Re: How to implement the virtual constructor behavour in C++
zho_zhang11790@yahoo.com wrote:
Hi, I know that in C++, there is no virtual constructor. But is there
any way to implement it by using the virtual function? For example:
We have a base class Shape, its constructor holds a string to indicate
what kind of shape we need to create. We also have three derived class
such as Square, Circle and Diamond. How to implement such kind of
behavour as following:
Square* ps = new Shape("Square");
Diamond* pd = new Shape("Diamond");
I guess the prototype design pattern may help. But I really don't know
how implement the above
behavour.
Any suggestions will be greatly appreciated
Joe
{ Look up the web for "virtual constructors"; FAQ 20.8 also mentions them.
-mod/sk }
In addition to the FAQ item mentioned above, to achieve what you expect
you can use a map<string, const Shape*>, which contains pointers to
prototype objects.
class Shape {
public:
virtual ~Shape() { }
virtual Shape* create() const = 0;
static void Register(const string& name, const Shape* shape)
{ prototypes[name] = shape; }
static Shape* Create(const string& name)
{ const Shape* p = prototypes[name]; assert(p);
return p->create(); }
private:
static map<string, const Shape*> prototypes;
};
class Circle : public Shape {
public:
virtual Shape* create() const { return new Circle(); }
};
// ...
Shape::Register("Circle", new Circle());
// ...
Shape* pc = Shape::Create("Circle");
// returns a Shape*; use a cast to convert it to a Circle*
--
Seungbeom Kim
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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