Virtual destructors only in selected bases

Question

This is a cornercase not covered by this (although interesting info is provided therein). My code looks like the following :

struct concept {
    virtual ~concept() = default; 
}; 

struct policy {
protected : 
    ~policy() = default; 
}; 

struct implementation final : concept, policy {
}; 

If I'm only using this hierarchy through pointers to concept :

unique_ptr<concept> = make_unique<implementation>(); 

is the above safe ?

I believe it is because if someone attempted to delete via a pointer to policy the protected destructor wouldn't allow it (a trick from Modern C++ design) but does the rest work fine? (namely when the pointer to concept is deleted are the destructors of the hierarchy guaranteed to be called correctly ? )

Answer 1

Since ~concept() is virtual, when you delete the object via a concept pointer, it will call ~implementation().

I don't see any reason why the rest will not work like they are supposed to.