Help with function pointers | Together C & C++ | Page 1

nocturne urchin Dec 14, 2022, 6:38 PM

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Hi, I have a menu system on my application and trying to implement function pointers to do this. I am getting error on this code: Where am I going wrong?

woeful nimbusBOT Dec 14, 2022, 6:38 PM

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nocturne urchin Dec 14, 2022, 6:38 PM

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!format

woeful nimbusBOT Dec 14, 2022, 6:38 PM

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void (VehicleList::*getMenuChoice(int choice))() {
  void (VehicleList::*options[6])() = {
      &VehicleList::addVehicle,
      &VehicleList::removeVehicle,
      &VehicleList::searchForCar,
      &VehicleList::searchForBike,
      &VehicleList::sortVehiclesByRegistrationName,
      &VehicleList::sortVehiclesByCostPerDay};

  return options[choice - 1];
}

JAMMEISTER

nocturne urchin Dec 14, 2022, 6:39 PM

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in the header -

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!format

woeful nimbusBOT Dec 14, 2022, 6:39 PM

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void (*getMenuChoice(int choice))();

JAMMEISTER

nocturne urchin Dec 14, 2022, 6:39 PM

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and calling it by doing this -

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getMenuChoice(input)();

nocturne urchin Dec 14, 2022, 6:39 PM

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nocturne urchin getMenuChoice(input)();

!format

woeful nimbusBOT Dec 14, 2022, 6:39 PM

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nocturne urchin !format

Nothing to format

nocturne urchin Dec 14, 2022, 6:39 PM

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Where am I going wrong?

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I think i am calling it wrong

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Not a clue

vital briar Dec 14, 2022, 7:19 PM

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yeah you are calling it wrong

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void (VehicleList::*getMenuChoice(int choice))() here you're returning a pointer to member function

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;compile

#include<iostream>
struct S{
    void foo(){
        std::cout<<"foo called\n";
    }
    void bar(){
        std::cout<<"bar called\n";
    }
};
void(S::*g(int x))(){
    void(S::*o[])(){&S::foo,&S::bar};
    return o[x];
}
int main(){
    S s;
    (s.*g(0))();
    (s.*g(1))();
}

idle jayBOT Dec 14, 2022, 7:22 PM

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Program Output

foo called
bar called

vital briar Dec 14, 2022, 7:22 PM

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here is a similarly structure function and how to call the returned function

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since it is a pointer to member function you need to provide an object to call it on

nocturne urchin Dec 14, 2022, 8:33 PM

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thankyou 🙂

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then calling it

(vehicleList->*vehicleList->getMenuChoice(input))();

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!format

woeful nimbusBOT Dec 14, 2022, 8:36 PM

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I have done it like this now - it works, is it about write?

void (VehicleList::*VehicleList::getMenuChoice(int choice))() {
  void (VehicleList::*options[6])() = {
      &VehicleList::addVehicle,
      &VehicleList::removeVehicle,
      &VehicleList::searchForCar,
      &VehicleList::searchForBike,
      &VehicleList::sortVehiclesByRegistrationName,
      &VehicleList::sortVehiclesByCostPerDay};

  return options[choice - 1];
}

JAMMEISTER

nocturne urchin Dec 14, 2022, 8:37 PM

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nocturne urchin then calling it (vehicleList->*vehicleList->getMenuChoice(input))();

!format

woeful nimbusBOT Dec 14, 2022, 8:37 PM

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nocturne urchin !format

Nothing to format

nocturne urchin Dec 14, 2022, 8:37 PM

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woeful nimbus I have done it like this now - it works, is it about write? ```cpp void (Vehicl...

(vehicleList->*vehicleList->getMenuChoice(input))();

vital briar Dec 14, 2022, 8:41 PM

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nocturne urchin then calling it (vehicleList->*vehicleList->getMenuChoice(input))();

seems like a strange function but looks like this would work

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you don't really use the VehicleList in getMenuChoice

nocturne urchin Dec 14, 2022, 8:41 PM

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I dont get why I need the exta VehicleList::

vital briar Dec 14, 2022, 8:42 PM

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in the definition?

nocturne urchin Dec 14, 2022, 8:42 PM

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Because If i remove it

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hang on

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1 sec

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!format

woeful nimbusBOT Dec 14, 2022, 8:43 PM

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void (*VehicleList::getMenuChoice(int choice))() {
  void (VehicleList::*options[6])() = {
      &VehicleList::addVehicle,
      &VehicleList::removeVehicle,
      &VehicleList::searchForCar,
      &VehicleList::searchForBike,
      &VehicleList::sortVehiclesByRegistrationName,
      &VehicleList::sortVehiclesByCostPerDay};

  return options[choice - 1];
}

JAMMEISTER

nocturne urchin Dec 14, 2022, 8:43 PM

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(vehicleList->*getMenuChoice(input))();

vital briar Dec 14, 2022, 8:43 PM

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that means something very different

nocturne urchin Dec 14, 2022, 8:43 PM

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would you mind explaining?

