Basics of formatting at compile-time based on compile-time API

include/fmt/compile.h

@@ -394,7 +394,7 @@ template <typename Char> struct text {
   using char_type = Char;
 
   template <typename OutputIt, typename... Args>
-  OutputIt format(OutputIt out, const Args&...) const {
+  constexpr OutputIt format(OutputIt out, const Args&...) const {
     return write<Char>(out, data);
   }
 };
@@ -413,7 +413,7 @@ template <typename Char> struct code_unit {
   using char_type = Char;
 
   template <typename OutputIt, typename... Args>
-  OutputIt format(OutputIt out, const Args&...) const {
+  constexpr OutputIt format(OutputIt out, const Args&...) const {
     return write<Char>(out, value);
   }
 };
@@ -426,7 +426,7 @@ template <typename Char, typename T, int N> struct field {
   using char_type = Char;
 
   template <typename OutputIt, typename... Args>
-  OutputIt format(OutputIt out, const Args&... args) const {
+  constexpr OutputIt format(OutputIt out, const Args&... args) const {
     // This ensures that the argument type is convertile to `const T&`.
     const T& arg = get<N>(args...);
     return write<Char>(out, arg);
@@ -461,7 +461,7 @@ template <typename L, typename R> struct concat {
   using char_type = typename L::char_type;
 
   template <typename OutputIt, typename... Args>
-  OutputIt format(OutputIt out, const Args&... args) const {
+  constexpr OutputIt format(OutputIt out, const Args&... args) const {
     out = lhs.format(out, args...);
     return rhs.format(out, args...);
   }
@@ -617,8 +617,8 @@ FMT_INLINE std::basic_string<Char> format(const CompiledFormat& cf,
 
 template <typename OutputIt, typename CompiledFormat, typename... Args,
           FMT_ENABLE_IF(detail::is_compiled_format<CompiledFormat>::value)>
-OutputIt format_to(OutputIt out, const CompiledFormat& cf,
-                   const Args&... args) {
+constexpr OutputIt format_to(OutputIt out, const CompiledFormat& cf,
+                             const Args&... args) {
   return cf.format(out, args...);
 }
 #  endif  // __cpp_if_constexpr
@@ -654,8 +654,8 @@ FMT_INLINE std::basic_string<typename S::char_type> format(const S&,
 template <typename OutputIt, typename CompiledFormat, typename... Args,
           FMT_ENABLE_IF(std::is_base_of<detail::basic_compiled_format,
                                         CompiledFormat>::value)>
-OutputIt format_to(OutputIt out, const CompiledFormat& cf,
-                   const Args&... args) {
+constexpr OutputIt format_to(OutputIt out, const CompiledFormat& cf,
+                             const Args&... args) {
   using char_type = typename CompiledFormat::char_type;
   using context = format_context_t<OutputIt, char_type>;
   return detail::cf::vformat_to<context>(out, cf,
@@ -664,7 +664,7 @@ OutputIt format_to(OutputIt out, const CompiledFormat& cf,
 
 template <typename OutputIt, typename S, typename... Args,
           FMT_ENABLE_IF(detail::is_compiled_string<S>::value)>
-OutputIt format_to(OutputIt out, const S&, const Args&... args) {
+FMT_CONSTEXPR OutputIt format_to(OutputIt out, const S&, const Args&... args) {
   constexpr auto compiled = detail::compile<Args...>(S());
   return format_to(out, compiled, args...);
 }

include/fmt/core.h

@@ -96,6 +96,12 @@
 #  define FMT_CONSTEXPR_DECL
 #endif
 
+#if __cplusplus >= 202002L
+#  define FMT_CONSTEXPR20 constexpr
+#else
+#  define FMT_CONSTEXPR20 inline
+#endif
+
 #ifndef FMT_OVERRIDE
 #  if FMT_HAS_FEATURE(cxx_override_control) || \
       (FMT_GCC_VERSION >= 408 && FMT_HAS_GXX_CXX11) || FMT_MSC_VER >= 1900
@@ -279,6 +285,14 @@ struct monostate {};
 
 namespace detail {
 
+constexpr bool is_constant_evaluated() FMT_DETECTED_NOEXCEPT {
+#ifdef __cpp_lib_is_constant_evaluated
+  return std::is_constant_evaluated();
+#else
+  return false;
+#endif
+}
+
 // A helper function to suppress "conditional expression is constant" warnings.
 template <typename T> constexpr T const_check(T value) { return value; }
 

include/fmt/format.h

@@ -390,7 +390,7 @@ inline buffer_appender<T> reserve(buffer_appender<T> it, size_t n) {
   return it;
 }
 
-template <typename Iterator> inline Iterator& reserve(Iterator& it, size_t) {
+template <typename Iterator> constexpr Iterator& reserve(Iterator& it, size_t) {
   return it;
 }
 
@@ -414,7 +414,7 @@ inline std::back_insert_iterator<Container> base_iterator(
 }
 
 template <typename Iterator>
-inline Iterator base_iterator(Iterator, Iterator it) {
+constexpr Iterator base_iterator(Iterator, Iterator it) {
   return it;
 }
 
