How to achieve “virtual template function” in C++

Question

first off: I have read and I know now that a virtual template member function is not (yet?) possible in C++. A workaround would be to make the class a template and then use

Answer 1

Per Mikael's post, I have made another offshoot, using the CRTP and following Eigen's style of using derived() for an explicit subclass reference:

// Adaptation of Visitor Pattern / CRTP from:
// http://stackoverflow.com/a/5872633/170413

#include 
using std::cout;
using std::endl;

class Base {
public:
  virtual void tpl(int x) = 0;
  virtual void tpl(double x) = 0;
};

// Generics for display
template
struct trait {
  static inline const char* name() { return "T"; }
};
template<>
struct trait {
  static inline const char* name() { return "int"; }
};
template<>
struct trait {
  static inline const char* name() { return "double"; }
};

// Use CRTP for dispatch
// Also specify base type to allow for multiple generations
template
class BaseImpl : public BaseType {
public:
  void tpl(int x) override {
    derived()->tpl_impl(x);
  }
  void tpl(double x) override {
    derived()->tpl_impl(x);
  }
private:
  // Eigen-style
  inline DerivedType* derived() {
    return static_cast(this);
  }
  inline const DerivedType* derived() const {
    return static_cast(this);
  }
};

// Have Child extend indirectly from Base
class Child : public BaseImpl {
protected:
  friend class BaseImpl;
  template
  void tpl_impl(T x) {
    cout << "Child::tpl_impl<" << trait::name() << ">(" << x << ")" << endl;
  }
};

// Have SubChild extend indirectly from Child
class SubChild : public BaseImpl {
protected:
  friend class BaseImpl;
  template
  void tpl_impl(T x) {
    cout << "SubChild::tpl_impl<" << trait::name() << ">(" << x << ")" << endl;
  }
};


template
void example(BaseType *p) {
  p->tpl(2);
  p->tpl(3.0);
}

int main() {
  Child c;
  SubChild sc;

  // Polymorphism works for Base as base type
  example(&c);
  example(&sc);
  // Polymorphism works for Child as base type
  example(&sc);
  return 0;
}

Output:

Child::tpl_impl(2)
Child::tpl_impl(3)
SubChild::tpl_impl(2)
SubChild::tpl_impl(3)
SubChild::tpl_impl(2)
SubChild::tpl_impl(3)

This snippet may be found in source here: repro:c808ef0:cpp_quick/virtual_template.cc