C++17: std::char_traits<>::{compare,length} is constexpr.

fmtlib · Apr 22, 2021 · 2e049b4 · 2e049b4
1 parent 1d4199f
commit 2e049b4
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Showing 3 changed files with 20 additions and 6 deletions.
diff --git a/include/fmt/color.h b/include/fmt/color.h
@@ -432,7 +432,7 @@ template <typename Char> struct ansi_color_escape {
   FMT_CONSTEXPR operator const Char*() const FMT_NOEXCEPT { return buffer; }
 
   FMT_CONSTEXPR const Char* begin() const FMT_NOEXCEPT { return buffer; }
-  FMT_CONSTEXPR const Char* end() const FMT_NOEXCEPT {
+  FMT_CONSTEXPR_CHAR_TRAITS const Char* end() const FMT_NOEXCEPT {
     return buffer + std::char_traits<Char>::length(buffer);
   }
 

diff --git a/include/fmt/core.h b/include/fmt/core.h
@@ -43,6 +43,22 @@
 #  define FMT_HAS_GXX_CXX11 0
 #endif
 
+// Check if constexpr std::char_traits<>::compare,length is supported.
+// libstdc++: present on GCC 7 and newer and __cplusplus >= 201703L
+// MSVC, libc++: always if __cplusplus >= 201703L
+// NOTE: FMT_GCC_VERSION  - is not libstdc++ version.
+//       _GLIBCXX_RELEASE - is present in GCC 7 libstdc++ and newer.
+#if __cplusplus >= 201703L
+#  ifndef __GLIBCXX__
+#    define FMT_CONSTEXPR_CHAR_TRAITS constexpr
+#  elif defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE >= 7
+#    define FMT_CONSTEXPR_CHAR_TRAITS constexpr
+#  endif
+#endif
+#ifndef FMT_CONSTEXPR_CHAR_TRAITS
+#  define FMT_CONSTEXPR_CHAR_TRAITS
+#endif
+
 #ifdef __NVCC__
 #  define FMT_NVCC __NVCC__
 #else
@@ -415,9 +431,7 @@ template <typename Char> class basic_string_view {
     the size with ``std::char_traits<Char>::length``.
     \endrst
    */
-#if __cplusplus >= 201703L  // C++17's char_traits::length() is constexpr.
-  constexpr
-#endif
+  FMT_CONSTEXPR_CHAR_TRAITS
       FMT_INLINE
       basic_string_view(const Char* s)
       : data_(s) {
@@ -457,7 +471,7 @@ template <typename Char> class basic_string_view {
   }
 
   // Lexicographically compare this string reference to other.
-  int compare(basic_string_view other) const {
+  FMT_CONSTEXPR_CHAR_TRAITS int compare(basic_string_view other) const {
     size_t str_size = size_ < other.size_ ? size_ : other.size_;
     int result = std::char_traits<Char>::compare(data_, other.data_, str_size);
     if (result == 0)

diff --git a/include/fmt/format.h b/include/fmt/format.h
@@ -2149,7 +2149,7 @@ FMT_CONSTEXPR OutputIt write(OutputIt out, Char value) {
 }
 
 template <typename Char, typename OutputIt>
-FMT_CONSTEXPR OutputIt write(OutputIt out, const Char* value) {
+FMT_CONSTEXPR_CHAR_TRAITS OutputIt write(OutputIt out, const Char* value) {
   if (!value) {
     FMT_THROW(format_error("string pointer is null"));
   } else {