Comparing properties of templated child classes from base class pointers - c++

Question

I have a class template that derives from a parent class. I am storing the children in a vector as Parent*. I can later use typeid to determine if two objects are the same type. What I want to do is compare a property of two objects that are the same type. For simplicity, I have omitted storing the objects in a vector, but the concept is demonstrated below:
#include <iostream>
#include <typeinfo>
#include <vector>
class Parent{ public: virtual ~Parent(){}};
template<typename T>
class TypedChild : public Parent
{
public:
virtual ~TypedChild(){}
T getValue() {return mValue;}
private:
T mValue;
};
int main()
{
Parent* child1 = new TypedChild<int>();
Parent* child2 = new TypedChild<float>();
std::vector<Parent*> objects;
objects.push_back(child1);
objects.push_back(child2);
if(typeid(*(objects[0])) == typeid(*(objects[1])))
if(objects[0]->getValue() == objects[1]->getValue()) // compiler error: Parent has no member named getValue
std::cout << "Success";
return 0;
}
Of course in this example I could dynamic_cast to TypedChild<int> before calling getValue(), but in the real case where the objects are in a vector I don't know their types, I just know that they are the same type so their getValue() functions should return the same type and can hence be compared.
Is there any way to do this comparison?

Answer 1

With your use case, being able to avoid a dynamic_cast will be hard, if not impossible. If you want to get the value of just one object, you'll need to use dynamic_cast, such as:
Parent* child = ...;
auto typedChild = dynamic_cast<TypedChild*>(child):
if ( typedChild )
{
int val = typedChild->getValue();
}
If you want to compare two objects for equality, the best case scenario is to have a virtual operator==() function.
class Parent
{
public:
virtual ~Parent(){}
virtual bool operator==(Parent const& rhs) const = 0;
};
template<typename T>
class TypedChild : public Parent
{
public:
virtual ~TypedChild(){}
T getValue() {return mValue;}
virtual bool operator==(Parent const& rhs) const
{
auto derivedRHS = dynamic_cast<TypedChild<T> const*>(&rhs);
if ( !derivedRHS )
{
return false;
}
return (this->mValue == derivedRHS->mValue);
}
private:
T mValue;
};