CMake: Enforce minimum C++ mode support

CMakeLists.txt

@@ -14,6 +14,12 @@ read_version("${CMAKE_CURRENT_SOURCE_DIR}/version.pri" VER_MAJOR VER_MINOR VER_B
 
 project(qBittorrent VERSION ${VER_MAJOR}.${VER_MINOR}.${VER_BUGFIX}.${VER_BUILD})
 
+# check for invalid compiler version/CXX standard as early as possible
+include(FunctionQbtCXXCompilerAndModeCheck)
+qbt_minimum_cxx_mode_check(14)
+message(STATUS "Building in C++${CMAKE_CXX_STANDARD} mode.\n"
+            "Make sure libtorrent was built with the same C++ mode for ABI compatibility.")
+
 set(PROJECT_VERSION "${VER_MAJOR}.${VER_MINOR}.${VER_BUGFIX}")
 
 if (NOT VER_BUILD EQUAL 0)

cmake/Modules/FunctionQbtCXXCompilerAndModeCheck.cmake

@@ -0,0 +1,24 @@
+# Function for ensuring the build does not use a lower than the required minimum C++ mode, _min_std.
+# It fails the build if:
+# - the compiler does not fully support _min_std
+# - the user specified a mode lower than _min_std via the CMAKE_CXX_STANDARD variable
+# If both checks are successful, it sets the following variables at the PARENT_SCOPE:
+# - CMAKE_CXX_STANDARD with the value _min_std, if it was not already set to a value >= _min_std
+# - CMAKE_CXX_STANDARD_REQUIRED with the value ON
+
+function(qbt_minimum_cxx_mode_check _min_std)
+    # ensure the compiler fully supports the minimum required C++ mode.
+    if(NOT CMAKE_CXX${_min_std}_STANDARD__HAS_FULL_SUPPORT)
+        message(FATAL_ERROR "${PROJECT_NAME} requires a compiler with full C++${_min_std} support")
+    endif()
+
+    # now we know that the compiler fully supports the minimum required C++ mode,
+    # but we must still prevent the user or compiler from forcing/defaulting to an insufficient C++ mode
+    if(NOT CMAKE_CXX_STANDARD)
+        set(CMAKE_CXX_STANDARD ${_min_std} PARENT_SCOPE)
+    elseif((CMAKE_CXX_STANDARD VERSION_LESS ${_min_std}) OR (CMAKE_CXX_STANDARD VERSION_EQUAL 98))
+        message(FATAL_ERROR "${PROJECT_NAME} has to be built with at least C++${_min_std} mode.")
+    endif()
+
+    set(CMAKE_CXX_STANDARD_REQUIRED ON PARENT_SCOPE)
+endfunction()

cmake/Modules/MacroQbtCompilerSettings.cmake

@@ -24,11 +24,6 @@ macro(qbt_set_compiler_options)
             #"-Wno-error=sign-conversion -Wno-error=float-equal"
         )
 
-        # GCC 4.8 has problems with std::array and its initialization
-        if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9)
-            list(APPEND _GCC_COMMON_CXX_FLAGS "-Wno-error=missing-field-initializers")
-        endif()
-
         include(CheckCXXCompilerFlag)
         # check for -pedantic
         check_cxx_compiler_flag(-pedantic _PEDANTIC_IS_SUPPORTED)

src/CMakeLists.txt

@@ -1,14 +1,3 @@
-set(CMAKE_CXX_STANDARD_REQUIRED True)
-
-# If C++14 is available, use it as libtorent ABI depends on 11/14 version
-if (cxx_std_14 IN_LIST CMAKE_CXX_COMPILE_FEATURES)
-    message(STATUS "Building in C++14 mode")
-    set(CMAKE_CXX_STANDARD "14")
-else()
-    message(STATUS "Building in C++11 mode")
-    set(CMAKE_CXX_STANDARD "11")
-endif()
-
 include(MacroQbtCompilerSettings)
 qbt_set_compiler_options()