How to make a lambda function friend of a class?

I was trying to make a class that takes as argument in the constructor a lambda function, and I want this function to be friend with the class. The code for the class looks like this:

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

class foo
{
public:
    foo(func f)
    {
        this->f = f;
    }

    func f;
private:
    int value_I_want_to_modify; //an int I want to change from the function I've passed in the constructor
}

And in the main() I would write something like this:

int main()
{
    //this will give an error because I cannot access private members from outside class
    foo v
    {
        [&v](void) { v.value_I_want_to_modify = 0 };
    }
}

Now I would like the function to be friend with the class but I can't find a way to do it.

Solution

How to make a lambda function friend of a class?

You cannot. It's a catch-22 problem.

If you define the lambda function before defining the class, you can't access the member variable of the class.

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

class foo;

// Trying to define the lambda function before the class.
// Can't use f.value_I_want_to_modify since foo is not defined yet.
auto lambda_function = [](foo& f) { f.value_I_want_to_modify = 0;}

class foo
{
   public:
      foo(func f)
      {
         this->f = f;
      }

      func f;
   private:
      int value_I_want_to_modify;
};

int main()
{
    foo v{lambda_function};
}

If you define the lambda function after defining the class, you can't make the lambda function a friend of the class.

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

class foo
{
   public:
      foo(func f)
      {
         this->f = f;
      }

      func f;
   private:
      int value_I_want_to_modify;
};

int main()
{
   foo f
   {
      // Can't make the lambda function a friend of foo
      // since it cannot be declared before the class definition.
      [&f](void) { f.value_I_want_to_modify = 0;}
   }
}

The easiest work around is to modify the lambda function to accept an int& as argument and modify its value.

#include <functional>

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

class foo
{
   public:
      foo(func f)
      {
         this->f = f;
         this->f(value_I_want_to_modify);
      }

   private:

      func f;
      int value_I_want_to_modify;
};

int main()
{
   foo v{ [](int& out) { out = 0;} };
}