C++ Inheritance with pure virtual functions - c++

Question

I'm trying to create a class that serves as a base object, which will then be sub-classed (=implemented) to serve various purposes.
I want to define one or more pure virtual functions, so that however subclasses the base class, is required and does not forget to implement them.
There is one caveat, the pure virtual function's signature includes the type of the base object. Once sub-classed, the function definition doesn't match the base classes definition anymore of course. E.g.:
class BaseItem
{
public:
virtual std::string getDifferences(const BaseItem& item) = 0;
}
So, in the derived class I'd like to do:
class DerivedClass : public BaseItem
{
public:
virtual std::string getDifferences(const DerivedClass& item) = 0;
private:
std::string derivedItemCustomObject;
}
which of course the compiler won't accept. I could make it a BaseItem of course, but then I can't utilize any objects in the derived class.
Do I have to use casting to accomplish this?
Please let me know if my intent/question is not clear.

Answer 1

There is NO need to change the function signature. Look at following:
class BaseItem
{public:
virtual std::string getDifferences(const BaseItem& item) = 0;
};
class DerivedClass : public BaseItem
{public:
virtual std::string getDifferences(const BaseItem& item) // keep it as it's
{
const DerivedClass& derivedItem = static_cast<const DerivedClass&>(item);
}
};
Can use static_cast<> without any fear because, DerivedClass::getDifferences() is called only for DerivedClass object. To illustrate,
BaseItem *p = new DerivedClass;
DerivedClass obj;
p->getDifferences(obj); // this always invoke DerivedClass::getDifferences
If you worry that sometime you might end up passing any other derived class object as an argument to the method, then use dynamic_cast<> instead and throw exception if that casting fails.

Answer 2

It's unclear what you're trying to achieve. Suppose that the compiler allowed you to do this (or you do this by the means of a cast), then it would open the following hole in the type system:
class BaseItem
{
public:
virtual std::string getDifferences(const BaseItem& item) = 0;
};
class DerivedClass : public BaseItem
{
public:
virtual std::string getDifferences(const DerivedClass& item)
{
item.f();
// ...
}
void f() const {}
};
class DerivedClass2 : public BaseItem
{
public:
virtual std::string getDifferences(const DerivedClass2& item) { ... }
};
void g()
{
BaseItem* x = new DerivedClass;
// oops, calls DerivedClass::f on an instance of DerivedClass2
x->getDifferences(DerivedClass2());
}
Your design is probably wrong.

Answer 3

I assume that the compiler accept but DerivedClass::getDifferences doesn't override BaseItem::getDifferences. Here is a way to achieve what you apparently want
template <typename T>
class DerivedHelper: public BaseItem {
public:
virtual std::string getDifferences(const BaseItem& item) {
getDifferences(dynamic_cast<const T&>(item));
}
virtual std::string getDifferences(const T& item) = 0;
};
class DerivedClass : public DerivedHelper<DerivedClass>
{
public:
// not more needed but providing it will hide getDifferences(const BaseItem& item)
// helping to statically catch some cases where a bad argument type is used.
virtual std::string getDifferences(const DerivedClass& item) = 0;
private:
std::string derivedItemCustomObject;
};
but be aware that there is a runtime check which will throw exceptions if the argument isn't of the correct class.

Answer 4

One way to accomplish this is to use a template and have the parameter be the type of the derived type
template <typename T>
class BaseItem {
public:
virtual std::string getDifferences(const T& item) = 0;
};
class DerivedClass : public BaseItem<DerivedClass> {
public:
virtual std::string getDifferences(const DerivedClass& item) {
// Implement it here
}
};

Answer 5

You should use cast from BaseItem to DerivedClass + runtime check if given BaseItem is a DerivedClass instance.