Policy class design but without making the whole user class a template

Question

Consider the following code where the Writer_I acts as an interface. Other classes which fulfil the contract of writing element types in correct form can derive

Answer 1

Your design constraint is that Calculator needs to not be a template, and has to be initialized with a writer.

That means its interface with the writer has to be dynamic. It can be through a virtual interface class, by storing function pointers, or by being passed pointers later, or similar.

As you don't want the polymorphic interface of writer to be fixed, that rules out a virtual interface.

Now, we can do this manually.

void header() {
  for (int i = 0; i < 10; i++) {
    write_i(i);
  }
}

void footer() {
  write_i(-100.0f);
}

those are the calls we need to type erase. We need to type erase down to their signatures, and remember how to do it later.

template
struct tag_t { using type=T; };
template
constexpr tag_t tag = {};

template
struct any_type_erase;
template
struct any_type_erase {
  std::function operation;

  any_type_erase() = default;
  any_type_erase(any_type_erase const&) = default;
  any_type_erase(any_type_erase &&) = default;
  any_type_erase& operator=(any_type_erase const&) = default;
  any_type_erase& operator=(any_type_erase &&) = default;

  template
  any_type_erase(tag_t, F&& f) {
    operation = [f=std::forward(f)](Any& object, Args&&...args)->R {
      return f(*std::any_cast(&object), std::forward(args)...);
    };
  }

  R operator()(Any& any, Args...args)const {
    return operation(any, std::forward(args)...);
  }
};

any_type_erase is a bit of a helper to do the boxing of the operation. For a const operation, pass in std::any const as the 2nd argument.

Add these members:

std::any writer;
any_type_erase print_header;
any_type_erase print_footer;

template
static auto invoke_writer() {
  return [](auto& writer, auto&&..args) {
    writer.write(decltype(args)(args)...);
  };
}

template
Calculator(Writer_I& writer) :
  writer(writer),
  print_header( tag>, invoke_writer() ),
  print_footer( tag>, invoke_writer() )
{}

void header() {
  for (int i = 0; i < 10; i++) {
    print_header( writer, i );
  }
}

void footer() {
  print_footer( writer, -100.0f );
}