Question

I would like to store pointers to a Base class in a vector, but then use them as function arguments where they act as a specific class, see here:

#include <iostream>
#include <vector>

class Base {};

template<class T>
class Derived : public Base {};

void Foo(Derived<int>* d) {
  std::cerr << "Processing int" << std::endl;
}

void Foo(Derived<double>* d) {
  std::cerr << "Processing double" << std::endl;
}

int main() {
  std::vector<Base*> vec;
  vec.push_back(new Derived<int>());
  vec.push_back(new Derived<double>());
  Foo(vec[0]);
  Foo(vec[1]);
  delete vec[0];
  delete vec[1];
  return 0;
}

This doesn’t compile:

error: call of overloaded 'Foo(Base*&)' is ambiguous

Is it possible to make it work? I need to process the elements of the vector differently, according to their int, double, etc. types.

Answer 1

You’ll need to use method polymorphism, as it’s dynamic, rather than function overloading, which is static (compile-time). To overload on a templated type, you’ll need to use template specialization.

Example:

#include <iostream>
#include <vector>

class Base {
  public:
    virtual void Foo() {
      std::cerr << "Processing base" << std::endl;
    }
};

template<class T>
class Derived : public Base {};

template <>
class Derived <int> : public Base {
  public:
    void Foo() {
      std::cerr << "Processing int" << std::endl;
    }
};

template <>
class Derived <double> : public Base {
  public:
    void Foo() {
      std::cerr << "Processing double" << std::endl;
    }
};

int main() {
  std::vector<Base*> vec;
  vec.push_back(new Derived<int>());
  vec.push_back(new Derived<double>());
  vec[0]->Foo();
  vec[1]->Foo();
  delete vec[0];
  delete vec[1];
  return 0;
}