), e) +
                                (sign_exists ? 2 : 1);
-            token.erase(offset, zero_prefix);
+            token.erase(static_cast<typename std::string::size_type>(offset),
+                        zero_prefix);
         }
         return token;
     }
@@ -609,7 +622,7 @@ struct serializer
 template<typename C,
          template<typename ...> class M, template<typename ...> class V>
 std::string
-format(const basic_value<C, M, V>& v, std::size_t w = 80,
+format(const basic_value<C, M, V>& v, std::size_t w = 80u,
        int fprec = std::numeric_limits<toml::floating>::max_digits10,
        bool force_inline = false)
 {
@@ -638,8 +651,8 @@ std::basic_ostream<charT, traits>&
 operator<<(std::basic_ostream<charT, traits>& os, const basic_value<C, M, V>& v)
 {
     // get status of std::setw().
-    const std::size_t w     = os.width();
-    const int         fprec = os.precision();
+    const auto w     = static_cast<std::size_t>(os.width());
+    const int  fprec = static_cast<int>(os.precision());
     os.width(0);
 
     if(!v.comments().empty())
diff --git a/toml/source_location.hpp b/toml/source_location.hpp
index 48396a0..051f4e5 100644
--- a/toml/source_location.hpp
+++ b/toml/source_location.hpp
@@ -48,9 +48,9 @@ struct source_location
     {
         if(reg)
         {
-            line_num_    = std::stoul(reg->line_num());
-            column_num_  = reg->before() + 1;
-            region_size_ = reg->size();
+            line_num_    = static_cast<std::uint_least32_t>(std::stoul(reg->line_num()));
+            column_num_  = static_cast<std::uint_least32_t>(reg->before() + 1);
+            region_size_ = static_cast<std::uint_least32_t>(reg->size());
             file_name_   = reg->name();
             line_str_    = reg->line();
         }
diff --git a/toml/value.hpp b/toml/value.hpp
index 86fc51d..56bb948 100644
--- a/toml/value.hpp
+++ b/toml/value.hpp
@@ -601,7 +601,7 @@ class basic_value
         : type_(value_t::floating),
           region_info_(std::make_shared<region_base>(region_base{}))
     {
-        assigner(this->floating_, f);
+        assigner(this->floating_, static_cast<floating>(f));
     }
 
     template<typename T, typename Container, typename std::enable_if<
@@ -611,7 +611,7 @@ class basic_value
           region_info_(std::make_shared<detail::region<Container>>(std::move(reg))),
           comments_(region_info_->comments())
     {
-        assigner(this->floating_, f);
+        assigner(this->floating_, static_cast<floating>(f));
     }
 
     template<typename T, typename std::enable_if<
@@ -621,7 +621,7 @@ class basic_value
         this->cleanup();
         this->type_ = value_t::floating;
         this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->floating_, f);
+        assigner(this->floating_, static_cast<floating>(f));
         return *this;
     }