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I am a newbie trying to code a Menu class and I want it to be able to call functions that are part of other classes. I have sought thoroughly on the internet, but the answers that I've found doesn't fit my needs.

Here is a straightforward piece of code that defines my problem:

#include <iostream>

class Class1;//forward declaration
typedef void (Class1::*FunctionPtr_t)();//type: Pointer to a function in Class1 scope

class Class1
{
public:
    void function1()
    {std::cout << "function1 executed!";}//Test function to execute from Class2
};

class Class2
{    
public:
    FunctionPtr_t myfcnptr = NULL;//Pointer to a function in Class1
};

int main()
{
  Class2 myclass2;

  myclass2.myfcnptr = &Class1::function1;//Assign function to pointer (Everything OK here)
  (myclass2.*myfcnptr)();//Compilation Error: 'myfcnptr' was not declared in this scope
}

I have messed around, made both classes friend and used scope operators almost everywhere. I know the solution must be quite easy and that I'm making a ridiculous mistake, but I don't catch it! Thank you for your patience ;)

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  • 1
    you need an object of Class1 to call a member function of that class. Do you really need to mess around with function pointers? Commented Jun 3, 2016 at 13:17
  • you should explain better your original problem and what you want to achieve. This looks like a xy problem because it is not clear why you need function pointers and the code you posted makes (no offense) little sense to me Commented Jun 3, 2016 at 13:20
  • no offense intended ;). I am trying to create a menu for an Arduino project and each menu option triggers a function. The menu class is generic so it has to be capable of manage a generic array of functions, which will ultimately be specified in the construction process. However some of the functions belong to other classes. As an example, the menu access a database which is managed by a database class, though my menu class will be passed a reference to a function (for example readData) and has to be able to call it. Commented Jun 4, 2016 at 10:41
  • Are there any workaround to deal with this kind of situation? Please remember that as I am programming for an Arduino, I can't implement the Standard Library. Commented Jun 4, 2016 at 10:41
  • you would not need to implement the standard library, others did that already for you :P (unfortunately not available for arduino). What is "the situation"? What do you actually need? Calling a (non-static) member function without an object does not make sense in the first place. Commented Jun 4, 2016 at 10:44

4 Answers 4

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The correct syntax would be (myclass2.*myclass2.myfcnptr)();.

But in your case, Class1 and Class2 are unrelated, so Class2 cannot sensibly call a member function from Class1. You need an instance of Class1, or something derived from Class1, to perform that call.

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2 Comments

SOLVED->Quite near!!! The correct syntax is (myclass1.*myclass2.myfcnptr)(); , where myclass1 is an instantiation of Class1. I'm sure you meant that, but anyway you pushed me in the right way ;) Thank you!!
@Leo I've split my answer between syntax and semantics, but maybe that makes it unclear...
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Ultimately, you will need to create a relationship between the two classes Class1 and Class2 by utilizing C++'s class inheritance system; as previously mentioned, Class2 simply cannot call a Class1 member function without having a relationship. You could, perhaps, cause Class2 to extend Class1 by writing something like this on the first line of your class definition:

class Class2 : public Class1
{  // ...

You would also want to mark the original function in the parent class Class1 as virtual and then provide a definition of the function in the child class Class2.

A good resource to learn about C++'s class inheritance, polymorphism, the virtual keyword, etc., would be http://www.cplusplus.com/doc/tutorial/polymorphism/.

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1

If no instantiation of Class1 is desired, it is also possible to make the method function1 static and call it directly from a Class2 instance. The following code will compile:

#include <iostream>


typedef void (*FunctionPtr_t)();//PAY ATTENTION TO THE NEW typedef !

class Class1
{
public:
    static void function1() //STATIC METHOD
    {std::cout << "function1 executed!";}//Test function to execute from Class2
};

class Class2
{    
public:
    FunctionPtr_t myfcnptr = NULL;//Pointer to a function in Class1
};

int main()
{
  Class2 myclass2;

  myclass2.myfcnptr = &Class1::function1;//Assign function to pointer
  (*(myclass2.myfcnptr))();
}
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You are trying to call a member function via a member function pointer, but you are not specifying the instance of Class1 to call against.

For example:

FunctionPtr_t func = &Class1::function1;
Class1 instance;
(instance.*func)();

Notice that I've had to specify the instance to call against. The pointer just references a function. This is different to languages like C# where you can reference a function and that reference also tracks the object containing the function.

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