Can C++ policy classes be used to specify existence / non-existence of constructors?

Question

Suppose I have:

struct Magic {
  Magic(Foo* foo);
  Magic(Bar* bar);
};

Is there a way to make Magic a template, and define template classes s.t.

typedef Magic<FooPolicy, ...> MagicFoo;
typedef Magic<BarPolicy, ...> MagicBar;
typedef Magic<..., ...> MagicNone;
typedef Magic<FooPolicy, BarPolicy> MagicAll;

s.t. MagicFoo & MagicAll have the Foo* constructor; MagicBar & MagicAll has the Bar* constructor; and MagicNone nas neither the Foo* nor the Bar* constructor?

Basically I want constructors to exist or not exist based on policy classes.

Answer 1

That looks like an application for subclasses, not policy classes. MagicFoo and MagicBar seem to want to be subclasses of Magic, which itself might have a protected constructor.

Answer 2

You can have a template definition for all policies and a specialization for MagicNone. An example will be:

template<class T> 
 struct Magic {
  Magic(T *o) {}
};

struct None {};

// specialize for MagicNone
template<> struct Magic<None> {
  Magic() {} // default ctor
};

int main()
{
  int a = 32;
  Magic<int> mi(&a);
  Magic<None> m;
}

Answer 3

You can write a constructor accepting anything, and then delegate to whatever the policies provide:

// "Tag" and "No" are used to make the class/function unique 
// (makes the using declarations work with GCC). 
template<int Tag>
struct No { void init(No); };

template<typename P1 = No<0>, typename P2 = No<1>, typename P3 = No<2> >
struct Magic : P1, P2, P3 {
  template<typename T>
  Magic(T t) {
    init(t);
  }

private:
  using P1::init;
  using P2::init;
  using P3::init;
};

Now, once you forward the argument, the compiler will figure out the best match among the policies:

struct IntPolicy { void init(int) { std::cout << "called int!"; } };
struct FloatPolicy { void init(float) { std::cout << "called float!"; } };
Magic<IntPolicy, FloatPolicy> m(0), n(0.0f);