diff --git a/include/libipc/imp/fmt.h b/include/libipc/imp/fmt.h
new file mode 100644
index 0000000..af0a5c4
--- /dev/null
+++ b/include/libipc/imp/fmt.h
@@ -0,0 +1,176 @@
+/**
+ * \file libipc/fmt.h
+ * \author mutouyun (orz@orzz.org)
+ * \brief String formatting.
+ * 
+ * \remarks The current performance is not high, 
+ * because I use std::sprintf directly for formatting for convenience.
+ */
+#pragma once
+
+#include <string>
+#include <utility>
+#include <type_traits>
+#include <chrono>   // std::chrono::time_point
+#include <tuple>
+#include <cstddef>
+#include <ctime>    // std::tm, std::localtime
+
+#include "libipc/imp/fmt_cpo.h"
+#include "libipc/imp/span.h"
+#include "libipc/imp/detect_plat.h"
+#include "libipc/imp/export.h"
+
+namespace ipc {
+
+/**
+ * \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 {
+    using arg_t = decltype(arg);
+    return fmt_ref<arg_t> {{fstr}, static_cast<arg_t>(arg)};
+  };
+}
+
+/**
+ * \brief String formatting function.
+ * 
+ * \param args arguments that support the fmt output
+ * \return an empty string if the fmt output fails 
+ */
+template <typename... A>
+LIBIPC_NODISCARD std::string fmt(A &&...args) {
+  std::string joined;
+  fmt_context ctx(joined);
+  if (fmt_to(ctx, std::forward<A>(args)...)) {
+    return ctx.finish() ? joined : "";
+  }
+  return {};
+}
+
+/// \brief String types.
+IPC_EXPORT bool to_string(fmt_context &ctx, char const *       a) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, std::string const &a) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, char const *       a, span<char const> fstr) noexcept;
+       inline bool to_string(fmt_context &ctx, std::string const &a, span<char const> fstr) noexcept { return to_string(ctx, a.c_str(), fstr); }
+
+/// \brief Character to string conversion.
+IPC_EXPORT bool to_string(fmt_context &ctx, char     a) noexcept;
+#if defined(LIBIPC_CPP_20)
+       inline bool to_string(fmt_context &ctx, char8_t  a) noexcept { return to_string(ctx, (char)a); }
+#endif // defined(LIBIPC_CPP_20)
+IPC_EXPORT bool to_string(fmt_context &ctx, wchar_t  a) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, char16_t a) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, char32_t a) noexcept;
+
+/// \brief Conversion of numeric types to strings.
+IPC_EXPORT bool to_string(fmt_context &ctx,   signed short     a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, unsigned short     a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx,   signed int       a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, unsigned int       a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx,   signed long      a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, unsigned long      a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx,   signed long long a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, unsigned long long a, span<char const> fstr = {}) noexcept;
+    inline bool to_string(fmt_context &ctx,   signed char      a, span<char const> fstr = {}) noexcept { return to_string(ctx,      (int)a, fstr); }
+    inline bool to_string(fmt_context &ctx, unsigned char      a, span<char const> fstr = {}) noexcept { return to_string(ctx, (unsigned)a, fstr); }
+
+/// \brief Conversion of floating point type to strings.
+IPC_EXPORT bool to_string(fmt_context &ctx,      double a, span<char const> fstr = {}) noexcept;
+IPC_EXPORT bool to_string(fmt_context &ctx, long double a, span<char const> fstr = {}) noexcept;
+       inline bool to_string(fmt_context &ctx, float       a, span<char const> fstr = {}) noexcept { return to_string(ctx, (double)a, fstr); }
+
+/// \brief Pointer.
+IPC_EXPORT bool to_string(fmt_context &ctx, std::nullptr_t) noexcept;
+template <typename T,
+          typename = std::enable_if_t<std::is_same<T, void>::value>>
+IPC_EXPORT bool to_string(fmt_context &ctx, T const volatile *a) noexcept;
+
+/// \brief Date and time.
+IPC_EXPORT bool to_string(fmt_context &ctx, std::tm const &a, span<char const> fstr = {}) noexcept;
+
+namespace detail_fmt {
+
+/**
+ * \brief Convert std::time_t to std::string.
