cl: c.Func (llgo.funcAddr); demo: cppintf (how to use c++ interface)

_demo/cppintf/foo/bar/bar.cpp

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#define interface struct
+
+interface ICallback {
+	virtual int valA() = 0;
+	virtual double valB() = 0;
+};
+
+extern "C" void f(ICallback* cb) {
+	printf("Hello %d, %lf!\n", cb->valA(), cb->valB());
+}

_demo/cppintf/foo/foo.go

@@ -0,0 +1,22 @@
+package foo
+
+import (
+	"unsafe"
+)
+
+const (
+	LLGoFiles   = "bar/bar.cpp"
+	LLGoPackage = "link"
+)
+
+type Callback struct {
+	Vptr *CallbackVtbl
+}
+
+type CallbackVtbl struct {
+	ValA unsafe.Pointer
+	ValB unsafe.Pointer
+}
+
+//go:linkname F C.f
+func F(cb *Callback)