How to make the lambda a friend of a class?

Question

Let's say, I have a class:

class A {
  int a;
};

And I have a lambda:

auto function = [](A* a) {
  a->a;  // <== gives an error in this line.
};

function(new A);

Is there any way to use a private member/method inside a lambda? - It's not necessary to pass the pointer to the lambda - it may be a capture-by or something else.

All reasonable schemes are welcome.

Answer 1

You can do it by creating a friend function that returns the lambda function. It inherits the friend access:

struct A {
  friend std::function<void(A&, int)> f();

  private:
    int i;
    void test() {std::cout << "test: " << i << "\n";}
};

std::function<void(A&, int)> f() {
  return [] (A &a, int i) {a.i = i; a.test(); };
}

int main() {
    A a;
    f()(a, 13);

    return 0;
}

Answer 2

using std::function takes extra resource, so I recomendet using friend/or method function to access private member (friend function implicit inlined):

class A{
    int a;

    friend int access_member(A*a){ return a->a;}
};

-----------------------------------------
auto function = [](A*a){   return access_member(a); }

Live example

EDIT: I personally like std::function, but don't forgot, std::function always takes extra memory resources, and may not inlined , so if you may implement your source without std::function, don't use std::function. See, How is std::function implemented? Also, Lambda to std::function conversion performance