+ * \return an empty string if the conversion fails
+ */
+inline bool time_to_string(fmt_context &ctx, std::time_t tt, span<char const> fstr) noexcept {
+#if defined(LIBIPC_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(ctx, tm, fstr);
+#else
+  return to_string(ctx, *std::localtime(&tt), fstr);
+#endif
+}
+
+} // namespace detail_fmt
+
+template <class Clock, class Duration>
+bool to_string(fmt_context &ctx, std::chrono::time_point<Clock, Duration> const &a, span<char const> fstr = {}) noexcept {
+  return detail_fmt::time_to_string(ctx, std::chrono::system_clock::to_time_t(a), fstr);
+}
+
+/**
+ * \brief Predefined fmt_to method
+ */
+namespace detail_tag_invoke {
+
+template <typename T>
+auto tag_invoke(decltype(ipc::fmt_to), fmt_context &ctx, T &&arg) noexcept
+  -> decltype(ipc::to_string(ctx, std::forward<T>(arg))) {
+  return ipc::to_string(ctx, std::forward<T>(arg));
+}
+
+template <typename T>
+auto tag_invoke(decltype(ipc::fmt_to), fmt_context &ctx, fmt_ref<T> arg) noexcept
+  -> decltype(ipc::to_string(ctx, static_cast<T>(arg.param), arg.fstr)) {
+  return ipc::to_string(ctx, static_cast<T>(arg.param), arg.fstr);
+}
+
+template <typename T>
+bool tag_invoke(decltype(ipc::fmt_to), fmt_context &ctx, span<T> s) {
+  if (s.empty()) {
+    return false;
+  }
+  if (!fmt_to(ctx, s[0])) {
+    return false;
+  }
+  for (std::size_t i = 1; i < s.size(); ++i) {
+    if (!fmt_to(ctx, ' ', s[i])) return false;
+  }
+  return true;
+}
+
+template <typename Tp, std::size_t... I>
+bool unfold_tuple_fmt_to(fmt_context &ctx, Tp const &tp, std::index_sequence<I...>) {
+  return fmt_to(ctx, std::get<I>(tp)...);
+}
+
+template <typename... T>
+bool tag_invoke(decltype(ipc::fmt_to), fmt_context &ctx, std::tuple<T...> const &tp) {
+  return unfold_tuple_fmt_to(ctx, tp, std::index_sequence_for<T...>{});
+}
+
+} // namespace detail_tag_invoke
+} // namespace ipc
diff --git a/include/libipc/imp/fmt_cpo.h b/include/libipc/imp/fmt_cpo.h
new file mode 100644
index 0000000..e7be313
--- /dev/null
+++ b/include/libipc/imp/fmt_cpo.h
@@ -0,0 +1,66 @@
+/**
+ * \file libipc/fmt_cpo.h
+ * \author mutouyun (orz@orzz.org)
+ * \brief String formatting CPO.
+ */
+#pragma once
+
+#include <string>
+#include <cstddef>
+#include <array>
+
+#include "libipc/imp/generic.h"
+#include "libipc/imp/detect_plat.h"
+#include "libipc/imp/span.h"
+#include "libipc/imp/export.h"
+
+namespace ipc {
+
+/**
+ * \class class IPC_EXPORT fmt_context
+ * \brief The context of fmt.
+ */
+class IPC_EXPORT fmt_context {
+  std::array<char, 2048U> sbuf_; ///< stack buffer
+
+  std::string &joined_;
+  std::size_t  offset_;
+
+public:
+  fmt_context(std::string &j) noexcept;
+
+  std::size_t capacity() noexcept;
+  void reset() noexcept;
+  bool finish() noexcept;
+  span<char> buffer(std::size_t sz) noexcept;
+  void expend(std::size_t sz) noexcept;
+  bool append(span<char const> const &str) noexcept;
+};
+
+/// \brief Supports custom fmt_to methods for ipc::fmt.
