cmake: Improve tiff_target_compile_as_cxx implementation

cmake/CompilerChecks.cmake

@@ -145,16 +145,40 @@ foreach(flag ${test_flags})
     endif (${test_c_flag})
 endforeach(flag ${test_flags})
 
-# Compile C files as C++ for compatibility checking
-if(cxx-compat-mode)
-    if(CMAKE_C_COMPILER_ID STREQUAL "GNU" OR
-            CMAKE_C_COMPILER_ID MATCHES "Clang")
-        # -x c++ forces C++ mode, -std=c++17 for modern C++ compliance
-        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -x c++ -std=c++17")
-        message(STATUS "C++ compatibility mode enabled: compiling C files as C++17")
-    elseif(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
-        # /TP forces C++ mode for all source files
-        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /TP /std:c++17")
-        message(STATUS "C++ compatibility mode enabled: compiling C files as C++17")
+# Function to compile a target's C sources as C++ for compatibility checking
+# This is a better approach than modifying CMAKE_C_FLAGS globally, as it:
+# - Doesn't break CMake's feature detection tests
+# - Uses proper target properties instead of global flags
+# - Follows CMake best practices
+function(tiff_target_compile_as_cxx target_name)
+    if(NOT cxx-compat-mode)
+        return()
     endif()
-endif()
+
+    # Get all source files for the target
+    get_target_property(target_sources ${target_name} SOURCES)
+
+    # Filter for C source files (*.c)
+    set(c_sources "")
+    foreach(source ${target_sources})
+        if(source MATCHES "\\.c$")
+            list(APPEND c_sources ${source})
+        endif()
+    endforeach()
+
+    if(c_sources)
+        # Set these C files to be compiled as C++
+        set_source_files_properties(${c_sources}
+            PROPERTIES
+                LANGUAGE CXX
+        )
+        # Use target properties for C++ standard (more portable than COMPILE_FLAGS)
+        set_target_properties(${target_name}
+            PROPERTIES
+                CXX_STANDARD 17
+                CXX_STANDARD_REQUIRED ON
+                CXX_EXTENSIONS OFF
+        )
+        message(STATUS "C++ compatibility mode: compiling ${target_name} C sources as C++17")
+    endif()
+endfunction()

contrib/addtiffo/CMakeLists.txt

@@ -26,6 +26,9 @@ add_executable(addtiffo addtiffo.c tif_overview.c tif_ovrcache.c tif_ovrcache.h)
 set_target_properties(addtiffo PROPERTIES LINKER_LANGUAGE CXX)
 target_link_libraries(addtiffo tiff tiff_port)
 
+# Apply C++ compatibility mode if enabled
+tiff_target_compile_as_cxx(addtiffo)
+
 if(WEBP_SUPPORT AND EMSCRIPTEN)
   # Emscripten is pretty finnicky about linker flags.
   # It needs --shared-memory if and only if atomics or bulk-memory is used.

contrib/addtiffo/addtiffo.c

@@ -60,13 +60,11 @@
  */
 
 #include "tiffio.h"
+#include "tif_ovrcache.h"
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-void TIFFBuildOverviews(TIFF *, int, int *, int, const char *,
-                        int (*)(double, void *), void *);
-
 /************************************************************************/
 /*                                main()                                */
 /************************************************************************/

contrib/addtiffo/tif_overview.c

@@ -67,8 +67,17 @@
 
 #define TIFF_DIR_MAX 65534
 
-void TIFFBuildOverviews(TIFF *, int, int *, int, const char *,
-                        int (*)(double, void *), void *);
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+    void TIFFBuildOverviews(TIFF *, int, int *, int, const char *,
+                            int (*)(double, void *), void *);
+
+#ifdef __cplusplus
+}
+#endif
 
 /************************************************************************/
 /*                         TIFF_WriteOverview()                         */

contrib/dbs/CMakeLists.txt

@@ -38,6 +38,11 @@ add_executable(tiff-rgb tiff-rgb.c)
 set_target_properties(tiff-rgb PROPERTIES LINKER_LANGUAGE CXX)
 target_link_libraries(tiff-rgb tiff tiff_port ${CMath_LIBRARIES})
 
