diff --git a/include/libimp/fmt.h b/include/libimp/fmt.h
new file mode 100644
index 0000000..9906982
--- /dev/null
+++ b/include/libimp/fmt.h
@@ -0,0 +1,95 @@
+/**
+ * @file libimp/fmt.h
+ * @author mutouyun (orz@orzz.org)
+ * @brief String formatting.
+ * @date 2022-11-26
+ */
+#pragma once
+
+#include <string>
+#include <utility>
+#include <type_traits>
+#include <chrono>   // std::chrono::time_point
+#include <cstddef>
+#include <ctime>    // std::tm
+
+#include "libimp/def.h"
+#include "libimp/span.h"
+#include "libimp/detect_plat.h"
+#include "libimp/export.h"
+
+LIBIMP_NAMESPACE_BEG_
+
+template <typename T>
+struct fmt_ref {
+  span<char const> fstr;
+  T param;
+};
+
+template <std::size_t N>
+auto spec(char const (&fstr)[N]) noexcept {
+  return [&fstr](auto &&arg) noexcept {
+    using arg_t = decltype(arg);
+    return fmt_ref<arg_t> {{fstr}, static_cast<arg_t>(arg)};
+  };
+}
+
+template <typename... A>
+std::string fmt(A &&...args) {
+  std::string joined;
+  LIBIMP_UNUSED auto unfold = {
+    joined.append(to_string(std::forward<A>(args)))...
+  };
+  return joined;
+}
+
+/// @brief Return the string directly.
+LIBIMP_EXPORT std::string const &to_string(std::string const &a) noexcept;
+LIBIMP_EXPORT std::string to_string(std::string const &a, span<char const> fstr) noexcept;
+
+/// @brief Character to string conversion.
+/// @return an empty string if the conversion fails
+LIBIMP_EXPORT std::string to_string(char a) noexcept;
+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;
+#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;
+LIBIMP_EXPORT std::string to_string(unsigned int a, span<char const> fstr = {}) noexcept;
+LIBIMP_EXPORT std::string to_string(signed long a, span<char const> fstr = {}) noexcept;
+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;
+
+/// @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;
+
+/// @brief Pointer.
+LIBIMP_EXPORT std::string to_string(std::nullptr_t) noexcept;
+template <typename T,
+          typename = std::enable_if_t<std::is_same<T, void>::value>>
+LIBIMP_EXPORT std::string to_string(T *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;
+} // 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 {
+  return detail::time_to_string(std::chrono::system_clock::to_time_t(a), fstr);
+}
+
+LIBIMP_NAMESPACE_END_
diff --git a/src/libimp/fmt.cpp b/src/libimp/fmt.cpp
new file mode 100644
index 0000000..f9da2e1
--- /dev/null
+++ b/src/libimp/fmt.cpp
@@ -0,0 +1,235 @@
+#define _CRT_SECURE_NO_WARNINGS
+#include "libimp/fmt.h"
+
+#include <cstdio>   // std::snprintf
+#include <iomanip>  // std::put_time
+#include <sstream>  // std::ostringstream
+
+#include "libimp/codecvt.h"
+
+LIBIMP_NAMESPACE_BEG_
+
+/**
+ * @brief Format conversions helpers.
+ * @see http://personal.ee.surrey.ac.uk/Personal/R.Bowden/C/printf.html
+ *      https://en.cppreference.com/w/cpp/io/c/fprintf
+ */
+namespace {
+
+span<char const> normalize(span<char const> a) {
+  if (a.empty()) return {};
+  return a.first(a.size() - (a.back() == '\0' ? 1 : 0));
+}
+
+std::string as_string(span<char const> a) {
+  if (a.empty()) return {};
+  a = normalize(a);
+  return std::string(a.data(), a.size());
+}
+
+std::string fmt_of(span<char const> fstr, span<char const> s) {
+  return '%' + as_string(fstr) + as_string(s);
+}
+
+std::string fmt_of_unsigned(span<char const> fstr, span<char const> l) {
+  if (fstr.empty()) {
+    return fmt_of(l, "u");
+  }
+  fstr = normalize(fstr);
+  switch (fstr.back()) {
+    case 'o':
+    case 'x':
+    case 'X':
