const reference to a function in a template parameter?

Question

How do I declare that I want a const function reference (in a template parameter)? eg,

template< bool (&func)(int arg) >
void foo(int stuff);

but const?

More concretely, if I try to compile the following with icpc :

template<bool (&func)(int arg)>
bool id(int arg) {
  return func(arg);
}

class Foo {
 public:
  Foo() {};
  virtual ~Foo() {};
  bool bar(int arg) { return true; }
  bool bar2(int arg) {
    return id<bar>(arg);
  };
};

int main() {
  return 0;
}

I get

$ icpc foo.cpp
foo.cpp(12): error: no instance of function template "id" matches the argument list
            argument types are: (int)
      return id<bar>(arg);
             ^
compilation aborted for foo.cpp (code 2)

or, with g++ , I get

$ g++ foo.cpp
foo.cpp: In member function ‘bool Foo::bar2(int)’:
foo.cpp:13:23: error: no matching function for call to ‘id(int&)’
     return id<bar>(arg);
                       ^
foo.cpp:13:23: note: candidate is:
foo.cpp:2:6: note: template<bool (& func)(int)> bool id(int)
 bool id(int arg) {
      ^
foo.cpp:2:6: note:   template argument deduction/substitution failed:
foo.cpp:13:23: error: could not convert template argument ‘Foo::bar’ to ‘bool (&)(int)’
     return id<bar>(arg);
                       ^

But if instead I move bar to toplevel, as in

template<bool (&func)(int arg)>
bool id(int arg) {
  return func(arg);
}

bool bar(int arg) { return true; }

class Foo {
 public:
  Foo() {};
  virtual ~Foo() {};
  bool bar2(int arg) {
    return id<bar>(arg);
  };
};

int main() {
  return 0;
}

it compiles fine. Why does this happen, and how can I fix it without making bar a global?

Note: In my original code, I was getting "(not const-qualified) cannot be initialized with a value of type" errors: (with icpc )

CollisionWorld.cpp(73): error: a reference of type "bool (&)(const Line &, vec_dimension={double}, vec_dimension={double}, vec_dimension={double}, vec_dimension={double})" (not const-qualified) cannot be initialized with a value of type "bool (const Line &, vec_dimension={double}, vec_dimension={double}, vec_dimension={double}, vec_dimension={double})"
    QuadTree<Line, vec_dimension, line_inside_box_with_time> *quad_tree =
                                  ^

(with g++ )

CollisionWorld.cpp:73:58: error: could not convert template argument ‘CollisionWorld::line_inside_box_with_time’ to ‘bool (&)(const Line&, double, double, double, double)’
   QuadTree<Line, vec_dimension, line_inside_box_with_time> *quad_tree =
                                                          ^

Answer 1

The problem is that the template expects a free function, not a member funciton. That is why it works when you put bar() out of Foo,

Try it like this:

template<typename C, bool (C::*func)(int arg)>
bool id(C *mthis, int arg) {
  return (mthis->*func)(arg);
}

class Foo {
 public:
  Foo() {};
  virtual ~Foo() {};
  bool bar(int arg) { return true; }
  bool bar2(int arg) {
    return id<Foo, &Foo::bar>(this, arg);
  };
};

Answer 2

To call a member function, you need two things: the this pointer and the function. Therefore it is not possible as easy as you write it. id would need the this pointer!

The template definition would look like this:

template<bool (Foo::*func)(int)>

But still, you cannot implement a true id function that works both on functions and member functions.