@@ -587,14 +587,17 @@ using needs_conversion = bool_constant<
 
 template <typename OutChar, typename InputIt, typename OutputIt,
           FMT_ENABLE_IF(!needs_conversion<InputIt, OutChar>::value)>
-OutputIt copy_str(InputIt begin, InputIt end, OutputIt it) {
+FMT_CONSTEXPR OutputIt copy_str(InputIt begin, InputIt end, OutputIt it) {
   while (begin != end) *it++ = *begin++;
   return it;
 }
 
 template <typename OutChar, typename InputIt,
           FMT_ENABLE_IF(!needs_conversion<InputIt, OutChar>::value)>
-inline OutChar* copy_str(InputIt begin, InputIt end, OutChar* out) {
+FMT_CONSTEXPR20 OutChar* copy_str(InputIt begin, InputIt end, OutChar* out) {
+  if (is_constant_evaluated()) {
+    return copy_str<OutChar, InputIt, OutChar*>(begin, end, out);
+  }
   return std::uninitialized_copy(begin, end, out);
 }
 
@@ -943,17 +946,7 @@ FMT_EXTERN template struct basic_data<void>;
 // This is a struct rather than an alias to avoid shadowing warnings in gcc.
 struct data : basic_data<> {};
 
-#ifdef FMT_BUILTIN_CLZLL
-// Returns the number of decimal digits in n. Leading zeros are not counted
-// except for n == 0 in which case count_digits returns 1.
-inline int count_digits(uint64_t n) {
-  // https://github.com/fmtlib/format-benchmark/blob/master/digits10
-  auto t = bsr2log10(FMT_BUILTIN_CLZLL(n | 1) ^ 63);
-  return t - (n < data::zero_or_powers_of_10_64_new[t]);
-}
-#else
-// Fallback version of count_digits used when __builtin_clz is not available.
-inline int count_digits(uint64_t n) {
+template <typename T> FMT_CONSTEXPR int count_digits_fallback(T n) {
   int count = 1;
   for (;;) {
     // Integer division is slow so do it for a group of four digits instead
@@ -967,10 +960,25 @@ inline int count_digits(uint64_t n) {
     count += 4;
   }
 }
+
+#ifdef FMT_BUILTIN_CLZLL
+// Returns the number of decimal digits in n. Leading zeros are not counted
+// except for n == 0 in which case count_digits returns 1.
+FMT_CONSTEXPR20 int count_digits(uint64_t n) {
+  if (is_constant_evaluated()) {
+    return count_digits_fallback(n);
+  }
+  // https://github.com/fmtlib/format-benchmark/blob/master/digits10
+  auto t = bsr2log10(FMT_BUILTIN_CLZLL(n | 1) ^ 63);
+  return t - (n < data::zero_or_powers_of_10_64_new[t]);
+}
+#else
+// Fallback version of count_digits used when __builtin_clz is not available.
+FMT_CONSTEXPR int count_digits(uint64_t n) { return count_digits_fallback(n); }
 #endif
 
 #if FMT_USE_INT128
-inline int count_digits(uint128_t n) {
+FMT_CONSTEXPR int count_digits(uint128_t n) {
   int count = 1;
   for (;;) {
     // Integer division is slow so do it for a group of four digits instead
@@ -987,7 +995,7 @@ inline int count_digits(uint128_t n) {
 #endif
 
 // Counts the number of digits in n. BITS = log2(radix).
-template <unsigned BITS, typename UInt> inline int count_digits(UInt n) {
+template <unsigned BITS, typename UInt> FMT_CONSTEXPR int count_digits(UInt n) {
   int num_digits = 0;
   do {
     ++num_digits;
@@ -1007,7 +1015,10 @@ template <> int count_digits<4>(detail::fallback_uintptr n);
 
 #ifdef FMT_BUILTIN_CLZ
 // Optional version of count_digits for better performance on 32-bit platforms.
-inline int count_digits(uint32_t n) {
+FMT_CONSTEXPR20 int count_digits(uint32_t n) {
+  if (is_constant_evaluated()) {
+    return count_digits_fallback(n);
+  }
   auto t = bsr2log10(FMT_BUILTIN_CLZ(n | 1) ^ 31);
   return t - (n < data::zero_or_powers_of_10_32_new[t]);
 }
@@ -1067,11 +1078,20 @@ template <typename Iterator> struct format_decimal_result {
 // buffer of specified size. The caller must ensure that the buffer is large
 // enough.
 template <typename Char, typename UInt>
-inline format_decimal_result<Char*> format_decimal(Char* out, UInt value,
-                                                   int size) {
+FMT_CONSTEXPR20 format_decimal_result<Char*> format_decimal(Char* out,
+                                                            UInt value,
+                                                            int size) {
   FMT_ASSERT(size >= count_digits(value), "invalid digit count");
   out += size;
   Char* end = out;
+  if (is_constant_evaluated()) {
+    while (value >= 10) {
+      *--out = static_cast<Char>('0' + value % 10);
+      value /= 10;
+    }
+    *--out = static_cast<Char>('0' + value);
+    return {out, end};
+  }
   while (value >= 100) {
     // Integer division is slow so do it for a group of two digits instead
     // of for every digit. The idea comes from the talk by Alexandrescu
@@ -2049,7 +2069,7 @@ OutputIt write(OutputIt out, string_view value) {
 }
 
