抽象类c ++的对象作为函数的参数

Question

I have the following code:

class Region {
public:
    virtual Region* clone() const = 0;
    virtual ~Region() {}
    bool contains(double x, double y) const = 0;
}

class UnionRegion : public Region {
    shared_ptr<Region> r1;
    shared_ptr<Region> r2;
public:
    UnionRegion(const Region& r1, const Region& r2) :
           r1(r1.clone()), r2(r2.clone()) {}
    .
    .
    .
}

Region is an abstract class. I do not understand how the ctor of UnionRegion can receive an object of type Region , because how can we create an object of an abstract class?

Also what is the point of holding a shared_ptr to Region ? I understand that because Region is abstract we need a pointer, but Region doesn't even have private members, so basically I can do nothing with that pointer.

Thanks!

Answer 1

Region is an abstract class, but it is never instantiated. A reference to const Region binds to an object of any subclass of Region (much as a pointer to const Region would). So UnionRegion 's constructor takes references to any two objects that are subclasses of Region .

Answer 2

Since UnionRegion inherits from Region the UnionRegion(const Region& r1, const Region& r2) can be called with either other UnionRegion parameters or any other class type which inherits from Region . Since this is a constructor I assume there might be also some other class type which inherits from Region and is used to construct UnionRegion .

Answer 3

how can we create an object of an abstract class?

You can not create an object of an abstract class, you can only use abstract class reference/pointer which refer to derived concrete class objects.

what is the point of holding a shared_ptr to Region?

If you want a class member which its concrete type is unknown, yo have to use pointer or reference to hold it. Then later you can invoke override virtual functions to have polymorphism actions.

The one which need to negotiate is: why your UnionRegion which is a derived type need to hold a pointer(shared_ptr) to its base? It's more like design behavior.