Remove unused code and migrate to CMake build system.

cpp/classes/main.cpp

@@ -0,0 +1,24 @@
+#include "ClassA.h"
+
+int main() {
+    ClassA a{};
+    a.Print();
+
+    ClassA *pA = nullptr;
+    pA = new ClassA();
+    pA->Print();
+    // (*pA).Print(); // same line as above, but more complex to write
+    delete pA;
+    pA = nullptr;
+
+    a.CompareTo(pA);
+
+    IComparable *pComparable = nullptr;
+    pComparable = new ClassA();
+    pComparable->CompareTo(pA);
+    // pA = dynamic_cast<ClassA *>(pComparable);
+    delete pComparable;
+    pComparable = nullptr;
+
+    return 0;
+}