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Function pointers inside classes

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Join Date: Feb 2005

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winbatch

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Function pointers inside classes

Apr 18th, 2006

So I'm trying to learn how to use function pointers (ie to pass the address of a function to another function to execute it - like a call back). I have it working when I use it as follows:

Classes.h:

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#include <iostream>
using namespace std;
class Class1
{
		public:
		void callBack( string source, void (*myFunc) (string))
		{
				cout<<"About to do call back..."<<endl;
				myFunc( source );
		}
};
#include <iostream>
using namespace std;
class Class1
{
		public:
		void callBack( string source, void (*myFunc) (string))
		{
				cout<<"About to do call back..."<<endl;
				myFunc( source );
		}
};

Classes.cpp:

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#include "Classes.h"
void thisWorks(string works )
{
		cout<<"THIS ONE WORKS"<<works<<endl;
}
int main()
{
		Class1 c1;
		c1.callBack( "TESTING", &thisWorks );
		return 0;
}
#include "Classes.h"
void thisWorks(string works )
{
		cout<<"THIS ONE WORKS"<<works<<endl;
}
int main()
{
		Class1 c1;
		c1.callBack( "TESTING", &thisWorks );
		return 0;
}

./Classes
About to do call back...
THIS ONE WORKSTESTING

However, I would like to figure out how to get it to work where I don't have any 'standalone/global' functions, just classes:

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Classes.h
#include <iostream>
using namespace std;
class Class1
{
		public:
		void callBack( string source, void (*myFunc) (string))
		{
				cout<<"About to do call back..."<<endl;
				myFunc( source );
		}
};
class Class2
{
        public:
        Class2( string myString)
        {
                Class1 class1;
                class1.callBack( myString, &test );
        }
        void test( string whatever )
        {
                cout<<"Call back function called!"<<whatever<<endl;
        }
};
Classes.h
#include <iostream>
using namespace std;
class Class1
{
		public:
		void callBack( string source, void (*myFunc) (string))
		{
				cout<<"About to do call back..."<<endl;
				myFunc( source );
		}
};
class Class2
{
        public:
        Class2( string myString)
        {
                Class1 class1;
                class1.callBack( myString, &test );
        }
        void test( string whatever )
        {
                cout<<"Call back function called!"<<whatever<<endl;
        }
};

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Classes.cpp
#include "Classes.h"
int main()
{
		Class2 class2("TESTING" );
		return 0;
}
Classes.cpp
#include "Classes.h"
int main()
{
		Class2 class2("TESTING" );
		return 0;
}

However, when I compile it, I get:
"Classes.h", line 18: Error: Formal argument myFunc of type void(*)(std::basic_string<char, std::char_traits<char>, std::allocator<char>>) in call to Class1::callBack(std::basic_string<char, std::char_traits<char>, std::allocator<char>>, void(*)(std::basic_string<char, std::char_traits<char>, std::allocator<char>>)) is being passed void(Class2:

)(std::basic_string<char, std::char_traits<char>, std::allocator<char>>).

Based on some googling, I'm pretty sure I have to create a typedef to void of my class, but I'm a bit confused. (Note I have a much larger example, the above is to try and illustrate just the problem).

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Re: Function pointers inside classes

Apr 19th, 2006

http://www.parashift.com/c++-faq-lit...o-members.html

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand

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Re: Function pointers inside classes

Apr 19th, 2006

Good to see that you are back Dave.

バルサミコ酢やっぱいらへんで

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winbatch

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Re: Function pointers inside classes

Apr 19th, 2006

•

Originally Posted by Dave Sinkula

http://www.parashift.com/c++-faq-lit...o-members.html

Dave,

I was looking at that - went over my head. Is there an easy answer to my particular example? Part of the problem seems to be that I'm INSIDE my class when I'm trying to pass another member function to the other class. In all the other examples, I'm in main() and I'm trying to pass a member of a class (rather than being inside the class itself..) sorry if I can't convey this clearly...

Winbatch

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Re: Function pointers inside classes

Apr 19th, 2006

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Originally Posted by winbatch

Dave,

I was looking at that - went over my head.

Mine as well, too much of the time. I can't even recall some similar stuff elseweb that I contributed to fairly recently.

•

Originally Posted by winbatch

Is there an easy answer to my particular example?

Not from me at the moment.

•

Originally Posted by winbatch

Part of the problem seems to be that I'm INSIDE my class when I'm trying to pass another member function to the other class. In all the other examples, I'm in main() and I'm trying to pass a member of a class (rather than being inside the class itself..) sorry if I can't convey this clearly...

Member functions are not like regular functions. So pointer to member functions are not like pointers to regular functions. The implicit this makes things different... or ugly... or difficult.

I may try to find that other thread. But I know I struggled with that FAQ for quite a while before I posted anything that looked useful.

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Re: Function pointers inside classes

Apr 19th, 2006

Dave,

Don't lose sleep over it. But if you're up anyway, yes please find the other thread

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Re: Function pointers inside classes

Apr 19th, 2006

http://cboard.cprogramming.com/showthread.php?t=77687
Seems as though I basically punted.

[It seems my edit may have been after your reply, so I un-edited it and replied with that edit. Apologies -- continuity.]

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Re: Function pointers inside classes

Apr 19th, 2006

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[It seems my edit may have been after your reply, so I un-edited it and replied with that edit. Apologies -- continuity.]

Say what Dr. Dave?

I had trouble following your other thread too. guess I'm biting off more than I can chew.

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Re: Function pointers inside classes

Apr 20th, 2006

u are doing fine, as far as i can see, just declare test as static, next while passing a function u need not give &, and pass test like Class2::test in the callback funtion.... function

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Re: Function pointers inside classes

#10

Apr 20th, 2006

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Originally Posted by dubeyprateek

u are doing fine, as far as i can see, just declare test as static, next while passing a function u need not give &, and pass test like Class2::test in the callback funtion.... function

That works, unfortunately, I don't want it to be a static function as I want it to use the instance of the class that I'm working in.