Calling private function in lambda from outside the class

Question

In my current project I am trying to pass a private member function as parameter to another function. In my code, the other function is a member function of a different class, but for keeping it simple, here it is a free function.

void outside_function(std::function<void(int)> func) {
  // do something with func
}


class MyClass {
public:
  void run();

private:
  bool my_func(double); // defined in source file
};

Now, from inside run I want to put my_func into outside_function as argument. Since the signature of run does not fit the requirements for the parameter, I cannot simply pass it. Using the adapter pattern was my first try, which was when I was reminded that member functions implicitly take the this pointer as first argument. This answer suggests using a lambda expression, which is what I did.

void MyClass::run() {
  outside_function([this](int a) -> void {
    double d = static_cast<double>(a);
    this->my_func(d);
  });
}

Why does this work? From my point of understanding, outside_function does not have access to my_func. Why don't I have to make my_func public first? Is it something the compiler does or some C++ rule I don't know?

Additionally, are there any catches to this approach that might break my code?

Answer 1

private access specifier only restrict an object name to be visible (it cannot be looked up) outside the class. The object itself is like any other member.

[class.access]/1

A member of a class can be

(1.1) private; that is, its name can be used only by members and friends of the class in which it is declared.

(1.2) protected; that is, its name can be used only by members and friends of the class in which it is declared, by classes derived from that class, and by their friends (see [class.protected]).

(1.3) public; that is, its name can be used anywhere without access restriction.