The mechanics of extension via free functions or member functions

Question

Loads of C++ libraries, the standard included, allow you to adapt your objects for use in the libraries. The choice is often between a member function or a free function in the

Answer 1

Altought, sometimes, developers can used free functions or class functions, interchangeably, there are some situations, to use one another.

(1) Object / Class functions ("methods), are prefered when most of its purpouse affect only the object, or objects are inteded to compose other objects.

// object method
MyListObject.add(MyItemObject);
MyListObject.add(MyItemObject);
MyListObject.add(MyItemObject);

(2) Free ("global" or "module") functions are prefered, when involves several objects, and the objects are not part / composed of each other. Or, when the function uses plain data (structs without methods, primitive types).

MyStringNamespace.MyStringClass A = new MyStringNamespace.MyStringClass("Mercury");
MyStringNamespace.MyStringClass B = new MyStringNamespace.MyStringClass("Jupiter"); 
// free function
bool X = MyStringNamespace.AreEqual(A, B);

When some common module function access objects, in C++, you have the "friend keyword" that allow them to access the objects methods, without regarding scope.

class MyStringClass {
  private:
    // ...
  protected:
    // ...
  // not a method, but declared, to allow access
  friend:
    bool AreEqual(MyStringClass A, MyStringClass B);
}

bool AreEqual(MyStringClass A, MyStringClass B) { ... }

In "almost pure object oriented" programming languages like Java or C#, where you can't have free functions, free functions are replaced with static methods, which makes stuff more complicated.