Example
// 3. Covariant smart pointer result (automated cleanup).
#include <memory>
using std::unique_ptr;
template< class Type >
auto up( Type* p ) { return unique_ptr<Type>( p ); }
class Top
{
private:
virtual Top* virtual_clone() const = 0;
public:
unique_ptr<Top> clone() const
{ return up( virtual_clone() ); }
virtual ~Top() = default; // Necessary for `delete` via Top*.
};
class D : public Top
{
private:
D* /* ← Covariant return */ virtual_clone() const override
{ return new D( *this ); }
public:
unique_ptr<D> /* ← Apparent covariant return */ clone() const
{ return up( virtual_clone() ); }
};
class DD : public D
{
private:
int answer_ = 42;
DD* /* ← Covariant return */ virtual_clone() const override
{ return new DD( *this ); }
public:
int answer() const
{ return answer_;}
unique_ptr<DD> /* ← Apparent covariant return */ clone() const
{ return up( virtual_clone() ); }
};
#include <iostream>
using namespace std;
int main()
{
auto p1 = unique_ptr<DD>(new DD());
auto p2 = p1->clone();
// Correct dynamic type DD for *p2 is guaranteed by the static type checking.
cout << p2->answer() << endl; // "42"
// Cleanup is automated via unique_ptr.
}
