CODEWITHSUNDEEP
c++function-pointersstdlist
the_naive
the_naive

Reputation: 3064

Is it possible to have a list of pointers to member functions of one class as a member in another class where these functions will be called?

Let's say we have a class A, which has several setter functions with the identical function signature, like the following code:

class A
{
public:
    void setter1(int whatever)
    {
        // set Something ...
    }
    
    void setter2(int whatever)
    {
        // set Something ...
    }
    
    void setter3(int whatever)
    {
        // set Something ...
    }
};

Another class B has an object of class A. Another class C has an object of B. In C, I try to declare the list of function pointers of A and try to call them through B's object.

Following is the example how I called:

class B
{
public:
    A objectA;
};
    
class C
{
    C()
    {
        for (auto fnc: fncList)
        {
            someFunc();
        }
    }
    
    void someFunc()
    {
        auto setterFnc = fncList .front();
        fncList.pop_front();
        objectB.objectA. // --> here I can't find the right syntax to call the function;
    } 

    std::list<void (A::*)(int)> fncList = {&A::setter1, &A::setter2, A::setter2};
};

Is there any other way to declare the pointer-to-function list so that I can use it from any function of the class A through its object in B from C?

Upvotes: 3

Views: 203

Answers (2)

Alan
Alan

Reputation: 1

Somewhere inside one of the functions from class B, all these functions of class A will be called one by one.

Yes it is possible as shown below. You have to have an object or a pointer to an object of your class type for calling the member functions using a pointer to those member functions.

C++17

Here you can use std::invoke to use the pointer to member function to make the code more readable.

#include <functional> 
#include <list>
struct A
{

    void setter1(int whatever){}
    void setter2(int whatever){}
    void setter3(int whatever){}

};
struct B
{
   std::list<void (A::*)(int)> fncList = {&A::setter1, &A::setter2, &A::setter3};
   //lets add a ctor
   B( A &a, int i)
   {
          for(auto ptr: fncList)
          {
                std::invoke(ptr, a, i);  //calls each of the setter
          }
         
   }
};

Demo

Prior C++17

Here we replace the call std::invoke(ptr, a, i) with:

(a.*ptr)(i);

Demo

Upvotes: 3

YSC
YSC

Reputation: 40160

Yes it is possible. The syntax is gross though.

static constexpr void (A::* const setters_of_a[])(int)

This declares a core constant array setters_of_a of pointers-to-member to A of type function taking an int and returning void.

(a.*setter)(42);

This calls the member-function referenced by pointer-to-member setter on object a.

#include <iostream>
struct A
{
    void f(int) { std::cout << "A::f()\n";}
    void g(int) { std::cout << "A::g()\n";}
};

struct B
{
    A a;
    static constexpr void (A::* const setters_of_a[])(int) = { &A::f, &A::g };
    void set_a()
    {
        for (auto setter : setters_of_a) {
            (a.*setter)(42);
        }
    }
};

int main()
{
    B b;
    b.set_a();
}

Demo on Compiler Explorer

Upvotes: 2

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