diff --git a/benchmark/benchmark_fmt.cpp b/benchmark/benchmark_fmt.cpp
new file mode 100644
index 0000000..0693203
--- /dev/null
+++ b/benchmark/benchmark_fmt.cpp
@@ -0,0 +1,69 @@
+#include <tuple>
+
+#include "benchmark/benchmark.h"
+
+#include "fmt/format.h"
+#include "fmt/chrono.h"
+
+#include "libimp/fmt.h"
+
+namespace {
+
+void imp_fmt_string(benchmark::State &state) {
+  for (auto _ : state) {
+    imp::fmt("hello world. hello world. hello world. hello world. hello world.");
+  }
+}
+
+void fmt_format_string(benchmark::State &state) {
+  for (auto _ : state) {
+    std::ignore = fmt::format("hello world. hello world. hello world. hello world. hello world.");
+  }
+}
+
+void imp_fmt_int(benchmark::State &state) {
+  for (auto _ : state) {
+    imp::fmt(654321);
+  }
+}
+
+void fmt_format_int(benchmark::State &state) {
+  for (auto _ : state) {
+    std::ignore = fmt::format("{}", 654321);
+  }
+}
+
+void imp_fmt_float(benchmark::State &state) {
+  for (auto _ : state) {
+    imp::fmt(654.321);
+  }
+}
+
+void fmt_format_float(benchmark::State &state) {
+  for (auto _ : state) {
+    std::ignore = fmt::format("{}", 654.321);
+  }
+}
+
+void imp_fmt_chrono(benchmark::State &state) {
+  for (auto _ : state) {
+    imp::fmt(std::chrono::system_clock::now());
+  }
+}
+
+void fmt_format_chrono(benchmark::State &state) {
+  for (auto _ : state) {
+    std::ignore = fmt::format("{}", std::chrono::system_clock::now());
+  }
+}
+
+} // namespace
+
+BENCHMARK(imp_fmt_string);
+BENCHMARK(fmt_format_string);
+BENCHMARK(imp_fmt_int);
+BENCHMARK(fmt_format_int);
+BENCHMARK(imp_fmt_float);
+BENCHMARK(fmt_format_float);
+BENCHMARK(imp_fmt_chrono);
+BENCHMARK(fmt_format_chrono);
diff --git a/benchmark/benchmark_log.cpp b/benchmark/benchmark_log.cpp
index e7db362..9f5fe09 100644
--- a/benchmark/benchmark_log.cpp
+++ b/benchmark/benchmark_log.cpp
@@ -5,14 +5,14 @@
 
 namespace {
 
-void BM_imp_log_no_output(benchmark::State& state) {
+void imp_log_no_output(benchmark::State &state) {
   imp::log::grip log {__func__, {}};
   for (auto _ : state) {
     log.debug("hello log.");
   }
 }
 
-void BM_imp_log_gripper(benchmark::State& state) {
+void imp_log_gripper(benchmark::State &state) {
   imp::log::grip log {__func__, {}};
   for (auto _ : state) {
     log.info("hello log.");
@@ -21,5 +21,5 @@ void BM_imp_log_gripper(benchmark::State& state) {
 
 } // namespace
 
-BENCHMARK(BM_imp_log_no_output);
-BENCHMARK(BM_imp_log_gripper);
\ No newline at end of file
+BENCHMARK(imp_log_no_output);
+BENCHMARK(imp_log_gripper);
\ No newline at end of file
diff --git a/include/libimp/fmt.h b/include/libimp/fmt.h
index 67f32cc..89dc7b7 100644
--- a/include/libimp/fmt.h
+++ b/include/libimp/fmt.h
@@ -3,6 +3,9 @@
  * @author mutouyun (orz@orzz.org)
  * @brief String formatting.
  * @date 2022-11-26
+ * 
+ * @remarks The current performance is not high, 
+ * because I use std::sprintf directly for formatting for convenience.
  */
 #pragma once
 
@@ -11,7 +14,7 @@
 #include <type_traits>
 #include <chrono>   // std::chrono::time_point
 #include <cstddef>
-#include <ctime>    // std::tm
+#include <ctime>    // std::tm, std::localtime
 
 #include "libimp/def.h"
 #include "libimp/fmt_cpo.h"
@@ -21,12 +24,23 @@
 
 LIBIMP_NAMESPACE_BEG_
 
+/**
+ * @brief The format string reference wrapper.
+ */
 template <typename T>
 struct fmt_ref {
   span<char const> fstr;
   T param;
 };
 