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sorry to be a pain

vital briar Dec 14, 2022, 8:44 PM

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the return type is a pointer to function

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the VehicleList:: here VehicleList::getMenuChoice means to define a method for VehicleList named getMenuChoice

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so the whole declaration void (*VehicleList::getMenuChoice(int choice))() means: defining a method named getMenuChoice on the class VehicleList that takes an int named choice and returns a pointer to function taking no arguments and returning nothing

uncut marlin Dec 14, 2022, 8:46 PM

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this feels like it would be better as a more or less plain switch case

vital briar Dec 14, 2022, 8:46 PM

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VehicleList::* means pointer to member of VehicleList, and * means pointer

uncut marlin Dec 14, 2022, 8:47 PM

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your choice is just an int, so switch is appropriate

nocturne urchin Dec 14, 2022, 8:47 PM

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Yeah, but need to use function pointers somewhere in my program

uncut marlin Dec 14, 2022, 8:47 PM

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not sure what benefit this list of pointers approach provides?

uncut marlin Dec 14, 2022, 8:47 PM

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nocturne urchin Yeah, but need to use function pointers somewhere in my program

hmm, is that like part of the assignment?

nocturne urchin Dec 14, 2022, 8:47 PM

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yeah

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This is literally the only use case I can think of

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so this

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!format

woeful nimbusBOT Dec 14, 2022, 8:50 PM

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void (VehicleList::*getMenuChoice(int choice))() {
  void (VehicleList::*options[6])() = {
      &VehicleList::addVehicle,
      &VehicleList::removeVehicle,
      &VehicleList::searchForCar,
      &VehicleList::searchForBike,
      &VehicleList::sortVehiclesByRegistrationName,
      &VehicleList::sortVehiclesByCostPerDay};

  return options[choice - 1];
}

JAMMEISTER

nocturne urchin Dec 14, 2022, 8:51 PM

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I should be able to call doing this

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(vehicleList->*getMenuChoice(input))();

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?

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Okay, so obviously this works

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!format

woeful nimbusBOT Dec 14, 2022, 8:53 PM

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void (VehicleList::*VehicleList::getMenuChoice(int choice))() {
  void (VehicleList::*options[6])() = {
      &VehicleList::addVehicle,
      &VehicleList::removeVehicle,
      &VehicleList::searchForCar,
      &VehicleList::searchForBike,
      &VehicleList::sortVehiclesByRegistrationName,
      &VehicleList::sortVehiclesByCostPerDay};

  return options[choice - 1];
}

JAMMEISTER

nocturne urchin Dec 14, 2022, 8:54 PM

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and just to be clear, this function return a function pointer to a vehicleList member function that takes no parameters and has a void return type?

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in which I call this method doing this

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(vehicleList->*vehicleList->getMenuChoice(input))();

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Which works, but I am confused as to why I need the second *vehicleList when calling the method

woeful nimbusBOT Dec 14, 2022, 9:00 PM

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@nocturne urchin Has your question been resolved? If so, run !solved :)

uncut marlin Dec 14, 2022, 9:02 PM

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are there any other stipulations about this function pointer you're required to use?

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because, like... every function is already a pointer

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void foo() {} now foo is a pointer

nocturne urchin Dec 14, 2022, 9:03 PM

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Nope. Just mentions use function pointers in the application

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I will try and figure out a different way to use them 🙂

vital briar Dec 14, 2022, 9:04 PM

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nocturne urchin Which works, but I am confused as to why I need the second *vehicleList when cal...

you created a method on VehicleList to get the pointer to member function
so vehicleList->getMenuChoice(input) gets this pointer to member function, and then you use it by doing vehicleList->*... and then call that function

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you could make it something other than a member function since it doesn't use this, a function just named getMenuChoice and do (vehicleList->*getMenuChoice(input))()

nocturne urchin Dec 14, 2022, 9:06 PM

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Okay, I got you. Thanks for the help kind sir!

woeful nimbusBOT Dec 15, 2022, 9:46 AM

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#Help with function pointers