Idiom "type erasure" on the example of QWidget

Type erasure idiom

The article assumes that the reader has basic knowledge of C++

Straight to the point.

Suppose you need to dynamically (like me, on button click) change the text
several objects such as:

1. QLabel
2. QPushButton
3. QLineEdit

Most of you know that these widgets inherit from QWidget, so why not declare a method that takes a pointer to this base class and then call the setText method?

QWidget * base_pointer = new QLabel();
base_pointer->setText("ERROR");

The problem is that QWidget doesn't define the setText() method, so you can't do that.

My friend advised me 2 options for solving this problem:

• Via the "type erasure" idiom
• Through signals and slots

You already know for sure about signals and slots, and therefore I will talk about 1 method. Briefly.

The type erasure idiom allows you to erase the type of any object by hiding it behind
object of a more general type.

Hello templates

How it looks in words:

1. We create an abstract base class A, in it we declare the virtual function SetText

2. We inherit from this class the already template class B, and it only has:

3. field in the form of a pointer to a template type

4. implementation of the virtual method SetText

5. In the client class, we declare a pointer of type A

6. Use :)

How it looks in practice:

• An abstract base class and its successor:

namespace om_animation {
class AbstractWritableWidget {
public:
    ~AbstractWritableWidget() {}
    virtual void SetText(const QString& text) = 0;
};
 
template <class Widget>
class WritableWidget : public AbstractWritableWidget {
public:
    WritableWidget(Widget* writable_widget) : writable_widget_(writable_widget) {}
    ~WritableWidget() {}
 
    void SetText(const QString& text) override {
        writable_widget_->setText(text);
  }
 
private:
    Widget* writable_widget_ = nullptr;
};
}

• We use (just an example):

class Client {
public:
    template <typename T>
    void Run(T * type){
        // do what you want
        writable_widget_ = new WritableWidget<T> (type);
        writable_widget_->SetText("Success");
        // do what you want
    }
 
private:
     AbstractWritableWidget* writable_widget = nullptr;
}

• Real code:

template <typename Widget>
void om_animation::TextAnimator::RunAnimation(Widget* widget) {
  if (WritableMatcher::IsWidgetWritable(widget->metaObject()->className())) {
    if (writable_widget_) {
      delete writable_widget_;
    }
    writable_widget_ = new WritableWidget<Widget>(widget);
    timer_->start(animation_delay_);
  } else {
    throw std::logic_error(
        "incompatible type for text animation, add type to "
        "WritableMatcher class");
  }
}

In real code, I also made a class with static methods to check which class supports
setText method in Qt library. There's just a vector under the hood. Nothing complicated.
Through the metaobject, we get the name of the class, compare it with the one already in the list. Yes - yes, no - no.

Further I have made simple check of the pointer, and actually already use.
If the object was not found, the client receives an exception.

That's all. I find this idiom very cool.

Thank you for your attention.