Mastering the C++17 STL

std::any versus polymorphic class types

std::any occupies a position in between the compile-time polymorphism of std::variant<A, B, C> and the runtime polymorphism of polymorphic inheritance hierarchies and dynamic_cast. You might wonder whether std::any interacts with the machinery of dynamic_cast at all. The answer is "no, it does not"--nor is there any standard way to get that behavior. std::any is one hundred percent statically type-safe: there is no way to break into it and get a "pointer to the data" (for example, a void *) without knowing the exact static type of that data:

    struct Animal {      virtual ~Animal() = default;    };    struct Cat : Animal {};    void test()    {      std::any a = Cat{};      // The held object is a "Cat"... assert(a.type() == typeid(Cat)); ...