+namespace detail_tag_invoke {
+
+class fmt_to_t {
+  template <typename A1>
+  bool get_result(fmt_context &ctx, A1 && a1) const {
+    return ipc::tag_invoke(fmt_to_t{}, ctx, std::forward<A1>(a1));
+  }
+
+  template <typename A1, typename... A>
+  bool get_result(fmt_context &ctx, A1 && a1, A &&...args) const {
+    return get_result(ctx, std::forward<A1>(a1))
+        && get_result(ctx, std::forward<A>(args)...);
+  }
+
+public:
+  template <typename... A>
+  bool operator()(fmt_context &ctx, A &&...args) const {
+    return get_result(ctx, std::forward<A>(args)...);
+  }
+};
+
+} // namespace detail_tag_invoke
+
+constexpr detail_tag_invoke::fmt_to_t fmt_to{};
+
+} // namespace ipc
diff --git a/include/libipc/imp/generic.h b/include/libipc/imp/generic.h
index e31e404..50184cb 100644
--- a/include/libipc/imp/generic.h
+++ b/include/libipc/imp/generic.h
@@ -44,7 +44,7 @@ constexpr in_place_t in_place {};
  * \brief A general pattern for supporting customisable functions
  * \see https://www.open-std.org/jtc1/sc22/WG21/docs/papers/2019/p1895r0.pdf
  */
-namespace detail {
+namespace detail_tag_invoke {
 
 void tag_invoke();
 
@@ -57,15 +57,15 @@ struct tag_invoke_t {
   }
 };
 
-} // namespace detail
+} // namespace detail_tag_invoke
 
-constexpr detail::tag_invoke_t tag_invoke {};
+constexpr detail_tag_invoke::tag_invoke_t tag_invoke{};
 
 /**
  * \brief Circumventing forwarding reference may override copy and move constructs.
  * \see https://mpark.github.io/programming/2014/06/07/beware-of-perfect-forwarding-constructors/
  */
-namespace detail {
+namespace detail_not_match {
 
 template <typename T, typename... A>
 struct is_same_first : std::false_type {};
@@ -73,11 +73,11 @@ struct is_same_first : std::false_type {};
 template <typename T>
 struct is_same_first<T, T> : std::true_type {};
 
-} // namespace detail
+} // namespace detail_not_match
 
 template <typename T, typename... A>
 using not_match = 
-  typename std::enable_if<!detail::is_same_first<T, 
+  typename std::enable_if<!detail_not_match::is_same_first<T, 
   typename std::decay<A>::type...>::value, bool>::type;
 
 /**
diff --git a/src/libipc/imp/fmt.cpp b/src/libipc/imp/fmt.cpp
new file mode 100644
index 0000000..deb344c
--- /dev/null
+++ b/src/libipc/imp/fmt.cpp
@@ -0,0 +1,326 @@
+
+#include <cstdio>     // std::snprintf
+#include <iomanip>    // std::put_time
+#include <sstream>    // std::ostringstream
+#include <array>
+#include <cstring>    // std::memcpy
+#include <algorithm>  // std::min
+#include <initializer_list>
+#include <cstdint>
+
+#include "libipc/imp/fmt.h"
+#include "libipc/imp/codecvt.h"
+#include "libipc/imp/detect_plat.h"
+
+namespace ipc {
+
+/**
+ * \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 {
+
+struct sfmt_policy {
+  static constexpr std::size_t aligned_size = 32U;
+};
+
+template <typename Policy = sfmt_policy>
+span<char> local_fmt_sbuf() noexcept {
+  thread_local std::array<char, Policy::aligned_size> sbuf;
+  return sbuf;
+}
+
+span<char const> normalize(span<char const> const &a) {
+  if (a.empty()) return {};
+  return a.first(a.size() - (a.back() == '\0' ? 1 : 0));
+}
+
+span<char> smem_cpy(span<char> const &sbuf, span<char const> a) noexcept {
+  if (sbuf.empty()) return {};
+  a = normalize(a);
+  auto sz = (std::min)(sbuf.size() - 1, a.size());
+  if (sz != 0) std::memcpy(sbuf.data(), a.data(), sz);
+  return sbuf.first(sz);
+}
+
+span<char> sbuf_cpy(span<char> sbuf, span<char const> const &a) noexcept {
+  sbuf = smem_cpy(sbuf, a);
+  *sbuf.end() = '\0';
+  return sbuf;
+}
+
+span<char> sbuf_cat(span<char> const &sbuf, std::initializer_list<span<char const>> args) noexcept {
+  std::size_t remain = sbuf.size();
+  for (auto s : args) {
+    remain -= smem_cpy(sbuf.last(remain), s).size();