+/**
+ * @brief Conversion specifiers.
+ * 
+ * @remarks Just like printf, the format string is of the form
+ * [flags][field_width][.precision][conversion_character]
+ * 
+ * @see http://personal.ee.surrey.ac.uk/Personal/R.Bowden/C/printf.html
+ */
 template <std::size_t N>
 auto spec(char const (&fstr)[N]) noexcept {
   return [&fstr](auto &&arg) noexcept {
@@ -35,9 +49,16 @@ auto spec(char const (&fstr)[N]) noexcept {
   };
 }
 
-template <typename... A>
-std::string fmt(A &&...args) {
-  std::string joined;
+/// @brief String formatting function.
+
+template <typename A>
+std::string fmt(A &&a) {
+  return fmt_to_string(std::forward<A>(a));
+}
+
+template <typename A1, typename... A>
+std::string fmt(A1 &&a1, A &&...args) {
+  std::string joined(fmt(std::forward<A1>(a1)));
   LIBIMP_UNUSED auto unfold = {
     joined.append(fmt_to_string(std::forward<A>(args)))...
   };
@@ -45,24 +66,26 @@ std::string fmt(A &&...args) {
 }
 
 /// @brief Return the string directly.
-LIBIMP_EXPORT std::string to_string(std::string const &a) noexcept;
-LIBIMP_EXPORT std::string to_string(std::string &&a) noexcept;
-LIBIMP_EXPORT std::string to_string(std::string const &a, span<char const> fstr) noexcept;
+inline char const *to_string(char const *a) noexcept { return (a == nullptr) ? "" : a; }
+template <std::size_t N>
+inline char const *to_string(char const (&a)[N]) noexcept { return a; }
+inline std::string to_string(std::string const &a) noexcept { return a; }
+inline std::string to_string(std::string &&a) noexcept { return std::move(a); }
+LIBIMP_EXPORT std::string to_string(char const *a, span<char const> fstr) noexcept;
+inline std::string to_string(std::string const &a, span<char const> fstr) noexcept { return to_string(a.c_str(), fstr); }
 
 /// @brief Character to string conversion.
 /// @return an empty string if the conversion fails
-LIBIMP_EXPORT std::string to_string(char a) noexcept;
+inline std::string to_string(char a) noexcept { return {a}; }
 LIBIMP_EXPORT std::string to_string(wchar_t a) noexcept;
 LIBIMP_EXPORT std::string to_string(char16_t a) noexcept;
 LIBIMP_EXPORT std::string to_string(char32_t a) noexcept;
 #if defined(LIBIMP_CPP_20)
-LIBIMP_EXPORT std::string to_string(char8_t a) noexcept;
+LIBIMP_EXPORT std::string to_string(char8_t a) noexcept { return to_string((char)a); }
 #endif // defined(LIBIMP_CPP_20)
 
 /// @brief Conversion of numeric types to strings.
 /// @return an empty string if the conversion fails
-LIBIMP_EXPORT std::string to_string(signed char a, span<char const> fstr = {}) noexcept;
-LIBIMP_EXPORT std::string to_string(unsigned char a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(signed short a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(unsigned short a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(signed int a, span<char const> fstr = {}) noexcept;
@@ -71,26 +94,47 @@ LIBIMP_EXPORT std::string to_string(signed long a, span<char const> fstr = {}) n
 LIBIMP_EXPORT std::string to_string(unsigned long a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(signed long long a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(unsigned long long a, span<char const> fstr = {}) noexcept;
+inline std::string to_string(  signed char a, span<char const> fstr = {}) noexcept { return to_string(     (int)a, fstr); }
+inline std::string to_string(unsigned char a, span<char const> fstr = {}) noexcept { return to_string((unsigned)a, fstr); }
 
 /// @brief Conversion of floating point type to strings.
 /// @return an empty string if the conversion fails
-LIBIMP_EXPORT std::string to_string(float a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(double a, span<char const> fstr = {}) noexcept;
 LIBIMP_EXPORT std::string to_string(long double a, span<char const> fstr = {}) noexcept;
+inline std::string to_string(float a, span<char const> fstr = {}) noexcept { return to_string((double)a, fstr); }
 
 /// @brief Pointer.
-LIBIMP_EXPORT std::string to_string(std::nullptr_t) noexcept;
+inline std::string to_string(std::nullptr_t) noexcept { return "null"; }
 template <typename T,
           typename = std::enable_if_t<std::is_same<T, void>::value>>
 LIBIMP_EXPORT std::string to_string(T const volatile *a) noexcept;
 