+    case 'u': return fmt_of(fstr.first(fstr.size() - 1), l) + fstr.back();
+    default : return fmt_of(fstr, l) + 'u';
+  }
+}
+
+std::string fmt_of_signed(span<char const> fstr, span<char const> l) {
+  if (fstr.empty()) {
+    return fmt_of(l, "d");
+  }
+  fstr = normalize(fstr);
+  switch (fstr.back()) {
+    case 'o':
+    case 'x':
+    case 'X':
+    case 'u': return fmt_of_unsigned(fstr, l);
+    default : return fmt_of(fstr, l) + 'd';
+  }
+}
+
+std::string fmt_of_float(span<char const> fstr, span<char const> l) {
+  if (fstr.empty()) {
+    return fmt_of(l, "f");
+  }
+  fstr = normalize(fstr);
+  switch (fstr.back()) {
+    case 'e':
+    case 'E':
+    case 'g':
+    case 'G': return fmt_of(fstr.first(fstr.size() - 1), l) + fstr.back();
+    default : return fmt_of(fstr, l) + 'f';
+  }
+}
+
+template <typename A /*a fundamental or pointer type*/>
+std::string sprintf(std::string const &sfmt, A a) {
+  auto sz = std::snprintf(nullptr, 0, sfmt.c_str(), a);
+  if (sz <= 0) return {};
+  std::string des;
+  des.resize(sz + 1);
+  if (std::snprintf(&des[0], des.size(), sfmt.c_str(), a) < 0) {
+    return {};
+  }
+  des.pop_back(); // remove the terminated null character
+  return des;
+}
+
+template <typename F /*a function pointer*/, 
+          typename A /*a fundamental or pointer type*/>
+std::string sprintf(F fop, span<char const> fstr, span<char const> s, A a) noexcept {
+  LIBIMP_TRY {
+    return ::LIBIMP::sprintf(fop(fstr, s), a);
+  } LIBIMP_CATCH(...) {
+    return {};
+  }
+}
+
+} // namespace
+
+std::string const &to_string(std::string const &a) noexcept {
+  return 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(wchar_t a) noexcept {
+  LIBIMP_TRY {
+    std::string des;
+    cvt_sstr(std::wstring{a}, des);
+    return des;
+  } LIBIMP_CATCH(...) {
+    return {};
+  }
+}
+
+std::string to_string(char16_t a) noexcept {
+  LIBIMP_TRY {
+    std::string des;
+    cvt_sstr(std::u16string{a}, des);
+    return des;
+  } LIBIMP_CATCH(...) {
+    return {};
+  }
+}
+
+std::string to_string(char32_t a) noexcept {
+  LIBIMP_TRY {
+    std::string des;
+    cvt_sstr(std::u32string{a}, des);
+    return des;
+  } LIBIMP_CATCH(...) {
+    return {};
+  }
+}
+
+#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);
+}
+
+std::string to_string(unsigned short a, span<char const> fstr) noexcept {
+  return ::LIBIMP::sprintf(fmt_of_unsigned, fstr, "h", a);
+}
+
+std::string to_string(signed int a, span<char const> fstr) noexcept {
+  return ::LIBIMP::sprintf(fmt_of_signed, fstr, "", a);
+}
+
+std::string to_string(unsigned int a, span<char const> fstr) noexcept {
+  return ::LIBIMP::sprintf(fmt_of_unsigned, fstr, "", a);
+}
+
+std::string to_string(signed long a, span<char const> fstr) noexcept {
+  return ::LIBIMP::sprintf(fmt_of_signed, fstr, "l", a);
+}
+
+std::string to_string(unsigned long a, span<char const> fstr) noexcept {
+  return ::LIBIMP::sprintf(fmt_of_unsigned, fstr, "l", a);
+}
+
+std::string to_string(signed long long a, span<char const> fstr) noexcept {
+  return ::LIBIMP::sprintf(fmt_of_signed, fstr, "ll", a);
+}
+
+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);
+}
+
+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 *a) noexcept {
+  if (a == nullptr) {
+    return to_string(nullptr);
+  }
+  return ::LIBIMP::sprintf(fmt_of, "", "p", a);
+}
+
+std::string to_string(std::tm const &a, span<char const> fstr) noexcept {
+  if (fstr.empty()) {
+    fstr = "%Y-%m-%d %H:%M:%S";
+  }
+  LIBIMP_TRY {
+    std::ostringstream ss;
+    ss << std::put_time(&a, as_string(fstr).c_str());
