how to make new C return other static type than C

[Ali Çehreli]

On 12/09/2012 10:23 AM, deed wrote:
 > interface I
 > {
 > void setX(int x);
 > int getX();
 > }
 >
 > class C : I
 > {
 > int x, y;
 >
 > void setX(int x) { this.x = x; }
 > int getX() { return x; }
 > void setY(int y) { this.y = y }
 > int getY() { return y; }
 > }
 >
 >
 > void main()
 > {
 > auto obj = new C; // Want new C to instantiate obj with static type I.
 > obj.setX(3); // Ok, sets x to 3.
 > obj.getY(); // Error, not specified in the interface.
 > }

new always returns an object of the actual type the programmer 
requested. It is up to the programmer what interface the object needs to 
be used with:

     C obj = new C;

Now the code works because getY() is being called on a C, which does 
have the definition of that function.

 > - Is it possible to enforce, from within the class, a certain static
 > interface type or ancestor type when instantiating with new? If so, how
 > is it done?

Not possible.

 > - Also, is it possible for a class to implement multiple interfaces and
 > choose static type among them when instantiated, based on static class
 > state or arguments passed to the constructor?

Again, it is always the same type of object. That object can be used by 
its many interfaces.

Ali