cmake: require C++11 and CMake 3.5.1+

CMakeLists.txt

@@ -149,7 +149,6 @@ endif()
 # ----------------------------------------------------------------------------
 # Detect compiler and target platform architecture
 # ----------------------------------------------------------------------------
-OCV_OPTION(ENABLE_CXX11 "Enable C++11 compilation mode" "${OPENCV_CXX11}")
 include(cmake/OpenCVDetectCXXCompiler.cmake)
 ocv_cmake_hook(POST_DETECT_COMPILER)
 
@@ -1018,9 +1017,6 @@ 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(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})
 status("    C++ flags (Debug):"      ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_DEBUG})

cmake/OpenCVDetectCXXCompiler.cmake

@@ -166,14 +166,11 @@ if(CMAKE_VERSION VERSION_LESS "3.1")
   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(CMAKE_CXX11_COMPILE_FEATURES)
+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(CMAKE_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")
@@ -185,3 +182,6 @@ if(NOT HAVE_CXX11)
     endif()
   endif()
 endif()
+if(NOT HAVE_CXX11)
+  message(FATAL_ERROR "OpenCV 4.x requires C++11")
+endif()

cmake/OpenCVMinDepVersions.cmake

-set(MIN_VER_CMAKE 2.8.12.2)
+set(MIN_VER_CMAKE 3.5.1)
 set(MIN_VER_CUDA 6.5)
 set(MIN_VER_PYTHON2 2.6)
 set(MIN_VER_PYTHON3 3.2)

doc/tutorials/dnn/dnn_halide/dnn_halide.markdown

@@ -68,8 +68,6 @@ MSBuild.exe /m:4 /t:Build /p:Configuration=Release .\\ALL_BUILD.vcxproj
 ## Build OpenCV with Halide backend
 When you build OpenCV add the following configuration flags:
 
-- `ENABLE_CXX11` - enable C++11 standard
-
 - `WITH_HALIDE` - enable Halide linkage
 
 - `HALIDE_ROOT_DIR` - path to Halide build directory

samples/CMakeLists.txt

@@ -81,7 +81,11 @@ else()
 #  Standalone mode
 #
 #===================================================================================================
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.1)
+
+# Enable C++11
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
 
 project(samples C CXX)
 option(BUILD_EXAMPLES "Build samples" ON)

samples/cpp/example_cmake/CMakeLists.txt

 # cmake needs this line
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.1)
+
+# Enable C++11
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
 
 # Define project name
 project(opencv_example_project)
@@ -18,11 +22,6 @@ message(STATUS "    version: ${OpenCV_VERSION}")
 message(STATUS "    libraries: ${OpenCV_LIBS}")
 message(STATUS "    include path: ${OpenCV_INCLUDE_DIRS}")
 
-if(CMAKE_VERSION VERSION_LESS "2.8.11")
-  # Add OpenCV headers location to your include paths
-  include_directories(${OpenCV_INCLUDE_DIRS})
-endif()
-
 # Declare the executable target built from your sources
 add_executable(opencv_example example.cpp)