C++ 1. Base example without covariant returns, shows why they're desirable


Example

// 1. Base example not using language support for covariance, dynamic type checking.

class Top
{
public:
    virtual Top* clone() const = 0;
    virtual ~Top() = default;       // Necessary for `delete` via Top*.
};

class D : public Top
{

public:
    Top* clone() const override
    { return new D( *this ); }
};

class DD : public D
{
private:
    int answer_ = 42;

public:
    int answer() const
    { return answer_;}

    Top* clone() const override
    { return new DD( *this ); }
};

#include <assert.h>
#include <iostream>
#include <typeinfo>
using namespace std;

int main()
{
    cout << boolalpha;

    DD* p1 = new DD();
    Top* p2 = p1->clone();
    bool const  correct_dynamic_type = (typeid( *p2 ) == typeid( DD ));
    cout << correct_dynamic_type << endl;               // "true"

    assert( correct_dynamic_type ); // This is essentially dynamic type checking. :(
    auto p2_most_derived = static_cast<DD*>( p2 );
    cout << p2_most_derived->answer() << endl;          // "42"
    delete p2;
    delete p1;
}