refactor: use snprintf to show char in hex

instead of std::ostringstream.

toml/combinator.hpp

@@ -9,7 +9,10 @@
 #include <type_traits>
 #include <iterator>
 #include <limits>
+#include <array>
 #include <iomanip>
+#include <cstdio>
+#include <cassert>
 #include <cctype>
 
 // they scans characters and returns region if it matches to the condition.
@@ -38,10 +41,12 @@ inline std::string show_char(const char c)
     }
     else
     {
-        std::ostringstream oss;
+        std::array<char, 5> buf;
-        oss << "0x" << std::hex << std::setfill('0') << std::setw(2)
+        buf.fill('\0');
-            << static_cast<int>(c);
+        const auto r = std::snprintf(
-        return oss.str();
+                buf.data(), buf.size(), "0x%02x", static_cast<int>(c) & 0xFF);
+        assert(r == buf.size() - 1);
+        return std::string(buf.data());
     }
 }