Support compile-time format string checking with std::format (#2544)

* Support compile-time format string checking with std::format

* Fix pre-VS 17.5 compilation

* Fix compilation without wchar_t support

* What am I doing

* Bring back fmt optimization

* Move to_string_view to common.h

* Fix SPDLOG_CONSTEXPR_FUNC emitting duplicate symbol errors when building in C++11

* Also add inline on VS 2013

* Appender doesn't work on wide strings

CMakeLists.txt

@@ -83,7 +83,7 @@ option(SPDLOG_BUILD_WARNINGS "Enable compiler warnings" OFF)
 
 # install options
 option(SPDLOG_INSTALL "Generate the install target" ${SPDLOG_MASTER_PROJECT})
-option(SPDLOG_USE_STD_FORMAT "Use std::format instead of fmt library. No compile-time format string checking." OFF)
+option(SPDLOG_USE_STD_FORMAT "Use std::format instead of fmt library." OFF)
 option(SPDLOG_FMT_EXTERNAL "Use external fmt library instead of bundled" OFF)
 option(SPDLOG_FMT_EXTERNAL_HO "Use external fmt header-only library instead of bundled" OFF)
 option(SPDLOG_NO_EXCEPTIONS "Compile with -fno-exceptions. Call abort() on any spdlog exceptions" OFF)

appveyor.yml

@@ -92,7 +92,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'OFF'
       USE_STD_FORMAT: 'ON'
-      CXX_STANDARD: 23 # std::format is only available with /std:c++latest at the moment.
+      CXX_STANDARD: 20
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
     - GENERATOR: '"Visual Studio 17 2022" -A x64'
       BUILD_TYPE: Release
@@ -102,7 +102,7 @@ environment:
       WCHAR_FILES: 'ON'
       BUILD_EXAMPLE: 'OFF'
       USE_STD_FORMAT: 'ON'
-      CXX_STANDARD: 23 # std::format is only available with /std:c++latest at the moment.
+      CXX_STANDARD: 20
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
 build_script:
   - cmd: >-

include/spdlog/common.h

@@ -17,8 +17,13 @@
 #include <cstdio>
 
 #ifdef SPDLOG_USE_STD_FORMAT
+#    include <version>
+#    if __cpp_lib_format >= 202207L
+#        include <format>
+#    else
 #        include <string_view>
 #    endif
+#endif
 
 #ifdef SPDLOG_COMPILED_LIB
 #    undef SPDLOG_HEADER_ONLY
@@ -59,14 +64,14 @@
 #if defined(_MSC_VER) && (_MSC_VER < 1900)
 #    define SPDLOG_NOEXCEPT _NOEXCEPT
 #    define SPDLOG_CONSTEXPR
-#    define SPDLOG_CONSTEXPR_FUNC
+#    define SPDLOG_CONSTEXPR_FUNC inline
 #else
 #    define SPDLOG_NOEXCEPT noexcept
 #    define SPDLOG_CONSTEXPR constexpr
 #    if __cplusplus >= 201402L
 #        define SPDLOG_CONSTEXPR_FUNC constexpr
 #    else
-#        define SPDLOG_CONSTEXPR_FUNC
+#        define SPDLOG_CONSTEXPR_FUNC inline
 #    endif
 #endif
 
@@ -134,7 +139,11 @@ using string_view_t = std::string_view;
 using memory_buf_t = std::string;
 
 template<typename... Args>
+#    if __cpp_lib_format >= 202207L
+using format_string_t = std::format_string<Args...>;
+#    else
 using format_string_t = std::string_view;
+#    endif
 
 template<class T, class Char = char>
 struct is_convertible_to_basic_format_string : std::integral_constant<bool, std::is_convertible<T, std::basic_string_view<Char>>::value>
@@ -145,8 +154,12 @@ using wstring_view_t = std::wstring_view;
 using wmemory_buf_t = std::wstring;
 
 template<typename... Args>
+#        if __cpp_lib_format >= 202207L
+using wformat_string_t = std::wformat_string<Args...>;
+#        else
 using wformat_string_t = std::wstring_view;
 #        endif
+#    endif
 #    define SPDLOG_BUF_TO_STRING(x) x
 #else // use fmt lib instead of std::format
 namespace fmt_lib = fmt;
@@ -323,6 +336,44 @@ struct file_event_handlers
 
