Store derived class objects in base class variables

Question

I would like to store instances of several classes in a vector. Since all classes inherit from the same base class this should be possible.

Imagine this program:

Answer 1

TL;DR: You should not inherit from a publicly copyable/movable class.

---

It is actually possible to prevent object slicing, at compilation time: the base object should not be copyable in this context.

Case 1: an abstract base

If the base is abstract, then it cannot be instantiated and thus you cannot experience slicing.

Case 2: a concrete base

If the base is not abstract, then it can be copied (by default). You have two choices:

• prevent copy altogether
• allow copy only for children

Note: in C++11, the move operations cause the same issue.

// C++ 03, prevent copy
class Base {
public:

private:
    Base(Base const&);
    void operator=(Base const&);
};

// C++ 03, allow copy only for children
class Base {
public:

protected:
    Base(Base const& other) { ... }
    Base& operator=(Base const& other) { ...; return *this; }
};

// C++ 11, prevent copy & move
class Base {
public:
    Base(Base&&) = delete;
    Base(Base const&) = delete;
    Base& operator=(Base) = delete;
};

// C++ 11, allow copy & move only for children
class Base {
public:

protected:
    Base(Base&&) = default;
    Base(Base const&) = default;
    Base& operator=(Base) = default;
};