Re: currying pointer to member functions

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 7 Mar 2013 00:10:27 -0800 (PST)
Message-ID:
<kh6r6g$44c$1@dont-email.me>
On 2013-03-06 00:20, Jerry wrote:

I appreciate any advice about how to do this.

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 standard does not assign any meaning to it except that it says
that this expression is not valid. According to 5.5 p6:

"If the result of .* or ->* is a function, then that result can be
used only as the operand for the function call operator ()."

Which means that the application of the address-of operator is not
supported.

I make a class:

     template<class R, class O>
     class ArrowStarVoidFunc
     {
     private:
         R(O::*fptr)();
     public:
         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); }


The latter declaration is invalid: A non-member overload of
operator->* needs to take two parameters.

Which seems to work fine except it also executes the function. What I
want to do is the following, but what goes where the ???? is:

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


You need to store both the O* value and the R(O::*fptr)() in the proxy
result object.

Isn't boost::bind() providing what you are considering to realize? See

http://www.boost.org/doc/libs/1_53_0/libs/bind/bind.html#with_member_pointers

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


Good luck ;-)

HTH & Greetings from Bremen,

Daniel Kr?gler

--
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