Re: Call a member function only if it exists

From:
Barry <dhb2000@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 2 Jan 2008 11:41:10 CST
Message-ID:
<flf4ka$g4h$1@news.cn99.com>
Jens Breitbart wrote:

Hi,

I wrote a framework, which uses callback functions. Currently all
classes used with the framework have to implement the callback
function. I would like to define a default behavior for all classes,
that do not implement the function. I am aware that this could easily
be done, by defining a base class and require that all classes used
with the framework inherit for this base class, but I would prefer to
do this without inheritance. The code I have in mind looks similar to
the one below, but I failed to write the caller template.

void f () {
    // default behavior
}

struct A {
    void f () {
        // special behavior for objects of type A
    }
};

struct B {
};

int main () {
    A a;
    B b;

    //caller<A>::f(a); // should call a.f()
    //caller<B>::f(b); // should call f()

    return 0;
}

Any suggestion how to solve the problem would be highly appreciated.


the code is base on
http://groups.google.com/group/comp.lang.c++/browse_thread/thread/c0ab71fc3e1ff59

#include <iostream>

template<class T> struct has_member_f;

template<class R, class C>
class has_member_f<R C::*> {
private:
      template<R C::*> struct helper;
      template<class T> static char check(helper<&T::f>*);
      template<class T> static char (& check(...))[2];
public:
      static const bool value = (sizeof(check<C>(0)) == sizeof(char));
};

namespace boost { // no dependency of boost

template <bool B, class T = void>
struct enable_if_c {
      typedef T type;
};

template <>
struct enable_if_c<false, void> {};

} // namespace boost

void f() {
      std::cout << "::f()" << std::endl;
}

struct caller {
      template <class T>
      static void f(
          T& t,
          typename boost::enable_if_c<
              has_member_f<void (T::*)()>::value
          >::type* = 0) {
          t.f();
      }

      template <class T>
      static void f(
          T& t,
          typename boost::enable_if_c<
              !has_member_f<void (T::*)()>::value
          >::type* = 0) {
          ::f();
      }
};

// client code below
struct A { };

struct B {
      void f() {
          std::cout << "B::f" << std::endl;
      }
};

int main() {
      std::cout
          << has_member_f<void (A::*)()>::value << '\n'
          << has_member_f<void (B::*)()>::value << std::endl;

      A a;
      caller::f(a);
      B b;
      caller::f(b);
}

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