Make format work with C++17 std::string_view

fmt/format.h

@@ -45,6 +45,19 @@
 // The fmt library version in the form major * 10000 + minor * 100 + patch.
 #define FMT_VERSION 40001
 
+#if defined(__has_include)
+# define FMT_HAS_INCLUDE(x) __has_include(x)
+#else
+# define FMT_HAS_INCLUDE(x) 0
+#endif
+
+#if FMT_HAS_INCLUDE(<string_view>) && __cplusplus > 201402L
+# include <string_view>
+# define FMT_HAS_STRING_VIEW 1
+#else
+# define FMT_HAS_STRING_VIEW 0
+#endif
+
 #if defined _SECURE_SCL && _SECURE_SCL
 # define FMT_SECURE_SCL _SECURE_SCL
 #else
@@ -521,6 +534,26 @@ class BasicStringRef {
       const std::basic_string<Char, std::char_traits<Char>, Allocator> &s)
   : data_(s.c_str()), size_(s.size()) {}
 
+#if FMT_HAS_STRING_VIEW
+  /**
+    \rst
+    Constructs a string reference from a ``std::basic_string_view`` object.
+    \endrst
+   */
+  BasicStringRef(
+      const std::basic_string_view<Char, std::char_traits<Char>> &s)
+  : data_(s.data()), size_(s.size()) {}
+
+  /**
+   \rst
+   Converts a string reference to an ``std::string_view`` object.
+   \endrst
+  */
+  explicit operator std::basic_string_view<Char>() const FMT_NOEXCEPT {
+    return std::basic_string_view<Char>(data_, size_);
+  }
+#endif
+
   /**
     \rst
     Converts a string reference to an ``std::string`` object.
@@ -1299,6 +1332,9 @@ class MakeValue : public Arg {
   MakeValue(typename WCharHelper<wchar_t *, Char>::Unsupported);
   MakeValue(typename WCharHelper<const wchar_t *, Char>::Unsupported);
   MakeValue(typename WCharHelper<const std::wstring &, Char>::Unsupported);
+#if FMT_HAS_STRING_VIEW
+  MakeValue(typename WCharHelper<const std::wstring_view &, Char>::Unsupported);
+#endif
   MakeValue(typename WCharHelper<WStringRef, Char>::Unsupported);
 
   void set_string(StringRef str) {
@@ -1386,6 +1422,9 @@ class MakeValue : public Arg {
   FMT_MAKE_VALUE(unsigned char *, ustring.value, CSTRING)
   FMT_MAKE_VALUE(const unsigned char *, ustring.value, CSTRING)
   FMT_MAKE_STR_VALUE(const std::string &, STRING)
+#if FMT_HAS_STRING_VIEW
+  FMT_MAKE_STR_VALUE(const std::string_view &, STRING)
+#endif
   FMT_MAKE_STR_VALUE(StringRef, STRING)
   FMT_MAKE_VALUE_(CStringRef, string.value, CSTRING, value.c_str())
 
@@ -1398,6 +1437,9 @@ class MakeValue : public Arg {
   FMT_MAKE_WSTR_VALUE(wchar_t *, WSTRING)
   FMT_MAKE_WSTR_VALUE(const wchar_t *, WSTRING)
   FMT_MAKE_WSTR_VALUE(const std::wstring &, WSTRING)
+#if FMT_HAS_STRING_VIEW
+  FMT_MAKE_WSTR_VALUE(const std::wstring_view &, WSTRING)
+#endif
   FMT_MAKE_WSTR_VALUE(WStringRef, WSTRING)
 
   FMT_MAKE_VALUE(void *, pointer, POINTER)

test/format-test.cc

@@ -151,16 +151,31 @@ TEST(StringRefTest, Ctor) {
 
   EXPECT_STREQ("defg", StringRef(std::string("defg")).data());
   EXPECT_EQ(4u, StringRef(std::string("defg")).size());
+
+#if FMT_HAS_STRING_VIEW
+  EXPECT_STREQ("hijk", StringRef(std::string_view("hijk")).data());
+  EXPECT_EQ(4u, StringRef(std::string_view("hijk")).size());
+#endif
 }
 
 TEST(StringRefTest, ConvertToString) {
   std::string s = StringRef("abc").to_string();
   EXPECT_EQ("abc", s);
+
+#if FMT_HAS_STRING_VIEW
+  StringRef str_ref("defg");
+  std::string_view sv = static_cast<std::string_view>(str_ref);
+  EXPECT_EQ("defg", sv);
+#endif
 }
 
 TEST(CStringRefTest, Ctor) {
   EXPECT_STREQ("abc", CStringRef("abc").c_str());
   EXPECT_STREQ("defg", CStringRef(std::string("defg")).c_str());
+
+#if FMT_HAS_STRING_VIEW
+  EXPECT_STREQ("hijk", CStringRef(std::string_view("hijk")).c_str());
+#endif
 }
 
 #if FMT_USE_TYPE_TRAITS
@@ -1378,6 +1393,12 @@ TEST(FormatterTest, FormatCStringRef) {
   EXPECT_EQ("test", format("{0}", CStringRef("test")));
 }
 
+#if FMT_HAS_STRING_VIEW
+TEST(FormatterTest, FormatStringView) {
+  EXPECT_EQ("test", format("{0}", std::string_view("test")));
+}
+#endif
+
 void format_arg(fmt::BasicFormatter<char> &f, const char *, const Date &d) {
   f.writer() << d.year() << '-' << d.month() << '-' << d.day();
 }