+    return ss.str();
+  } LIBIMP_CATCH(...) {
+    return {};
+  }
+}
+
+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_
diff --git a/test/imp/test_imp_fmt.cpp b/test/imp/test_imp_fmt.cpp
new file mode 100644
index 0000000..5ecb906
--- /dev/null
+++ b/test/imp/test_imp_fmt.cpp
@@ -0,0 +1,87 @@
+#define _CRT_SECURE_NO_WARNINGS
+#include <iostream>
+
+#include "gtest/gtest.h"
+
+#include "libimp/fmt.h"
+
+TEST(fmt, operator) {
+  auto a = imp::spec("hello")(123);
+  EXPECT_STREQ(a.fstr.data(), "hello");
+  EXPECT_EQ(a.param , 123);
+
+  auto b = imp::spec("hello")("world");
+  EXPECT_STREQ(b.fstr.data(), "hello");
+  EXPECT_STREQ(b.param , "world");
+}
+
+TEST(fmt, to_string) {
+  /// @brief string
+  EXPECT_EQ(imp::to_string(""), "");
+  EXPECT_EQ(imp::to_string("%what%"), "%what%");
+  EXPECT_EQ(imp::to_string("%what%", "10") , "    %what%");
+  EXPECT_EQ(imp::to_string("%what%", "-10"), "%what%    ");
+
+  /// @brief character
+  EXPECT_EQ(imp::to_string('A'), "A");
+  EXPECT_EQ(imp::to_string(L'A'), "A");
+  EXPECT_EQ(imp::to_string(u'A'), "A");
+  EXPECT_EQ(imp::to_string(U'A'), "A");
+
+  /// @brief numeric
+  EXPECT_EQ(imp::to_string((signed char)123), "123");
+  EXPECT_EQ(imp::to_string((signed char)-321), "-65");
+  EXPECT_EQ(imp::to_string((unsigned char)123), "123");
+  EXPECT_EQ(imp::to_string((unsigned char)321), "65");
+  EXPECT_EQ(imp::to_string((short)123), "123");
+  EXPECT_EQ(imp::to_string((short)-321), "-321");
+  EXPECT_EQ(imp::to_string((unsigned short)123), "123");
+  EXPECT_EQ(imp::to_string((unsigned short)321), "321");
+  EXPECT_EQ(imp::to_string((short)123123), "-7949");
+  EXPECT_EQ(imp::to_string((short)-321321), "6359");
+  EXPECT_EQ(imp::to_string((unsigned short)123123), "57587");
+  EXPECT_EQ(imp::to_string((unsigned short)321321), "59177");
+  EXPECT_EQ(imp::to_string(123123), "123123");
+  EXPECT_EQ(imp::to_string(-321321), "-321321");
+  EXPECT_EQ(imp::to_string(123123u), "123123");
+  EXPECT_EQ(imp::to_string(321321u), "321321");
+  EXPECT_EQ(imp::to_string(123123ll), "123123");
+  EXPECT_EQ(imp::to_string(-321321ll), "-321321");
+  EXPECT_EQ(imp::to_string(123123ull), "123123");
+  EXPECT_EQ(imp::to_string(321321ull), "321321");
+  EXPECT_EQ(imp::to_string(123123, "x"), "1e0f3");
+  EXPECT_EQ(imp::to_string(123123u, "x"), "1e0f3");
+  EXPECT_EQ(imp::to_string(123123123123ll, "X"), "1CAAB5C3B3");
+  EXPECT_EQ(imp::to_string(123123123123ull, "X"), "1CAAB5C3B3");
+
+  /// @brief floating point
+  EXPECT_EQ(imp::to_string(123.123f, ".3"), "123.123");
+  EXPECT_EQ(imp::to_string(123.123, "010.5"), "0123.12300");
+  EXPECT_EQ(imp::to_string(123.123l, "010.6"), "123.123000");
+  EXPECT_EQ(imp::to_string(1.5, "e"), "1.500000e+00");
+  EXPECT_EQ(imp::to_string(1.5, "E"), "1.500000E+00");
+  double r = 0.0;
+  std::cout << imp::to_string(0.0/r) << "\n";
+  std::cout << imp::to_string(1.0/r) << "\n";
+  SUCCEED();
+
+  /// @brief pointer
+  EXPECT_EQ(imp::to_string(nullptr), "null");
+  int *p = (int *)0x0f013a04;
+  std::cout << imp::to_string((void *)p) << "\n";
+  SUCCEED();
+
+  /// @brief date and time
+  auto tp = std::chrono::system_clock::now();
+  auto tt = std::chrono::system_clock::to_time_t(tp);
+  auto tm = *std::localtime(&tt);
+  std::cout << imp::to_string(tm) << "\n";
+  EXPECT_EQ(imp::to_string(tm), imp::to_string(tp));
+  SUCCEED();
+}
+
+TEST(fmt, fmt) {
+  auto s = imp::fmt("hello", " ", "world", ".");
+  EXPECT_EQ(s, "hello world.");
+  std::cout << imp::fmt('[', std::chrono::system_clock::now(), "] ", s) << "\n";
+}
\ No newline at end of file