feat: Add KDBindings_QT_NO_EMIT for Qt compatibility

As mentioned in #79, KDBindings currently has issues in combination with
Qt's `emit` keyword.
So warn the user and add an option to set QT_NO_EMIT, which should solve
this issue for most users.

CMakeLists.txt

@@ -41,6 +41,7 @@ option(${PROJECT_NAME}_TESTS "Build the tests" ON)
 option(${PROJECT_NAME}_EXAMPLES "Build the examples" ON)
 option(${PROJECT_NAME}_DOCS "Build the API documentation" OFF)
 option(${PROJECT_NAME}_ERROR_ON_WARNING "Enable all compiler warnings and treat them as errors" OFF)
+option(${PROJECT_NAME}_QT_NO_EMIT "Qt Compatibility: Disable Qt's `emit` keyword" OFF)
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/ECM/modules)

README.md

@@ -21,6 +21,16 @@ Find more information at:
  * [detailed browsable API reference](https://docs.kdab.com/kdbindings/latest/getting-started/)
  * [our assorted example programs](https://github.com/KDAB/KDBindings/blob/main/examples)
 
+Compatibility with Qt
+=====================
+Because Qt defines `emit` as a keyword, Qt is by default incompatible with KDBindings, which uses `emit` as a function name.
+
+This can only be fixed by defining `QT_NO_EMIT` or `QT_NO_KEYWORDS` and simply omitting `emit` in your Qt code.
+
+To make this easy, KDBindings includes a CMake option: `KDBindings_QT_NO_EMIT`, which defines this for all dependents of KDBindings.
+
+See also: [#79](https://github.com/KDAB/KDBindings/issues/79)
+
 Contact
 =======
 * Visit us on GitHub: https://github.com/KDAB/KDBindings

src/kdbindings/CMakeLists.txt

@@ -40,6 +40,9 @@ set_target_properties(KDBindings PROPERTIES
 if(KDBINDINGS_ENABLE_WARN_UNUSED)
   target_compile_definitions(KDBindings INTERFACE KDBINDINGS_ENABLE_WARN_UNUSED=1)
 endif()
+if(KDBindings_QT_NO_EMIT)
+  target_compile_definitions(KDBindings INTERFACE QT_NO_EMIT)
+endif()
 
 target_include_directories(KDBindings INTERFACE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>

src/kdbindings/signal.h

@@ -17,6 +17,16 @@
 #include <type_traits>
 #include <utility>
 
+#ifdef emit
+static_assert(false, "KDBindings is not compatible with Qt's 'emit' keyword.\n"
+                     "To use KDBindings with Qt, please define QT_NO_EMIT to disable Qt's 'emit' keyword.\n"
+                     "If you're using CMake you can set KDBindings_QT_NO_EMIT to ON to add this.");
+
+// Undefine emit to suppress more compiler errors after the static_assert failure.
+// Otherwise the compiler would drown our custom error message with more errors.
+#undef emit
+#endif
+
 #include <kdbindings/connection_evaluator.h>
 #include <kdbindings/genindex_array.h>
 #include <kdbindings/utils.h>
@@ -29,7 +39,6 @@
  * All public parts of KDBindings are members of this namespace.
  */
 namespace KDBindings {
-
 /**
  * @brief A Signal provides a mechanism for communication between objects.
  *