C++ 2. Covariant result version of the base example, static type checking.


Example

// 2. Covariant result version of the base example, static type checking.

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

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

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

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

    DD* /* ← Covariant return */ clone() const override
    { return new DD( *this ); }
};

#include <iostream>
using namespace std;

int main()
{
    DD* p1 = new DD();
    DD* p2 = p1->clone();
    // Correct dynamic type DD for *p2 is guaranteed by the static type checking.

    cout << p2->answer() << endl;          // "42"
    delete p2;
    delete p1;
}