Access polymorphic objects through pointers and references

Reason

If you have a class with a virtual function, you don't (in general) know which class provided the function to be used.

Example

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

void use(B b)
{
    D d;
    B b2 = d;   // slice
    B b3 = b;
}

void use2()
{
    D d;
    use(d);   // slice
}

Both ds are sliced.

Exception

You can safely access a named polymorphic object in the scope of its definition, just don't slice it.

void use3()
{
    D d;
    d.f();   // OK
}

See also

A polymorphic class should suppress copying

Enforcement

Flag all slicing.