Flags slicing of member variables or vtable. Slicing happens when copying a derived object into a base object: the members of the derived object (both member variables and virtual member functions) will be discarded. This can be misleading especially for member function slicing, for example:

struct B { int a; virtual int f(); };
struct D : B { int b; int f() override; };

void use(B b) {  // Missing reference, intended?
  b.f();  // Calls B::f.

D d;
use(d);  // Slice.

See the relevant C++ Core Guidelines sections for details: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es63-dont-slice https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c145-access-polymorphic-objects-through-pointers-and-references