Question

I’m trying to use std::mem_fun_ref (Yes, the deprecated
version. Reasons below) to call a member function through a proxy.

template<typename T>
struct proxy {
  T& operator*() { return *t; }
  T* operator->() { return t; }
  // no address of etc
  T* t;
};

struct A {void foo() {}};

int main()
{
  A a;
  proxy<A> pa = {&a};
  std::mem_fun_ref_t<void, A>
    fn = std::mem_fun_ref(&A::foo);
  fn(pa); // borks
  return 0;
}

This works well with C++11 std::mem_fn but not boost::mem_fn, but
I can use neither of those, as I need to specify the type of the
binder in another place and the type of the resulting binder is
unspecified for boost::mem_fn. This wouldn’t be a problem if I could
use decltype but I can’t as the code needs to be compatible with
C++03.

What is the easiest way to work around this? A custom
mem_fun_through_proxy?

Edit: Another caveat is that the proxy class cannot be changed.

Answer 1

As Georg recommended, I implemented my own solution. Here is the short version:

// snipped solution: does not include const version and hack for void
// returns

#include <functional>

namespace mine {

template<typename Ret, typename T>
class mem_fun_ref_t : public std::unary_function<T, Ret>
{
public:
  explicit
  mem_fun_ref_t(Ret (T::*f)())
  : f(f) { }

  template<typename U>
  Ret
  operator()(U& u) const
  { return (*u.*f)(); }

  Ret
  operator()(T& t) const
  { return (t.*f)(); }

private:
  Ret (T::*f)();
};

} // mine

template<typename T>
struct proxy {
  T& operator*() { return *t; }
  T* operator->() { return t; }
  // no address of etc
  T* t;
};

struct X {
  int foo() {return 23;}
};

int main()
{
  mine::mem_fun_ref_t<int, X> fn(&X::foo);
  X x;
  // normal
  fn(x);
  proxy<X> px = {&x};
  fn(px);
  return 0;
}