Having a function as a property on a class - C++

Question

I am looking to make a class in C++ which can have a changeable function as one of its individual properties. Is there any standard way to achieve this? What would the syntax look like?

For example, something I have tried and want to do:

#include <iostream>

class Thing {
  private:
    
    int val;
    
    void func();
    
  public:
    
    Thing(int val = 0) {
      this->val = val;
    }
    
    void setFunc(void func()) { this->func() = func; }

    void useFunc() { this->func(); }
    
};

int main() {
  
  Thing thing1 = Thing(1);
  
  thing1.setFunc({
    
    std::cout << "i am thing 1, my value is: " << this->val;
    
  });
  
  thing1.useFunc();
  
  return 0;

}

I'm not sure what the syntax would be like for something like this, or if pointers are needed. This (probably obviously) returns a couple of errors, those being:

./main.cpp:16:46: error: expression is not assignable
    void setFunc(void func()) { this->func() = func; }
                                ~~~~~~~~~~~~ ^
./main.cpp:28:51: error: invalid use of 'this' outside of a non-static member function
    std::cout << "i am thing 1, my value is: " << this->val;
                                                  ^

I realize I could use a lot of properties that aren't functions instead, then have one method which uses these properties to determine the outcome, however I am not looking to do this, as it would require a lot more properties and I want to try something different for the sake of learning how to do it. In this example there'd be no point in doing what I am trying to do, but this is just an example.

Answer 1

Step 1: Define a function pointer!

class Thing {
  private:

    int val;
    // void func(); declares a function that will be defined later
    // not what we want.
    using func_ptr = void (*)(); // define a new type. It make everything 
                                 // that follows much easier 
    func_ptr func; // declares func to be a pointer to a function

  public:

    Thing(int val = 0): val(val) // use member initializer list
                                 // all members must be initialized
                                 // before entering the body of the
                                 // constructor. not that important
                                 // for an int, but very important for
                                 // types with expensive initialization
                                 // or no default initialization
    {
    }

    void setFunc(func_ptr func)  // pass a function pointer
    {
        this->func = func; // Do not call the function!
                           // avoid reusing names in the same scope.
                           // It only leads to trouble
    }

    void useFunc()
    {
        func(); // which func is not ambiguous here. No need for this
    }
};

Step 2: Assigning the function!

  thing1.setFunc({
    
    std::cout << "i am thing 1, my value is: " << this->val;
    
  });

Looks like you're trying to use a Lambda Expression. A couple of problems here.

1. Wrong syntax. C++ is NOT a language that rewards guessing. Get a good book or two and learn the syntax in a controlled manner. The alternative is slow, painful, and not worth doing.

Proper syntax looks like

  thing1.setFunc([](){

      std::cout << "i am thing 1, my value is: " << this->val;

    });

2. Unfortunately the Lambda has no clue that it is associated with thing1 nor is it a member of Thing so it cannot use this. This is a bit of a show-stopper. The Lambda need to Capture thing1, but as soon as you capture, you can't use function pointers anymore. They are too simple to deal with captures. Enter std::function.

Step 3: Start over with std::function

#include <iostream>
#include <functional>

class Thing {
  private:

    int val;
    using func_ptr = std::function<void ()>; 
    func_ptr func; 

  public:

    Thing(int val = 0): val(val) 
    {
    }

    void setFunc(func_ptr func)  
    {
        this->func = func; 
    }

    void useFunc()
    {
        func(); 
    }

    int get_val()
    {
        return val;
    }
};
int main() {

  Thing thing1 = Thing(1);
  thing1.setFunc([thing1](){ // captures by value. Not always what you 
                             // want, but generally the safest option.
                             // Just fine for a simple example like this
      std::cout << "i am thing 1, my value is: " << thing1.val;

    });

  thing1.useFunc();

  return 0;

}

And this still can't work because Thing::val is a private member and cannot be accessed from outside Thing or a friend of Thing. The Lambda is neither and cannot easily be shaped into one. Either A) the Lambda must call a Thing member function to do the real work, pointless because you'd discard the all of this rigamarole and simply directly call the member function, B) you add a getter for Thing::val and the Lambda uses the getter, or C) you make Thing::val public.

B looks like the best option. But what if the logic you need has to change thing1? That needs a...

Step 4: Find a simpler answer to the overarching problem.