Target wide C++ standard applying

also export this information via cmake configuration as a part of the
public interface.

CMakeLists.txt

@@ -16,8 +16,8 @@ set(AV_ENABLE_STATIC On CACHE BOOL "Enable static library build (On)")
 set(AV_ENABLE_SHARED On CACHE BOOL "Enable shared library build (On)")
 set(AV_BUILD_EXAMPLES On CACHE BOOL "Build example applications (On)")
 
-# Compiler-specific C++11 activation.
-set(CMAKE_CXX_STANDARD 17)
+# Compiler-specific C++ standard activation
+#set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED yes)
 
 # Warnings

src/CMakeLists.txt

@@ -38,6 +38,19 @@ foreach(TARGET ${AV_TARGETS})
           ${CMAKE_CURRENT_SOURCE_DIR})
     set_target_properties(${TARGET} PROPERTIES PUBLIC_HEADER "${AV_HEADERS}")
 
+    # If not CMAKE_CXX_STANARD provided globally (subproject), use latest supported one
+    if (NOT CMAKE_CXX_STANDARD)
+        if ("cxx_std_26" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+            target_compile_features(${TARGET} PUBLIC cxx_std_26)
+        elseif ("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+            target_compile_features(${TARGET} PUBLIC cxx_std_23)
+        elseif("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+            target_compile_features(${TARGET} PUBLIC cxx_std_20)
+        else()
+            target_compile_features(${TARGET} PUBLIC cxx_std_17)
+        endif()
+    endif()
+
     if(WIN32)
       target_link_libraries(${TARGET} PRIVATE ws2_32)
     endif()

src/avutils.h

@@ -45,6 +45,14 @@ extern "C" {
 #    define FF_ENABLE_DEPRECATION_WARNINGS
 #endif
 
+#if __has_include(<compare>)
+#  include <compare>
+#  if defined(__cpp_lib_three_way_comparison) && __cpp_lib_three_way_comparison >= 201907
+#    define AVCPP_USE_SPACESHIP_OPERATOR 1
+#  endif
+#endif
+
+
 //
 // Functions
 //

src/rational.h

@@ -5,11 +5,12 @@
 #include <memory>
 #include <type_traits>
 
-#if __cplusplus > 201703L
-#include <compare>
-#endif
-
 #include "ffmpeg.h"
+#include "avutils.h"
+
+#ifdef AVCPP_USE_SPACESHIP_OPERATOR
+#  include <compare>
+#endif
 
 namespace av
 {
@@ -50,7 +51,7 @@ class Rational
     Rational& operator=  (double value) noexcept;
 
     bool      operator== (const Rational   &other) const noexcept;
-#if __cplusplus > 201703L
+#ifdef AVCPP_USE_SPACESHIP_OPERATOR
     std::strong_ordering operator<=>(const Rational &other) const noexcept
     {
         switch (threewaycmp(other)) {