The Widget::O Idiom

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A well encapsulated design makes only the minimum necessary parts of a class public (public or protected). In other words, everything should be private by default and make things public (or protected) only if absolutely necessary.

In C++, one tool for this is to implement parts of your class as non-member non-friend functions. These functions should be put in a namespace so, they don’t litter the global namespace. If you have a class Widget, then usually people define a WidgetHelper namespace in which they put the Widget class and also its helper functions:

class Widget {
public:
  Widget() : width{}, height{} {}
  void setWidth(int w) { width = w; }
  void setHeight(int h) { height = h; }
private:
  int width;
  int height;
};

namespace WidgetHelper {
  void setSize(Widget& o, int w, int h) {
    o.setWidth(w);
    o.setHeight(h);
  }
}

int main() {
  auto w = Widget{};
  WidgetHelper::setSize(w, 10, 20);
  return 0;
}

A cleaner way is to use something I call the Widget::O idiom. Here it is how the above code looks like using the Widget::O idiom:

namespace Widget {
  class O {
  public:
    O() : width{}, height{} {}
    void setWidth(int w) { width = w; }
    void setHeight(int h) { height = h; }
  private:
    int width;
    int height;
  };

  void setSize(O& o, int w, int h) {
    o.setWidth(w);
    o.setHeight(h);
  }
}

int main() {
  auto w = Widget::O{};
  setSize(w, 10, 20); // Syntax with ADL.
  Widget::setSize(w, 30, 40); // Syntax without ADL.
  return 0;
}

The Widget::O idiom helps you in keeping all the functions and data related to Widget::O in the same Widget namespace, regardless of whether they are members or non-members. With this idiom, you can use also use ADL (Argument Dependent Lookup) if you prefer when calling the non-member functions.