+  }
+  auto sz = sbuf.size() - remain;
+  sbuf[sz] = '\0';
+  return sbuf.first(sz);
+}
+
+char const *as_cstr(span<char const> const &a) {
+  if (a.empty()) return "";
+  if (a.back() == '\0') return a.data();
+  return sbuf_cpy(local_fmt_sbuf(), a).data();
+}
+
+span<char> fmt_of(span<char const> const &fstr, span<char const> const &s) {
+  return sbuf_cat(local_fmt_sbuf(), {"%", fstr, s});
+}
+
+span<char> fmt_of_unsigned(span<char const> fstr, span<char const> 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 sbuf_cat(local_fmt_sbuf(), {"%", fstr.first(fstr.size() - 1), l, fstr.last(1)});
+    default : return sbuf_cat(local_fmt_sbuf(), {"%", fstr, l, "u"});
+  }
+}
+
+span<char> fmt_of_signed(span<char const> fstr, span<char const> 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 sbuf_cat(local_fmt_sbuf(), {"%", fstr, l, "d"});
+  }
+}
+
+span<char> fmt_of_float(span<char const> fstr, span<char const> 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 sbuf_cat(local_fmt_sbuf(), {"%", fstr.first(fstr.size() - 1), l, fstr.last(1)});
+    default : return sbuf_cat(local_fmt_sbuf(), {"%", fstr, l, "f"});
+  }
+}
+
+template <typename A /*a fundamental or pointer type*/>
+int sprintf(fmt_context &ctx, span<char const> const &sfmt, A a) {
+  for (std::int32_t sz = -1;;) {
+    auto sbuf = ctx.buffer(sz + 1);
+    if (sbuf.size() < std::size_t(sz + 1)) {
+      return -1;
+    }
+    sz = std::snprintf(sbuf.data(), sbuf.size(), sfmt.data(), a);
+    if (sz <= 0) {
+      return sz;
+    }
+    if (std::size_t(sz) < sbuf.size()) {
+      ctx.expend(sz);
+      return sz;
+    }
+  }
+}
+
+template <typename F /*a function pointer*/, 
+          typename A /*a fundamental or pointer type*/>
+bool sprintf(fmt_context &ctx, F fop, span<char const> const &fstr, span<char const> const &s, A a) noexcept {
+  LIBIPC_TRY {
+    return ipc::sprintf(ctx, fop(fstr, s), a) >= 0;
+  } LIBIPC_CATCH(...) {
+    return false;
+  }
+}
+
+} // namespace
+
+/// \brief The context of fmt.
+
+fmt_context::fmt_context(std::string &j) noexcept
+  : joined_(j)
+  , offset_(0) {}
+
+std::size_t fmt_context::capacity() noexcept {
+  return (offset_ < sbuf_.size()) ? sbuf_.size() : joined_.size();
+}
+
+void fmt_context::reset() noexcept {
+  offset_ = 0;
+}
+
+bool fmt_context::finish() noexcept {
+  LIBIPC_TRY {
+    if (offset_ < sbuf_.size()) {
+      joined_.assign(sbuf_.data(), offset_);
+    } else {
+      joined_.resize(offset_);
+    }
+    return true;
+  } LIBIPC_CATCH(...) {
+    return false;
+  }
+}
+
+span<char> fmt_context::buffer(std::size_t sz) noexcept {
+  auto roundup = [](std::size_t sz) noexcept {
+    constexpr std::size_t fmt_context_aligned_size = 512U;
+    return (sz & ~(fmt_context_aligned_size - 1)) + fmt_context_aligned_size;
+  };
+  auto sbuf = make_span(sbuf_);
+  LIBIPC_TRY {
+    if (offset_ < sbuf.size()) {
+      if ((offset_ + sz) < sbuf.size()) {
+        return sbuf.subspan(offset_);
+      } else {
+        /// \remark switch the cache to std::string
+        joined_.assign(sbuf.data(), offset_);
+        joined_.resize(roundup(offset_ + sz));
+      }
+    } else if ((offset_ + sz) >= joined_.size()) {
+      joined_.resize(roundup(offset_ + sz));
+    }
+    return {&joined_[offset_], joined_.size() - offset_};
+  } LIBIPC_CATCH(...) {
+    return {};
+  }
+}
+
+void fmt_context::expend(std::size_t sz) noexcept {
+  offset_ += sz;
+}
+
+bool fmt_context::append(span<char const> const &str) noexcept {
+  auto sz = str.size();
+  if (str.back() == '\0') --sz;
+  auto sbuf = buffer(sz);
+  if (sbuf.size() < sz) {
+    return false;
+  }
+  std::memcpy(sbuf.data(), str.data(), sz);
+  offset_ += sz;
+  return true;
+}
+
+/// \brief To string conversion.
