Design patterns Prototype Pattern Prototype Pattern (C++)
Example
class IPrototype {
public:
virtual ~IPrototype() = default;
auto Clone() const { return std::unique_ptr<IPrototype>{DoClone()}; }
auto Create() const { return std::unique_ptr<IPrototype>{DoCreate()}; }
private:
virtual IPrototype* DoClone() const = 0;
virtual IPrototype* DoCreate() const = 0;
};
class A : public IPrototype {
public:
auto Clone() const { return std::unique_ptr<A>{DoClone()}; }
auto Create() const { return std::unique_ptr<A>{DoCreate()}; }
private:
// Use covariant return type :)
A* DoClone() const override { return new A(*this); }
A* DoCreate() const override { return new A; }
};
class B : public IPrototype {
public:
auto Clone() const { return std::unique_ptr<B>{DoClone()}; }
auto Create() const { return std::unique_ptr<B>{DoCreate()}; }
private:
// Use covariant return type :)
B* DoClone() const override { return new B(*this); }
B* DoCreate() const override { return new B; }
};
class ChildA : public A {
public:
auto Clone() const { return std::unique_ptr<ChildA>{DoClone()}; }
auto Create() const { return std::unique_ptr<ChildA>{DoCreate()}; }
private:
// Use covariant return type :)
ChildA* DoClone() const override { return new ChildA(*this); }
ChildA* DoCreate() const override { return new ChildA; }
};
That allows to construct the derived class from a base class pointer:
ChildA childA;
A& a = childA;
IPrototype& prototype = a;
// Each of the following will create a copy of `ChildA`:
std::unique_ptr<ChildA> clone1 = childA.Clone();
std::unique_ptr<A> clone2 = a.Clone();
std::unique_ptr<IPrototype> clone3 = prototype.Clone();
// Each of the following will create a new default instance `ChildA`:
std::unique_ptr<ChildA> instance1 = childA.Create();
std::unique_ptr<A> instance2 = a.Create();
std::unique_ptr<IPrototype> instance3 = prototype.Create();
