Polymorphism & Pointers to arrays [duplicate]
-
05-07-2019 - |
Question
This question already has an answer here:
I have a class A:
class A
{
public:
virtual double getValue() = 0;
}
And a class B:
class B : public A
{
public:
virtual double getValue() { return 0.0; }
}
And then in main() I do:
A * var;
var = new B[100];
std::cout << var[0].getValue(); //This works fine
std::cout << var[1].getValue(); //This, or any other index besides 0, causes the program to quit
If instead I do:
B * var;
var = new B[100];
std::cout << var[0].getValue(); //This works fine
std::cout << var[1].getValue(); //Everything else works fine too
Everything compiles fine, but it seems as though there is something wrong with my polymorphism perhaps? I'm puzzled.
Solution
You can't treat arrays polymorphically, so while new B[100]
creates an array of B
objects and returns a pointer to the array - or equivalently the first element of the array - and while it is valid to assign this pointer to a pointer to a base class, it is not valid to treat this as a pointer into an array of A
objects.
The principal reason that you can't is that (typically) derived objects are a different size to their base classes, so attempting to access the array as an array of base class objects will not use the correct offset to get a pointer to the next base class subobject of the next member of the derived class array.
OTHER TIPS
There is no problem with the polymrphism but with the way you are dealing with memory. The [] operator will advance you through the array by the sizeof(A) bytes in the first case and the sizeof(B) bytes in the second case. Because the objects are of type B the A* is not pointing to the correct location in memory.
Here is another way of looking at it
char * var;
var = (char*) new B[100];
std::cout << ((A*)var[0]).getValue(); //This works fine
std::cout << ((A*)var[1]).getValue(); //This will fail
std::cout << ((A*)var[sizeof(B)]).getValue(); // should work
You didn't allocate the objects in the array:
for (int i=0;i<100;i++)
var[i] = new B;
(although I may be mixing up C++ and C#)