+
+bool to_string(fmt_context &ctx, char const *a) noexcept {
+  return to_string(ctx, a, {});
+}
+
+bool to_string(fmt_context &ctx, std::string const &a) noexcept {
+  return ctx.append(a);
+}
+
+bool to_string(fmt_context &ctx, char const *a, span<char const> fstr) noexcept {
+  if (a == nullptr) return false;
+  return ipc::sprintf(ctx, fmt_of, fstr, "s", a);
+}
+
+bool to_string(fmt_context &ctx, char a) noexcept {
+  return ipc::sprintf(ctx, fmt_of, {}, "c", a);
+}
+
+bool to_string(fmt_context &ctx, wchar_t a) noexcept {
+  LIBIPC_TRY {
+    std::string des;
+    cvt_sstr(std::wstring{a}, des);
+    return ctx.append(des);
+  } LIBIPC_CATCH(...) {
+    return false;
+  }
+}
+
+bool to_string(fmt_context &ctx, char16_t a) noexcept {
+  LIBIPC_TRY {
+    std::string des;
+    cvt_sstr(std::u16string{a}, des);
+    return ctx.append(des);
+  } LIBIPC_CATCH(...) {
+    return false;
+  }
+}
+
+bool to_string(fmt_context &ctx, char32_t a) noexcept {
+  LIBIPC_TRY {
+    std::string des;
+    cvt_sstr(std::u32string{a}, des);
+    return ctx.append(des);
+  } LIBIPC_CATCH(...) {
+    return false;
+  }
+}
+
+bool to_string(fmt_context &ctx, signed short a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_signed, fstr, "h", a);
+}
+
+bool to_string(fmt_context &ctx, unsigned short a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_unsigned, fstr, "h", a);
+}
+
+bool to_string(fmt_context &ctx, signed int a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_signed, fstr, "", a);
+}
+
+bool to_string(fmt_context &ctx, unsigned int a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_unsigned, fstr, "", a);
+}
+
+bool to_string(fmt_context &ctx, signed long a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_signed, fstr, "l", a);
+}
+
+bool to_string(fmt_context &ctx, unsigned long a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_unsigned, fstr, "l", a);
+}
+
+bool to_string(fmt_context &ctx, signed long long a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_signed, fstr, "ll", a);
+}
+
+bool to_string(fmt_context &ctx, unsigned long long a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_unsigned, fstr, "ll", a);
+}
+
+bool to_string(fmt_context &ctx, double a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_float, fstr, "", a);
+}
+
+bool to_string(fmt_context &ctx, long double a, span<char const> fstr) noexcept {
+  return ipc::sprintf(ctx, fmt_of_float, fstr, "L", a);
+}
+
+bool to_string(fmt_context &ctx, std::nullptr_t) noexcept {
+  return ctx.append("null");
+}
+
+template <>
+bool to_string<void, void>(fmt_context &ctx, void const volatile *a) noexcept {
+  if (a == nullptr) {
+    return to_string(ctx, nullptr);
+  }
+  return ipc::sprintf(ctx, fmt_of, "", "p", a);
+}
+
+bool to_string(fmt_context &ctx, std::tm const &a, span<char const> fstr) noexcept {
+  if (fstr.empty()) {
+    fstr = "%Y-%m-%d %H:%M:%S";
+  }
+  LIBIPC_TRY {
+    std::ostringstream ss;
+    ss << std::put_time(&a, as_cstr(fstr));
+    return ctx.append(ss.str());
+  } LIBIPC_CATCH(...) {
+    return {};
+  }
+}
+
+} // namespace ipc
diff --git a/test/imp/test_imp_fmt.cpp b/test/imp/test_imp_fmt.cpp
new file mode 100644
index 0000000..04acd6d
--- /dev/null
+++ b/test/imp/test_imp_fmt.cpp
@@ -0,0 +1,125 @@
+#define _CRT_SECURE_NO_WARNINGS
+#include <iostream>
+#include <tuple>
+#include <cstring>
+
+#include "test.h"
+
+#include "libipc/imp/fmt.h"
+
+TEST(fmt, spec) {
+  EXPECT_STREQ(ipc::spec("hello")(123).fstr.data(), "hello");
+  EXPECT_EQ(ipc::spec("hello")(123).param , 123);
