How do i pass a subclass by non-constant reference

Question

I am writing a unit test for A::some_method() which accepts class B as follows:

class A {
public:
    some_method(B (&obj)[a_big_number]);
} 

Class B (Bad Class) has an ugly interface:

class B {
public: 
    get_value(int index);
    set_value(int index, value);
}

The reason I don't like this interface is that get_value(i) does not map to the same element as set_value(i). Someone thought it was a good idea to have a complicated mapping to the underlying array when getting the value. This mapping is created and developed by some third class C, which is not relevant here.

This means I either need to reverse engineer this unrelated mapping in order enter test data into class B, or somehow bypass it.

To bypass the complicated mapping, I want to create a subclass called B_Mock, as follows:

class B_Mock: public B {
public:
    get_value(int index) {/*code that just returns the index referenced by set_value(index)*/}
}

And then instantiate it in my unit test as:

A a;
B_Mock inputs_outputs[1000];
/* code that repeatedly calls set_value() for each element of inputs_outputs with an initial value. */

Once inputs_outputs is set up, I want to call a.some_method(inputs_outputs) to verify that that some_method() works correctly.

Here are my issues:

• If I simply call a.some_method(inputs_outputs), I get the following compiler error:

  a reference of type "A (&)[1000]" (not const-qualified) cannot be initialized with a value of type "B_Mock [1000]"'

• I cannot change the interface of A::some_method() to A::some_method(const B (&obj)[a_big_number]), because some_method() calls obj[index].set_value().

• It does compile successfully if I change the interface of A::some_method() to A::some_method(const B obj[a_big_number]) and pass by value. I do not want to do this, since (1) obj is a rather large array of large objects, and (2) I need to access the modified data in inputs_outputs after a.some_method() operates on it, to verify that updates were made correctly. Passing by value will not return the modified objects.

How do I pass in my subclassed B_Mock to A.some_method() in a way that allows me to see the modified objects via the inputs_outputs parameter?