Re: currying pointer to member functions

SG <>
Thu, 7 Mar 2013 05:29:39 -0800 (PST)
On Mar 6, 12:20 am, Jerry wrote:

I can make this work:

  struct a
    int b;
    int c() {return b+1;}
    int d(int x) {return b+x;}

  int main()
    a m = { 1 }, n = { 2 };
    a *ps = &m;
    int (a::*pf)() = &a::c;
    std::cout << (ps->*pf)() << std::endl;
    return 0;

And it runs the function and everything works. But what I want to
do is curry the function so that I can store (ps->*pf) and then
later execute it. So what is the type of &(ps->*pf) ?

The type of (ps->*pf) is int() but the C++ standard says that the only
thing you can do with it is to actually call the function via the
function call operator. That makes the expression &(ps->*pf) illegal.

I make a class:

    template<class R, class O>
    class ArrowStarVoidFunc
        ArrowStarVoidFunc(R(O::*f)()) : fptr(f) {}
        R operator()(O* o) const { return (o->*fptr)(); }

    template<class R, class O>
    ArrowStarVoidFunc<R,O> operator->*(R(O::*f)())
        { return ArrowStarVoidFunc<R,O>(f); }

Which seems to work fine except it also executes the function.

This looks similar to what std::mem_fun already does.

What I
want to do is the following, but what goes where the ???? is:

    template<class R, class O>
    class ArrowStarVoidFunc
        ArrowStarVoidFunc(R(O::*f)()) : fptr(f) {}
        ???? operator()(O* o) const { return &(o->*fptr); }

Oh, and to make all this more interesting (i.e. complicated) I am
working with a compiler that is more than 10 years old and is only
compatible with C++98 - if I could a newer compiler I wouldn't be
asking this!

You can return a function object of a custom type:

     template<typename R, class O>
     class bound_mem_fun
         bound_mem_fun(R (O::*f)(), O* o)
             : f(f), o(o) {}
         R operator()() const
         { return (o->*f)(); }
         R (O::*f)();
         O* o;

It stores the function pointer as well as the object pointer. The C+
+98 standard library is short on this kind of wrapper. It only
provides bind1st and bind2nd which take binary functors and return
unary functors.

In addition, C++11 offers a more general std::bind as well as lambda
expressions which would be of help here if you were not restricted to
C ++98. But maybe you can make use of Boost.Bind instead. It's kind of
the predecessor to std::bind:

     O* o = ...;
     R (O::*f) = ...;

     boost::bind(f,o) ();
     ^^^^^^^^^^^^^^^^ ^^
      yields nullary invokes
          functor it


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