+  EXPECT_STREQ(ipc::spec("hello")("world").fstr.data(), "hello");
+  EXPECT_STREQ(ipc::spec("hello")("world").param , "world");
+}
+
+TEST(fmt, to_string) {
+  std::string joined;
+  ipc::fmt_context ctx(joined);
+
+  auto check = [&](auto &&txt, auto &&...val) {
+    ctx.reset();
+    EXPECT_TRUE(ipc::to_string(ctx, std::forward<decltype(val)>(val)...));
+    ctx.finish();
+    EXPECT_EQ(joined, std::forward<decltype(txt)>(txt));
+  };
+
+  /// \brief string
+  check("", "");
+  check("%what%", "%what%");
+  check("    %what%", "%what%", "10");
+  check("%what%    ", "%what%", "-10");
+
+  /// \brief character
+  check("A", 'A');
+  check("A", L'A');
+  check("A", u'A');
+  check("A", U'A');
+
+  /// \brief numeric
+  check("123"       , (signed char)123      );
+  check("-65"       , (signed char)-321     );
+  check("123"       , (unsigned char)123    );
+  check("65"        , (unsigned char)321    );
+  check("123"       , (short)123            );
+  check("-321"      , (short)-321           );
+  check("123"       , (unsigned short)123   );
+  check("321"       , (unsigned short)321   );
+  check("-7949"     , (short)123123         );
+  check("6359"      , (short)-321321        );
+  check("57587"     , (unsigned short)123123);
+  check("59177"     , (unsigned short)321321);
+  check("123123"    , 123123                );
+  check("-321321"   , -321321               );
+  check("123123"    , 123123u               );
+  check("321321"    , 321321u               );
+  check("123123"    , 123123ll              );
+  check("-321321"   , -321321ll             );
+  check("123123"    , 123123ull             );
+  check("321321"    , 321321ull             );
+  check("1e0f3"     , 123123, "x"           );
+  check("1e0f3"     , 123123u, "x"          );
+  check("1CAAB5C3B3", 123123123123ll, "X"   );
+  check("1CAAB5C3B3", 123123123123ull, "X"  );
+
+  /// \brief floating point
+  check("123.123"     , 123.123f, ".3");
+  check("0123.12300"  , 123.123, "010.5");
+  check("123.123000"  , 123.123l, "010.6");
+  check("1.500000e+00", 1.5, "e");
+  check("1.500000E+00", 1.5, "E");
+  double r = 0.0;
+  ctx.reset(); EXPECT_TRUE(ipc::to_string(ctx, 0.0/r)); ctx.finish();
+  std::cout << joined << "\n";
+  ctx.reset(); EXPECT_TRUE(ipc::to_string(ctx, 1.0/r)); ctx.finish();
+  std::cout << joined << "\n";
+
+  /// \brief pointer
+  check("null", nullptr);
+  int *p = (int *)0x0f013a04;
+  ctx.reset(); EXPECT_TRUE(ipc::to_string(ctx, (void *)p)); ctx.finish();
+  std::cout << joined << "\n";
+
+  /// \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);
+  ctx.reset(); EXPECT_TRUE(ipc::to_string(ctx, tm)); ctx.finish();
+  std::cout << joined << "\n";
+  std::string tm_str = joined;
+  ctx.reset(); EXPECT_TRUE(ipc::to_string(ctx, tp)); ctx.finish();
+  EXPECT_EQ(tm_str, joined);
+}
+
+TEST(fmt, fmt) {
+  char const txt[] = "hello world.";
+
+  /// \brief hello world
+  auto s = ipc::fmt("hello", " ", "world", ".");
+  EXPECT_EQ(s, txt);
+
+  /// \brief chrono
+  std::cout << ipc::fmt('[', std::chrono::system_clock::now(), "] ", s) << "\n";
+
+  /// \brief long string
+  s = ipc::fmt(ipc::spec("4096")(txt));
+  std::string test(4096, ' ');
+  std::memcpy(&test[test.size() - sizeof(txt) + 1], txt, sizeof(txt) - 1);
+  EXPECT_EQ(s, test);
+}
+
+namespace {
+
+class foo {};
+
+bool tag_invoke(decltype(ipc::fmt_to), ipc::fmt_context &, foo arg) noexcept(false) {
+  throw arg;
+  return {};
+}
+
+} // namespace
+
+TEST(fmt, throw) {
+  EXPECT_THROW(std::ignore = ipc::fmt(foo{}), foo);
+}