c++ member function pointer problem

Question

I'm new to c++ . I want to know about object pointer and pointer to member function . I wrote a code which is following:

code :

#include <iostream>
using namespace std;
class golu
{
   int i;
public:
   void man()
   {
      cout<<"\ntry to learn \n";
   }
};
int main()
{
   golu m, *n;
   void golu:: *t =&golu::man(); //making pointer to member function

   n=&m;//confused is it object pointer
   n->*t();
}

but when i compile it it shows me two error which is following:

pcc.cpp: In function ‘int main()’:
pcc.cpp:15: error: cannot declare pointer to ‘void’ member
pcc.cpp:15: error: cannot call member function ‘void golu::man()’ without object
pcc.cpp:18: error: ‘t’ cannot be used as a function.

my question are following :

What I'm doing wrong in this code ?
How to make object pointer ?
How to make pointer to member function of a class and how to use them ?

Please explain me these concept.

AProgrammer · Accepted Answer · 2011-06-07 08:56:35Z

8

Two errors corrected here:

int main()
{
   golu m, *n;
   void (golu::*t)() =&golu::man; 

   n=&m;
   (n->*t)();
}

you want a pointer to function
the priority of the operators is not the one you expected, I had to add parenthesis. n->*t(); is interpreted as (n->*(t())) while you want (n->*t)();

edited Jun 7, 2011 at 8:56

answered Jun 7, 2011 at 8:49

AProgrammer

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3 Comments

Golu Over a year ago

will you explain a bit about priorty? I don't understand the concept of priority in this context.

AProgrammer Over a year ago

Does the modified answer suit you?

AProgrammer Over a year ago

It is what makes you consider 1 + 2 * 3 is 7 and not 9. See en.wikipedia.org/wiki/Order_of_operations

Xeo · Accepted Answer · 2011-06-07 08:56:15Z

A member function pointer has the following form:

R (C::*Name)(Args...)

Where R is the return type, C is the class type and Args... are any possible parameters to the function (or none).

With that knowledge, your pointer should look like this:

void (golu::*t)() = &golu::man;

Note the missing () after the member function. That would try to call the member function pointer you just got and thats not possible without an object.
Now, that gets much more readable with a simple typedef:

typedef void (golu::*golu_memfun)();
golu_memfun t = &golu::man;

Finally, you don't need a pointer to an object to use member functions, but you need parenthesis:

golu m;
typedef void (golu::*golu_memfun)();
golu_memfun t = &golu::man;
(m.*t)();

The parenthesis are important because the () operator (function call) has a higher priority (also called precedence) than the .* (and ->*) operator.

Ali1S232 · Accepted Answer · 2011-06-07 08:52:16Z

2

'void golu:: *t =&golu::man();' should be changed to 'void (golu:: *t)() =&golu::man;' you are trying to use pointer to function not pointer to result of a static function!

edited Jun 7, 2011 at 8:52

answered Jun 7, 2011 at 8:46

Ali1S232

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Comments

iammilind · Accepted Answer · 2011-06-07 08:48:41Z

1

(1) Function pointer is not declared properly.

(2) You should declare like this:

void (golu::*t) () = &golu::man;

(3) Member function pointer should be used with object of the class.

answered Jun 7, 2011 at 8:48

iammilind

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