function pointer explanation

Question

I am learning C++ and just playing around with function pointers.

I have this very simple program, but I do not get any output when called. Would you mind explaining why? I think it's because inside the printTheNumbers, I never called the function itself I just make a reference to it?

void sayHello()
{
    cout << "hello there, I was accessed through another function\n";
}

void accessFunctionThroughPointer(int a, int b, void(*function)())
{

}

int main(int argc, const char * argv[])
{
    printTheNumbers(2, 3, sayHello);

    return 0;
}

no offense, but as you said you are still learning c++ (and posting beginner code that has nothing to do with the solution - addTheNumbers) it would be better for learning to play around with what you know (functions, etc) and learn dealing with strings, reading from commandline, parsing arguments and so on. IMHO function pointers are nice but more complicated and not often needed... — relascope
– relascope, Commented Apr 17, 2015 at 21:34
edited. I need to study this for a final not functions. Thanks i guess? @relascope — Beto
– Beto, Commented Apr 17, 2015 at 21:51

R Sahu · Accepted Answer · 2015-04-17 21:25:43Z

2

Your function isn't doing anything:

void printTheNumbers(int a, int b, void (*function)()){
    // there is no body here    
}

You need to actually call the passed-in function pointer:

void printTheNumbers(int a, int b, void (*function)()){
    function();
}

edited Apr 17, 2015 at 21:25

R Sahu

207k15 gold badges168 silver badges291 bronze badges

answered Apr 17, 2015 at 21:25

Barry

312k32 gold badges734 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Beto Over a year ago

how about if I make the sayHello() to return a string instead of void.

Darren Griffith · Accepted Answer · 2015-04-17 21:21:14Z

2

You pass in the sayHello function to printTheNumbers, but you never call the passed in function.

answered Apr 17, 2015 at 21:21

Darren Griffith

794 bronze badges

Comments

Barry · Accepted Answer · 2015-04-17 21:35:09Z

0

You need to call the function manually. Prove:

Code snippet

More over consider to use std::function. Example:

#include <functional>

void printTheNumbers(int a, int b, std::function<void()> function){ 
    function();
}

In most cases std::function is enough and more readable.

edited Apr 17, 2015 at 21:35

Barry

312k32 gold badges734 silver badges1.1k bronze badges

answered Apr 17, 2015 at 21:28

CyberGuy

2,8231 gold badge24 silver badges31 bronze badges

Collectives™ on Stack Overflow

function pointer explanation

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related