 namespace details {
 
+// to_string_view
+
+SPDLOG_CONSTEXPR_FUNC spdlog::string_view_t to_string_view(const memory_buf_t &buf) SPDLOG_NOEXCEPT
+{
+    return spdlog::string_view_t{buf.data(), buf.size()};
+}
+
+SPDLOG_CONSTEXPR_FUNC spdlog::string_view_t to_string_view(spdlog::string_view_t str) SPDLOG_NOEXCEPT
+{
+    return str;
+}
+
+#if defined(SPDLOG_WCHAR_FILENAMES) || defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT)
+SPDLOG_CONSTEXPR_FUNC spdlog::wstring_view_t to_string_view(const wmemory_buf_t &buf) SPDLOG_NOEXCEPT
+{
+    return spdlog::wstring_view_t{buf.data(), buf.size()};
+}
+
+SPDLOG_CONSTEXPR_FUNC spdlog::wstring_view_t to_string_view(spdlog::wstring_view_t str) SPDLOG_NOEXCEPT
+{
+    return str;
+}
+#endif
+
+#ifndef SPDLOG_USE_STD_FORMAT
+template<typename T, typename... Args>
+inline fmt::basic_string_view<T> to_string_view(fmt::basic_format_string<T, Args...> fmt)
+{
+    return fmt;
+}
+#elif __cpp_lib_format >= 202207L
+template<typename T, typename... Args>
+SPDLOG_CONSTEXPR_FUNC std::basic_string_view<T> to_string_view(std::basic_format_string<T, Args...> fmt) SPDLOG_NOEXCEPT
+{
+    return fmt.get();
+}
+#endif
+
 // make_unique support for pre c++14
 
 #if __cplusplus >= 201402L // C++14 and beyond

include/spdlog/details/fmt_helper.h

@@ -18,11 +18,6 @@ namespace spdlog {
 namespace details {
 namespace fmt_helper {
 
-inline spdlog::string_view_t to_string_view(const memory_buf_t &buf) SPDLOG_NOEXCEPT
-{
-    return spdlog::string_view_t{buf.data(), buf.size()};
-}
-
 inline void append_string_view(spdlog::string_view_t view, memory_buf_t &dest)
 {
     auto *buf_ptr = view.data();

include/spdlog/logger.h

@@ -87,7 +87,7 @@ public:
     template<typename... Args>
     void log(source_loc loc, level::level_enum lvl, format_string_t<Args...> fmt, Args &&... args)
     {
-        log_(loc, lvl, fmt, std::forward<Args>(args)...);
+        log_(loc, lvl, details::to_string_view(fmt), std::forward<Args>(args)...);
     }
 
     template<typename... Args>
@@ -180,7 +180,7 @@ public:
     template<typename... Args>
     void log(source_loc loc, level::level_enum lvl, wformat_string_t<Args...> fmt, Args &&... args)
     {
-        log_(loc, lvl, fmt, std::forward<Args>(args)...);
+        log_(loc, lvl, details::to_string_view(fmt), std::forward<Args>(args)...);
     }
 
     template<typename... Args>
@@ -394,12 +394,8 @@ protected:
         {
             // format to wmemory_buffer and convert to utf8
             wmemory_buf_t wbuf;
-#    ifdef SPDLOG_USE_STD_FORMAT
             fmt_lib::vformat_to(
                 std::back_inserter(wbuf), fmt, fmt_lib::make_format_args<fmt_lib::wformat_context>(std::forward<Args>(args)...));
-#    else
-            fmt::vformat_to(std::back_inserter(wbuf), fmt, fmt::make_format_args<fmt::wformat_context>(std::forward<Args>(args)...));
-#    endif
 
             memory_buf_t buf;
             details::os::wstr_to_utf8buf(wstring_view_t(wbuf.data(), wbuf.size()), buf);
@@ -408,27 +404,6 @@ protected:
         }
         SPDLOG_LOGGER_CATCH(loc)
     }
-
-    // T can be statically converted to wstring_view, and no formatting needed.
-    template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::wstring_view_t>::value, int>::type = 0>
-    void log_(source_loc loc, level::level_enum lvl, const T &msg)
-    {
-        bool log_enabled = should_log(lvl);
-        bool traceback_enabled = tracer_.enabled();
-        if (!log_enabled && !traceback_enabled)
-        {
-            return;
-        }
-        SPDLOG_TRY
-        {
-            memory_buf_t buf;
-            details::os::wstr_to_utf8buf(msg, buf);
-            details::log_msg log_msg(loc, name_, lvl, string_view_t(buf.data(), buf.size()));
-            log_it_(log_msg, log_enabled, traceback_enabled);
-        }
-        SPDLOG_LOGGER_CATCH(loc)
-    }
-
 #endif // SPDLOG_WCHAR_TO_UTF8_SUPPORT
 
     // log the given message (if the given log level is high enough),

include/spdlog/tweakme.h

@@ -82,8 +82,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 ///////////////////////////////////////////////////////////////////////////////
-// Uncomment to use C++20 std::format instead of fmt. This removes compile
+// Uncomment to use C++20 std::format instead of fmt.
-// time checking of format strings, but doesn't depend on the fmt library.
 //
 // #define SPDLOG_USE_STD_FORMAT
 ///////////////////////////////////////////////////////////////////////////////

tests/test_fmt_helper.cpp

@@ -2,7 +2,7 @@
 #include "includes.h"
 
 using spdlog::memory_buf_t;
-using spdlog::details::fmt_helper::to_string_view;
+using spdlog::details::to_string_view;
 
 void test_pad2(int n, const char *expected)
 {

tests/test_pattern_formatter.cpp

@@ -2,7 +2,7 @@
 #include "test_sink.h"
 
 using spdlog::memory_buf_t;
-using spdlog::details::fmt_helper::to_string_view;
+using spdlog::details::to_string_view;
 
 // log to str and return it
 template<typename... Args>