 /// @brief Date and time.
 LIBIMP_EXPORT std::string to_string(std::tm const &a, span<char const> fstr = {}) noexcept;
+
 namespace detail {
-LIBIMP_EXPORT std::string time_to_string(std::time_t tt, span<char const> fstr) noexcept;
+
+/**
+ * @brief Convert std::time_t to std::string.
+ * @return an empty string if the conversion fails
+ */
+inline std::string time_to_string(std::time_t tt, span<char const> fstr) noexcept {
+#if defined(LIBIMP_CC_MSVC)
+  /// @see https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
+  std::tm tm {};
+  if (::localtime_s(&tm, &tt) != 0) {
+    return {};
+  }
+  return to_string(tm, fstr);
+#else
+  return to_string(*std::localtime(&tt), fstr);
+#endif
+}
+
 } // namespace detail
+
 template <class Clock, class Duration>
-LIBIMP_EXPORT std::string to_string(std::chrono::time_point<Clock, Duration> const &a, span<char const> fstr = {}) noexcept {
+std::string to_string(std::chrono::time_point<Clock, Duration> const &a, span<char const> fstr = {}) noexcept {
   return detail::time_to_string(std::chrono::system_clock::to_time_t(a), fstr);
 }
 
diff --git a/src/libimp/fmt.cpp b/src/libimp/fmt.cpp
index 613f3ad..5a5a8e1 100644
--- a/src/libimp/fmt.cpp
+++ b/src/libimp/fmt.cpp
@@ -1,4 +1,3 @@
-#define _CRT_SECURE_NO_WARNINGS
 #include "libimp/fmt.h"
 
 #include <cstdio>   // std::snprintf
@@ -98,21 +97,9 @@ std::string sprintf(F fop, span<char const> fstr, span<char const> s, A a) noexc
 
 } // namespace
 
-std::string to_string(std::string const &a) noexcept {
-  return a;
-}
-
-std::string to_string(std::string &&a) noexcept {
-  return std::move(a);
-}
-
-std::string to_string(std::string const &a, span<char const> fstr) noexcept {
-  if (a.empty()) return {};
-  return ::LIBIMP::sprintf(fmt_of, fstr, "s", a.c_str());
-}
-
-std::string to_string(char a) noexcept {
-  return {a};
+std::string to_string(char const *a, span<char const> fstr) noexcept {
+  if (a == nullptr) return {};
+  return ::LIBIMP::sprintf(fmt_of, fstr, "s", a);
 }
 
 std::string to_string(wchar_t a) noexcept {
@@ -145,20 +132,6 @@ std::string to_string(char32_t a) noexcept {
   }
 }
 
-#if defined(LIBIMP_CPP_20)
-std::string to_string(char8_t a) noexcept {
-  return to_string((char)a);
-}
-#endif // defined(LIBIMP_CPP_20)
-
-std::string to_string(signed char a, span<char const> fstr) noexcept {
-  return to_string((int)a, fstr);
-}
-
-std::string to_string(unsigned char a, span<char const> fstr) noexcept {
-  return to_string((unsigned)a, fstr);
-}
-
 std::string to_string(signed short a, span<char const> fstr) noexcept {
   return ::LIBIMP::sprintf(fmt_of_signed, fstr, "h", a);
 }
@@ -191,10 +164,6 @@ std::string to_string(unsigned long long a, span<char const> fstr) noexcept {
   return ::LIBIMP::sprintf(fmt_of_unsigned, fstr, "ll", a);
 }
 
-std::string to_string(float a, span<char const> fstr) noexcept {
-  return to_string((double)a, fstr);
-}
-
 std::string to_string(double a, span<char const> fstr) noexcept {
   return ::LIBIMP::sprintf(fmt_of_float, fstr, "", a);
 }
@@ -203,10 +172,6 @@ std::string to_string(long double a, span<char const> fstr) noexcept {
   return ::LIBIMP::sprintf(fmt_of_float, fstr, "L", a);
 }
 
-std::string to_string(std::nullptr_t) noexcept {
-  return "null";
-}
-
 template <>
 std::string to_string<void, void>(void const volatile *a) noexcept {
   if (a == nullptr) {
@@ -228,12 +193,4 @@ std::string to_string(std::tm const &a, span<char const> fstr) noexcept {
   }
 }
 
-namespace detail {
-
-std::string time_to_string(std::time_t tt, span<char const> fstr) noexcept {
-  return to_string(*std::localtime(&tt), fstr);
-}
-
-} // namespace detail
-
 LIBIMP_NAMESPACE_END_