I have my base class as follows:
class point //concrete class
{
... //implementation
}
class subpoint : public point //concrete class
{
... //implementation
}
How do I cast from a point object to a subpoint object? I have tried all three of the following:
point a;
subpoint* b = dynamic_cast<subpoint*>(&a);
subpoint* b = (subpoint*)a;
subpoint b = (subpoint)a;
What is wrong with these casts?
How do I cast from a point object to a subpoint object?
You can't; unless either point
has a conversion operator, or subpoint
has a conversion constructor, in which case the object types can be converted with no need for a cast.
You could cast from a point
reference (or pointer) to a subpoint
reference (or pointer), if the referred object were actually of type subpoint
:
subpoint s;
point & a = s;
subpoint & b1 = static_cast<subpoint&>(a);
subpoint & b2 = dynamic_cast<subpoint&>(a);
The first (static_cast
) is more dangerous; there is no check that the conversion is valid, so if a
doesn't refer to a subpoint
, then using b1
will have undefined behaviour.
The second (dynamic_cast
) is safer, but will only work if point
is polymorphic (that is, if it has a virtual function). If a
refers to an object of incompatible type, then it will throw an exception.