+# Apply C++ compatibility mode to all contrib/dbs targets if enabled
+foreach(target tiff-bi tiff-grayscale tiff-palette tiff-rgb)
+  tiff_target_compile_as_cxx(${target})
+endforeach()
+
 if(WEBP_SUPPORT AND EMSCRIPTEN)
   # Emscripten is pretty finnicky about linker flags.
   # It needs --shared-memory if and only if atomics or bulk-memory is used.

contrib/iptcutil/CMakeLists.txt

@@ -25,3 +25,6 @@
 add_executable(iptcutil iptcutil.c)
 set_target_properties(iptcutil PROPERTIES LINKER_LANGUAGE CXX)
 target_link_libraries(iptcutil tiff tiff_port)
+
+# Apply C++ compatibility mode if enabled
+tiff_target_compile_as_cxx(iptcutil)

libtiff/CMakeLists.txt

@@ -115,6 +115,9 @@ else()
   set_property(SOURCE tif_unix.c APPEND PROPERTY COMPILE_DEFINITIONS ALLOW_TIFF_NON_EXT_ALLOC_FUNCTIONS)
 endif()
 
+# Apply C++ compatibility mode if enabled
+tiff_target_compile_as_cxx(tiff)
+
 target_include_directories(tiff
     PUBLIC
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>

port/CMakeLists.txt

@@ -39,6 +39,9 @@ if(NOT HAVE_GETOPT)
             ${CMAKE_CURRENT_SOURCE_DIR}/getopt.c)
   endif()
 
+  # Apply C++ compatibility mode if enabled
+  tiff_target_compile_as_cxx(tiff_port)
+
   target_include_directories(tiff_port PUBLIC
           ${CMAKE_CURRENT_BINARY_DIR}
           ${CMAKE_CURRENT_SOURCE_DIR})

test/CMakeLists.txt

@@ -238,6 +238,11 @@ target_link_libraries(test_RGBAImage PRIVATE tiff tiff_port)
 target_compile_definitions(test_RGBAImage PRIVATE SOURCE_DIR=\"${CMAKE_CURRENT_SOURCE_DIR}\")
 list(APPEND simple_tests test_RGBAImage)
 
+# Apply C++ compatibility mode to all test targets if enabled
+foreach(target ${simple_tests})
+  tiff_target_compile_as_cxx(${target})
+endforeach()
+
 if(WEBP_SUPPORT AND EMSCRIPTEN)
   # Emscripten is pretty finnicky about linker flags.
   # It needs --shared-memory if and only if atomics or bulk-memory is used.

tools/CMakeLists.txt

@@ -176,6 +176,38 @@ if(OPENGL_SUPPORT)
           RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")
 endif()
 
+# Apply C++ compatibility mode to all tool targets if enabled
+foreach(target fax2ps
+               fax2tiff
+               pal2rgb
+               ppm2tiff
+               raw2tiff
+               rgb2ycbcr
+               thumbnail
+               tiff2bw
+               tiff2pdf
+               tiff2ps
+               tiff2rgba
+               tiffcmp
+               tiffcp
+               tiffcrop
+               tiffdither
+               tiffdump
+               tiffinfo
+               tiffmedian
+               tiffset
+               tiffsplit)
+  tiff_target_compile_as_cxx(${target})
+endforeach()
+
+if(NOT CMAKE_CROSSCOMPILING)
+  tiff_target_compile_as_cxx(tiff_mkg3states)
+endif()
+
+if(OPENGL_SUPPORT)
+  tiff_target_compile_as_cxx(tiffgt)
+endif()
+
 if(WEBP_SUPPORT AND EMSCRIPTEN)
   # Emscripten is pretty finnicky about linker flags.
   # It needs --shared-memory if and only if atomics or bulk-memory is used.