Preventing slicing in copy constructor

Question

I want to copy a vector of type Foo objects but the objects can be several different derived types of Foo. I can't figure out how to copy without slicing. Here's my toy code

#include "stdafx.h"
#include <memory>
#include <vector>
#include <string>
#include <iostream>

class Foo
{
public:
    Foo() { m_x = "abc"; }
    Foo( const Foo &other ) { m_x = other.m_x; }
    virtual std::string ToString() { return m_x; }
    std::string m_x;
};

class FooDerivedA : public Foo
{
public:
    FooDerivedA() : Foo() { m_y = 123; }
    std::string ToString() { return m_x + ", " + std::to_string( m_y ); }
    int m_y;
};

class FooDerivedB : public Foo
{
public:
    FooDerivedB() : Foo() { m_z = true; }
    std::string ToString() { return m_x + ", " + std::to_string( m_z ); }
    bool m_z;
};

class Foos
{
public:
    Foos(){}
    Foos( const Foos &other )
    {
        for ( auto &foo : other.m_Foos )
        {
            // I believe this is slicing. How can I prevent this?
            auto f = std::unique_ptr<Foo>( new Foo( *foo ) ); 
            m_Foos.push_back( std::move( f ) );
        }
    }
    void Add( std::unique_ptr<Foo> foo ) { m_Foos.push_back( std::move( foo ) ); }
    std::string ToString() 
    {
        std::string s;
        for ( auto &foo : m_Foos )
        {
            s += foo->ToString() + "\n";
        }
        return s;
    }
private:
    std::vector<std::unique_ptr<Foo>> m_Foos;
};

int main()
{
    Foos f1;
    f1.Add( std::unique_ptr<FooDerivedA>( new FooDerivedA ) );
    auto f2 = Foos( f1 );
    std::cout << "f1:" << f1.ToString() << std::endl;
    std::cout << "f2:" << f2.ToString() << std::endl;
    system("pause");
    return 0;
}

I can't specify that the type should be FooDerivedA like:

auto f = std::unique_ptr<Foo>( new FooDerivedA( *foo ) ); 

because it might be FooDerivedB. How can I copy the data without slicing?

Answer 1

The classical method to solve this problem is to implement a virtual Foo *clone() const, which is then called instead of the copy constructor.

So, if we have an object of some (derived form of) Foo in x, we can create another one by:

 void someFunc(Foo *x)
 {
     Foo *copy_of_x = x->clone();
     ... 
     delete copy_of_x;  // Important so we don't leak!
 }

Note that since it's a virtual function, we can't call it in the constructor of foo or any of it's derived types, as virtual functions don't operate "correctly" inside constructors.

Answer 2

You may consider using Boost.Variant instead of pointers in your container. This avoid a lot of slicing and memory management issues. Also, you get a lot more from default constructors.

Here is a full rework of your example using this design:

#include <vector>
#include <iterator>
#include <string>
#include <boost/variant.hpp>

struct Foo
{
    Foo() : m_x("abc") {}
    std::string m_x;
};

struct FooDerivedA : Foo
{
    FooDerivedA() : m_y(123) {}
    int m_y;
};

struct FooDerivedB : Foo
{
    FooDerivedB() : m_z(true) {}
    bool m_z;
};

typedef boost::variant<FooDerivedA, FooDerivedB> a_foo;

struct to_string : boost::static_visitor<std::string>
{
    std::string operator()(Foo const& foo) const 
        {return foo.m_x;}
    std::string operator()(FooDerivedA const& foo) const
        {return foo.m_x + ", " + std::to_string(foo.m_y);}
    std::string operator()(FooDerivedB const& foo) const
        {return foo.m_x + ", " + std::to_string(foo.m_z);}
};

std::ostream& operator<<(std::ostream& os, a_foo const& foo)
{
    return os << boost::apply_visitor(to_string(), foo);
}

int main()
{
    std::vector<a_foo> f1;
    f1.push_back(FooDerivedA());
    f1.push_back(FooDerivedB());
    auto f2 = f1;
    std::ostream_iterator<a_foo> out_it(std::cout, "\n");
    std::cout << "f1:" << std::endl;
    std::copy(f1.begin(), f1.end(), out_it);
    std::cout << "f2:" << std::endl;
    std::copy(f2.begin(), f2.end(), out_it);
    return 0;
}