Question

I have implemented a Policy using the CRTP. The policy requires the Base class to have a function called foo:

template<typename Base>
struct Policy<Base> {
  // ...
  Base* b(){ return static_cast<Base*>(this); }
  void do(){ b()->foo(); }
};

I have one class called Widget that uses my policy. Widget implements foo and everything is fine:

struct Widget : Policy<Widget> {
  // ...
  void foo();
};

The problem: I also have a type called OldWidget that implements the functionality of foo in a function named oldFoo:

struct OldWidget : Policy<OldWidget> {
  // ...
  void oldFoo();
};

I don’t want to modify OldWidget (besides extending it with the policy). I don’t want to use an AdaptedOldWidget:

struct AdaptedOldWidget : OldWidget, Policy<AdaptedOldWidget> {
  void foo(){ oldFoo(); }
};

The best would be to extend my existing policy_traits class to something like:

template<typename T>
struct policy_traits {};

template<>
struct policy_traits<Widget> {
  // typedefs...
  member_function_name = foo;
};

template<>
struct policy_traits<OldWidget> {
  // typedefs
  member_function_name = oldFoo;
};

Such that I can implement the Policy like this:

template<typename Base>
struct Policy<Base> {
  // ...
  Base* b() { return static_cast<Base*>(this); }
  void do(){ b()->policy_traits<Base>::member_function_name(); }
};

Is there away to achieve something like this in C++?

Proposed solution: I could do the following:

template<typename Base>
struct Policy<Base> : Policy_Member_Traits<Base> {
  // ...
  Base* b(){ return static_cast<Base*>(this); }
  void do(){ foo_wrapper(); }
};

template<typename T> struct Policy_Member_Traits { };
template<> struct Policy_Member_Traits<Widget> { 
  void foo_wrapper(){ static_cast<T*>(this)->foo(); }
};
template<> struct Policy_Member_Traits<OldWidget> { 
  void foo_wrapper(){ static_cast<T*>(this)->oldFoo(); }
};

There must be hopefully a better easier way to achieve this.

Answer 1

Here’s an example how specializing selectively. First, some example classes:

#include <iostream>

struct Foo
{
    void foo() const { std::cout << "Foo::foo\n"; }
    void bar() const { std::cout << "Foo::foo\n"; }
};

struct Biz
{
    void old_foo() const { std::cout << "Fiz::old_foo\n"; }
    void bar() const { std::cout << "Fiz::foo\n"; }
};

struct Fiz
{
    void foo() const { std::cout << "Biz::foo\n"; }
    void old_bar() const { std::cout << "Biz::old_foo\n"; }
};

Now the trait:

template <typename T> struct Dispatch
{
    static void foo(T const & x) { x.foo(); }
    static void bar(T const & x) { x.bar(); }
};

template <> void Dispatch<Biz>::foo(Biz const & x) { x.old_foo(); }
template <> void Dispatch<Fiz>::bar(Fiz const & x) { x.old_bar(); }

And here’s a usage example:

template <typename T> void dispatch(T const & x)
{
    Dispatch<T>::foo(x);
    Dispatch<T>::bar(x);
}

int main()
{
    Foo f;
    Biz b;
    Fiz c;

    dispatch(f);
    dispatch(b);
    dispatch(c);
}