NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/lang/cplus/13225

Text File | 1992-09-02 | 2.8 KB | 108 lines

Path: sparky!uunet!sun-barr!cs.utexas.edu!devnull!thunder.Berkeley.EDU!rjf
From: [e-mail] (Russell Fleming)
Newsgroups: comp.lang.c++
Subject: Re: Pointer to member function as parameter
Keywords: pointer, member function
Message-ID: <[e-mail]>
Date: 2 Sep 92 19:05:41 GMT
References: <[e-mail]>
Sender: [e-mail]
Lines: 96

In article <[e-mail]>, [e-mail]
(Bruce Hickey) writes:
>
> I am attempting to pass a pointer to a member function as an argument to
> a function. Neither Stroustrup (2nd Ed) or Lippman (2nd Ed) cover this
> subject directly, to my knowledge.
>
> I am trying to create a pointer to a member function as well as use that
> pointer to call the function. The problem arises when I try to pass this
> pointer as a paramter to a function. There appears to be no reason why this
> is not possible.
>
> class mine {
> public:
>     double func( int );
> };
>
> double mine::func( int var1 )
> {
>     cerr << var1 << "\n";
>     return 0.1;
> }
>
> // This function wants a pointer to a member function.
> void freefunc( double (mine::*f)(int) )
> {
>     cerr << "calling func\n";
>     (*f)(2);            // How do I call the function?
> }
>
> int main()
> {
> mine test;
>
> double (mine::*pf)(int) = &mine::func;
>
> (test.*pf)(5);         // Call the member function through pointer
>
> freefunc(test.*pf);     // Attempt to pass pointer to function
>
> }
>

There is nothing wrong with your declaration of a pointer to the member
function.  The problem lies with your use of the pointer.  All
non-static member functions have an implied "this" argument which is
used to access the data associated with the object.  When you use a
ptr-to-mem-fcn to invoke the function, you must also provide the object
which the function is to act upon (ie. (this->*f)(2)).  Two solutions to
your problem are available.  The first elimintates the need for a
pointer.  Simply modify your code as follows:

void freefunc (mine& m)
{
    cerr << "calling func\n";
    m.func (2);
}

int main ()
{
    mine test;
    freefunc (test);
}

Alternatively, since the member function you are using does not (as
shown) rely on any data from the object,
you could re-write your code to use a pointer function as follows:

class mine {
public:
    static double func(int);
    };

double mine::func (int var1)
{
    cerr << var1 << "\n";
    return 0.1;
}

void freefunc (double (*f)(int))
{
    cerr << "calling func\n";
    (*f)(2);
}

int main ()
{
    mine test;
    double (*pf)(int) = &mine::func;
    freefunc (pf);
}

=================================================================
Rusty Fleming, Software Consultant @ Tandem Computers Inc.
email: [e-mail]            Austin, Texas
voice: (512) 244 - 8390              USA