 template <typename Char, typename OutputIt>
-OutputIt write(OutputIt out, basic_string_view<Char> value) {
+FMT_CONSTEXPR OutputIt write(OutputIt out, basic_string_view<Char> value) {
   auto it = reserve(out, value.size());
   it = copy_str<Char>(value.begin(), value.end(), it);
   return base_iterator(out, it);
@@ -2066,7 +2086,7 @@ template <typename Char, typename OutputIt, typename T,
           FMT_ENABLE_IF(is_integral<T>::value &&
                         !std::is_same<T, bool>::value &&
                         !std::is_same<T, Char>::value)>
-OutputIt write(OutputIt out, T value) {
+FMT_CONSTEXPR OutputIt write(OutputIt out, T value) {
   auto abs_value = static_cast<uint32_or_64_or_128_t<T>>(value);
   bool negative = is_negative(value);
   // Don't do -abs_value since it trips unsigned-integer-overflow sanitizer.
@@ -2085,19 +2105,19 @@ OutputIt write(OutputIt out, T value) {
 }
 
 template <typename Char, typename OutputIt>
-OutputIt write(OutputIt out, bool value) {
+constexpr OutputIt write(OutputIt out, bool value) {
   return write<Char>(out, string_view(value ? "true" : "false"));
 }
 
 template <typename Char, typename OutputIt>
-OutputIt write(OutputIt out, Char value) {
+FMT_CONSTEXPR OutputIt write(OutputIt out, Char value) {
   auto it = reserve(out, 1);
   *it++ = value;
   return base_iterator(out, it);
 }
 
 template <typename Char, typename OutputIt>
-OutputIt write(OutputIt out, const Char* value) {
+FMT_CONSTEXPR OutputIt write(OutputIt out, const Char* value) {
   if (!value) {
     FMT_THROW(format_error("string pointer is null"));
   } else {

test/compile-test.cc

@@ -7,6 +7,9 @@
 
 #include <string>
 #include <type_traits>
+#if __cplusplus >= 202002L
+#  include <string_view>
+#endif
 
 // Check that fmt/compile.h compiles with windows.h included before it.
 #ifdef _WIN32
@@ -171,3 +174,69 @@ TEST(CompileTest, TextAndArg) {
   EXPECT_EQ("42!", fmt::format(FMT_COMPILE("{}!"), 42));
 }
 #endif
+
+#if __cplusplus >= 202002L
+template <size_t max_string_length> struct test_string {
+  template <typename T> constexpr bool operator==(const T& rhs) const noexcept {
+    return (std::string_view(rhs).compare(buffer.data()) == 0);
+  }
+
+  std::array<char, max_string_length> buffer{};
+};
+
+template <size_t max_string_length, typename... Args>
+consteval auto test_format(auto format, const Args&... args) {
+  test_string<max_string_length> string{};
+  fmt::format_to(string.buffer.data(), format, args...);
+  return string;
+}
+
+TEST(CompileTimeFormattingTest, Bool) {
+  {
+    constexpr auto result = test_format<5>(FMT_COMPILE("{}"), true);
+    EXPECT_EQ(result, "true");
+  }
+  {
+    constexpr auto result = test_format<6>(FMT_COMPILE("{}"), false);
+    EXPECT_EQ(result, "false");
+  }
+}
+
+TEST(CompileTimeFormattingTest, Integer) {
+  {
+    constexpr auto result = test_format<3>(FMT_COMPILE("{}"), 42);
+    EXPECT_EQ(result, "42");
+  }
+  {
+    constexpr auto result = test_format<4>(FMT_COMPILE("{}"), 420);
+    EXPECT_EQ(result, "420");
+  }
+  {
+    constexpr auto result = test_format<6>(FMT_COMPILE("{} {}"), 42, 42);
+    EXPECT_EQ(result, "42 42");
+  }
+  {
+    constexpr auto result =
+        test_format<6>(FMT_COMPILE("{} {}"), uint32_t{42}, uint64_t{42});
+    EXPECT_EQ(result, "42 42");
+  }
+}
+
+TEST(CompileTimeFormattingTest, String) {
+  {
+    constexpr auto result = test_format<3>(FMT_COMPILE("{}"), "42");
+    EXPECT_EQ(result, "42");
+  }
+  {
+    constexpr auto result =
+        test_format<17>(FMT_COMPILE("{} is {}"), "The answer", "42");
+    EXPECT_EQ(result, "The answer is 42");
+  }
+}
+
+TEST(CompileTimeFormattingTest, Combination) {
+  constexpr auto result =
+      test_format<18>(FMT_COMPILE("{}, {}, {}"), 420, true, "answer");
+  EXPECT_EQ(result, "420, true, answer");
+}
+#endif