cmake: update CXX11 compiler flag

CMakeLists.txt

@@ -110,14 +110,12 @@ if(POLICY CMP0056)
   cmake_policy(SET CMP0056 NEW)
 endif()
 
-include(cmake/OpenCVUtils.cmake)
-
-if(OPENCV_CXX11)
-  #cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
-  set(CMAKE_CXX_STANDARD 11)
-  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+if(POLICY CMP0067)
+  cmake_policy(SET CMP0067 NEW)
 endif()
 
+include(cmake/OpenCVUtils.cmake)
+
 # must go before the project command
 ocv_update(CMAKE_CONFIGURATION_TYPES "Debug;Release" CACHE STRING "Configs" FORCE)
 if(DEFINED CMAKE_BUILD_TYPE)
@@ -149,6 +147,7 @@ endif()
 # ----------------------------------------------------------------------------
 # Detect compiler and target platform architecture
 # ----------------------------------------------------------------------------
+OCV_OPTION(ENABLE_CXX11 "Enable C++11 compilation mode" "${OPENCV_CXX11}")
 include(cmake/OpenCVDetectCXXCompiler.cmake)
 
 # Add these standard paths to the search paths for FIND_LIBRARY
@@ -305,7 +304,6 @@ OCV_OPTION(INSTALL_TESTS            "Install accuracy and performance test binar
 
 # OpenCV build options
 # ===================================================
-OCV_OPTION(OPENCV_CXX11               "Enable C++11 compilation mode"                            OFF )
 OCV_OPTION(ENABLE_CCACHE              "Use ccache"                                               (UNIX AND NOT IOS AND (CMAKE_GENERATOR MATCHES "Makefile" OR CMAKE_GENERATOR MATCHES "Ninja")) )
 OCV_OPTION(ENABLE_PRECOMPILED_HEADERS "Use precompiled headers"                                  ON IF (NOT IOS AND NOT CMAKE_CROSSCOMPILING) )
 OCV_OPTION(ENABLE_SOLUTION_FOLDERS    "Solution folder in Visual Studio or in other IDEs"        (MSVC_IDE OR CMAKE_GENERATOR MATCHES Xcode) )
@@ -963,8 +961,8 @@ string(STRIP "${OPENCV_COMPILER_STR}" OPENCV_COMPILER_STR)
 status("")
 status("  C/C++:")
 status("    Built as dynamic libs?:" BUILD_SHARED_LIBS THEN YES ELSE NO)
-if(OPENCV_CXX11)
-status("    C++11:" "YES")
+if(ENABLE_CXX11 OR HAVE_CXX11)
+status("    C++11:" HAVE_CXX11 THEN YES ELSE NO)
 endif()
 status("    C++ Compiler:"           ${OPENCV_COMPILER_STR})
 status("    C++ flags (Release):"    ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_RELEASE})

cmake/OpenCVDetectCXXCompiler.cmake

@@ -182,3 +182,23 @@ if(CMAKE_VERSION VERSION_LESS "3.1")
     endif()
   endforeach()
 endif()
+
+if(ENABLE_CXX11)
+  #cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
+  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+  set(CMAKE_CXX_EXTENSIONS OFF) # use -std=c++11 instead of -std=gnu++11
+  if(XCMAKE_CXX11_COMPILE_FEATURES)
+    set(HAVE_CXX11 ON)
+  endif()
+endif()
+if(NOT HAVE_CXX11)
+  ocv_check_compiler_flag(CXX "" HAVE_CXX11 "${OpenCV_SOURCE_DIR}/cmake/checks/cxx11.cpp")
+  if(NOT HAVE_CXX11 AND ENABLE_CXX11)
+    ocv_check_compiler_flag(CXX "-std=c++11" HAVE_STD_CXX11 "${OpenCV_SOURCE_DIR}/cmake/checks/cxx11.cpp")
+    if(HAVE_STD_CXX11)
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+      set(HAVE_CXX11 ON)
+    endif()
+  endif()
+endif()

cmake/checks/cxx11.cpp

@@ -0,0 +1,13 @@
+#if __cplusplus >= 201103L || (defined(_MSC_VER) && _MSC_VER >= 1600)
+// OK
+#else
+#error "C++11 is not supported"
+#endif
+
+static int test() { return 0; }
+
+int main()
+{
+    auto res = test();
+    return res;
+}