cmake: add HIDAPI_BUILD_AS_CXX to compile sources as C++

Add an opt-in CMake variable HIDAPI_BUILD_AS_CXX that, when set to
TRUE, attaches LANGUAGE CXX to each backend's C source file so it is
compiled with the C++ compiler instead of the C compiler. Useful as a
portability check, to confirm hidapi sources remain compatible with
consumers that embed them into a C++ translation unit.

The variable is intentionally not declared as a CMake option(), so it
is not surfaced in cmake-gui or `cmake -L` output. It is documented
under "Testing-only variables" in BUILD.cmake.md.

Wire it through every backend (linux, libusb, mac, netbsd, windows).
For the windows backend, both hid.c and hidapi_descriptor_reconstruct.c
are re-tagged.

CI: enable HIDAPI_BUILD_AS_CXX=ON on the ubuntu-cmake, macos-cmake and
windows-cmake jobs to guard against regressions. The MSVC variant also
sets CMAKE_CXX_STANDARD=20 so the designated initialisers in the
Windows backend compile as ISO C++ (g++ and Clang accept them as a GNU
extension in their default mode).

Assisted-by: Claude:claude-opus-4.7

Author: Youw

.github/workflows/builds.yml

@@ -90,6 +90,15 @@ jobs:
         cd build/static_test
         make install
 
+    - name: Check C++ portability build
+      run: |
+        rm -rf build/cxx
+        cmake -B build/cxx -S hidapisrc \
+          -DCMAKE_BUILD_TYPE=Release \
+          -DHIDAPI_BUILD_AS_CXX=ON \
+          "-DCMAKE_CXX_FLAGS=-Wall -Wextra -Werror"
+        cmake --build build/cxx
+
     - name: Check Meson build
       run: |
         meson setup build_meson hidapisrc
@@ -154,6 +163,15 @@ jobs:
         cd build/static_test
         make install
 
+    - name: Check C++ portability build
+      run: |
+        rm -rf build/cxx
+        cmake -B build/cxx -S hidapisrc \
+          -DCMAKE_BUILD_TYPE=Release \
+          -DHIDAPI_BUILD_AS_CXX=ON \
+          "-DCMAKE_CXX_FLAGS=-Wall -Wextra -Werror -Wformat-signedness"
+        cmake --build build/cxx
+
     - name: Check Meson build
       run: |
         meson setup build_meson hidapisrc
@@ -206,6 +224,14 @@ jobs:
       working-directory: build/msvc
       run: ctest -C RelWithDebInfo --no-compress-output --output-on-failure
 
+    - name: Configure CMake MSVC C++
+      shell: cmd
+      run: |
+        cmake -B build\msvc_cxx -S hidapisrc -DCMAKE_BUILD_TYPE=Release -DHIDAPI_BUILD_AS_CXX=ON -DCMAKE_CXX_STANDARD=20 "-DCMAKE_CXX_FLAGS=%MSVC_COMPILE_FLAGS%"
+    - name: Build CMake MSVC C++
+      working-directory: build/msvc_cxx
+      run: cmake --build . --config Release
+
     - name: Configure CMake NMake MSVC
       shell: cmd
       run: |

BUILD.cmake.md

@@ -101,6 +101,13 @@ currently this option is only available on Windows, since only Windows backend h
 
 </details><br>
 
+<details>
+  <summary>Testing-only variables</summary>
+
+  - `HIDAPI_BUILD_AS_CXX` - when set to TRUE, the HIDAPI C source files are compiled with a C++ compiler instead of a C compiler. Useful to confirm that HIDAPI's sources stay compatible with consumers that include them into a C++ translation unit. Not intended for production builds; defaults to FALSE.
+
+</details><br>
+
 To see all most-useful CMake variables available for HIDAPI, one of the most convenient ways is too use [`cmake-gui`](https://cmake.org/cmake/help/latest/manual/cmake-gui.1.html) tool ([example](https://cmake.org/runningcmake/)).
 
 _NOTE_: HIDAPI packages built by CMake can be used with `pkg-config`, as if built with [Autotools](BUILD.autotools.md).

libusb/CMakeLists.txt

@@ -4,6 +4,9 @@ add_library(hidapi_libusb
     ${HIDAPI_PUBLIC_HEADERS}
     hid.c
 )
+if(HIDAPI_BUILD_AS_CXX)
+    set_source_files_properties(hid.c PROPERTIES LANGUAGE CXX)
+endif()
 target_link_libraries(hidapi_libusb PUBLIC hidapi_include)
 
 if(TARGET usb-1.0)

linux/CMakeLists.txt

@@ -2,6 +2,9 @@ add_library(hidapi_hidraw
     ${HIDAPI_PUBLIC_HEADERS}
     hid.c
 )
+if(HIDAPI_BUILD_AS_CXX)
+    set_source_files_properties(hid.c PROPERTIES LANGUAGE CXX)
+endif()
 target_link_libraries(hidapi_hidraw PUBLIC hidapi_include)
 
 find_package(Threads REQUIRED)

mac/CMakeLists.txt

@@ -4,6 +4,9 @@ add_library(hidapi_darwin
     ${HIDAPI_PUBLIC_HEADERS}
     hid.c
 )
+if(HIDAPI_BUILD_AS_CXX)
+    set_source_files_properties(hid.c PROPERTIES LANGUAGE CXX)
+endif()
 
 find_package(Threads REQUIRED)
 

netbsd/CMakeLists.txt

@@ -2,6 +2,9 @@ add_library(hidapi_netbsd
     ${HIDAPI_PUBLIC_HEADERS}
     hid.c
 )
+if(HIDAPI_BUILD_AS_CXX)
+    set_source_files_properties(hid.c PROPERTIES LANGUAGE CXX)
+endif()
 target_link_libraries(hidapi_netbsd PUBLIC hidapi_include)
 
 find_package(Threads REQUIRED)

src/CMakeLists.txt

@@ -38,6 +38,12 @@ endif()
 if(NOT DEFINED CMAKE_POSITION_INDEPENDENT_CODE)
     set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 endif()
+if(NOT DEFINED HIDAPI_BUILD_AS_CXX)
+    set(HIDAPI_BUILD_AS_CXX FALSE)
+endif()
+if(HIDAPI_BUILD_AS_CXX)
+    enable_language(CXX)
+endif()
 
 get_directory_property(IS_EXCLUDE_FROM_ALL EXCLUDE_FROM_ALL)
 if(IS_EXCLUDE_FROM_ALL)

windows/CMakeLists.txt

@@ -18,6 +18,9 @@ add_library(hidapi_winapi
     ${HIDAPI_PUBLIC_HEADERS}
     ${SOURCES}
 )
+if(HIDAPI_BUILD_AS_CXX)
+    set_source_files_properties(hid.c hidapi_descriptor_reconstruct.c PROPERTIES LANGUAGE CXX)
+endif()
 target_link_libraries(hidapi_winapi
     PUBLIC hidapi_include
 )