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:

#include <iostream>
#include <vector>
using namespace std;

class Base
{
    public:
    virtual void identify ()
    {
        cout << "BASE" << endl;
    }
};

class Derived: public Base
{
    public:
    virtual void identify ()
    {
        cout << "DERIVED" << endl;
    }
};

int main ()
{
    Derived derived;
    
    vector<Base> vect;
    vect.push_back(derived);
    
    vect[0].identify();
    return 0;
}

I expected it to print "DERIVED", because the identify() method is virtual. Instead vect[0] seems to be a Base instance and it prints "BASE".

I guess I could write my own container (probably derived from vector) somehow that is capable of doing this (maybe holding only pointers...).

I just wanted to ask if there is a more C++'ish way to do this. AND I would like to be completely vector-compatible (just for convenience if other users should ever use my code).

Budgerigar

I wonder what happens if you create a base class with a pure virtual function identify and then make both your Base and your Derived inherit from that base class. – Cuff

@Mr Lister Thank you for your comment but this doesn't change the result. – Budgerigar

See also this. – Wil

Answer 1

What you are seeing is Object Slicing.
You are storing object of Derived class in an vector which is supposed to store objects of Base class, this leads to Object slicing and the derived class specific members of the object being stored get sliced off, thus the object stored in the vector just acts as object of Base class.

Solution:

You should store pointer to object of Base class in the vector:

vector<Base*> 

By storing a pointer to Base class there would be no slicing and you can achieve the desired polymorphic behavior as well.
Since you ask for a C++ish way of doing this, the right approach is to use a suitable Smart pointer instead of storing a raw pointer in the vector. That will ensure you do not have to manually manage the memory, RAII will do that for you automatically.

Burmeister

Instead of Vector<Base*> use Vector<unique_ptr<Base>>. Then instead of vec.push_back, use vec.emplace_back(new Derived()). – Knightly

@ShitalShah And if I want to assign something in this new Derived before store it in the vector? Now I have something like this: void Add(const CThing & ins){ things.push_back(ins); } – Heyerdahl

Answer 2

You're experiencing slicing. The vector copies the derived object, a new one of type Base is inserted.

Hereditary

Answer 3

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;
};

Fortson

I cannot get this to work. Could you please expand this example to work (both the C++11 and the classic versions). Unfortunately I cannot post what I've done so far. – Lippi

@Zingam: Without it is difficult to get to the heart of your issue; I advise that you post a new question and if you do not get useful answers do not hesitate to answer this comment and point me toward your question. – Fortson

Is there a reason why the C++ 11 version declares the constructors as public and not as private? Does it matter if they are delete'd and public or private? – Lippi

@Zingam: The accessibility of a deleted method should not matter at all, so it is mostly to group all special methods together rather than split them apart. – Fortson

Answer 4

I'd use vector<Base*> to store them. If you say vector<Base>, slicing will occur.

This does mean that you'd have to delete the actual objects yourself after you've removed the pointers from your vector, but otherwise you should be fine.

Susumu

Answer 5

// Below is the solution by using vector<Based*> vect,
// Base *pBase , and initialized pBase with
// with the address of derived which is
// of type Derived

#include <iostream>
#include <vector>

using namespace std;

class Base
{

public:

virtual void identify ()
{
    cout << "BASE" << endl;
}
};

class Derived: public Base
{
public:
virtual void identify ()
{
    cout << "DERIVED" << endl;
}
};

int main ()

{
Base *pBase; // The pointer pBase of type " pointer to Base"
Derived derived;
// PBase is initialized with the address of derived which is
// of type Derived

pBase = & derived;
// Store pointer to object of Base class in the vector:
vector<Base*> vect;
// Add an element to vect using pBase which is initialized with the address 
// of derived
vect.push_back(pBase);
vect[0]->identify();
return 0;
}

Rudderpost

Answer 6

As all others mentioned here, you cannot accomplish to insert an object of derived to a vector of base due to object slicing that would happen on copy construction.

If goal is to avoid memory allocation, you can use an std::variant, but the vector would no longer be of base class

using HierarchyItem = std::variant<Base, Derived>;

int main()
{
    vector<HierarchyItem> vect;
    vect.push_back(Derived());

    std::visit([](auto &&hier_item){ hier_item.identify(); }, vect[0]);

    return 0;
}

Parang