Backport: Use safe mechanism for converting between function pointers and void*.

Backport of changeset 7863:507307a6c315.

2 files changed, 14 insertions, 15 deletions

diff --git a/qt/ScintillaEditBase/PlatQt.cpp b/qt/ScintillaEditBase/PlatQt.cpp
index b4ea7a7d0..281fdba3d 100644
--- a/qt/ScintillaEditBase/PlatQt.cpp
+++ b/qt/ScintillaEditBase/PlatQt.cpp
@@ -1110,17 +1110,17 @@ public:
 	}
 	Function FindFunction(const char *name) override {
 		if (lib) {
-			// C++ standard doesn't like casts between function pointers and void pointers so use a union
-			union {
+			// Use memcpy as it doesn't invoke undefined or conditionally defined behaviour.
 #if QT_VERSION >= QT_VERSION_CHECK(5, 0, 0)
-				QFunctionPointer fp;
+			QFunctionPointer fp {};
 #else
-				void *fp;
+			void *fp = nullptr;
 #endif
-				Function f;
-			} fnConv;
-			fnConv.fp = lib->resolve(name);
-			return fnConv.f;
+			fp = lib->resolve(name);
+			Function f = nullptr;
+			static_assert(sizeof(f) == sizeof(fp), "size mismatch");
+			memcpy(&f, &fp, sizeof(f));
+			return f;
 		}
 		return nullptr;
 	}
diff --git a/win32/PlatWin.cxx b/win32/PlatWin.cxx
index 6553b7486..296f7f0cc 100644
--- a/win32/PlatWin.cxx
+++ b/win32/PlatWin.cxx
@@ -2851,13 +2851,12 @@ public:
 	// Use GetProcAddress to get a pointer to the relevant function.
 	Function FindFunction(const char *name) noexcept override {
 		if (h) {
-			// C++ standard doesn't like casts between function pointers and void pointers so use a union
-			union {
-				FARPROC fp;
-				Function f;
-			} fnConv;
-			fnConv.fp = ::GetProcAddress(h, name);
-			return fnConv.f;
+			// Use memcpy as it doesn't invoke undefined or conditionally defined behaviour.
+			FARPROC fp = ::GetProcAddress(h, name);
+			Function f = nullptr;
+			static_assert(sizeof(f) == sizeof(fp), "size mismatch");
+			memcpy(&f, &fp, sizeof(f));
+			return f;
 		} else {
 			return nullptr;
 		}