Widgets

23.1.1. Gestión normal de la memoria en C++

gtkmm le permite al programador controlar la vida (esto es, la construcción y la destrucción) de cualquier widget de la misma manera que cualquier otro objeto de C++. Esta flexibilidad le permite usar new y delete para crear y destruir objetos dinámicamente o para usar miembros de clase regulares (que se destruyen automáticamente cuando se destruye la clase) o usar instancias locales (que se destruyen cuando la instancia sale del alcance). Esta flexibilidad no está presente en algunos toolkits de IGU de C++, que sólo le permiten al programador usar un subconjunto de las características de gestión de memoria de C++.

Algunos ejemplos de gestión normal de la memoria en C++:

23.1.1.1. Widgets de alcance de clase

Si un programador no necesita asignación dinámica de memoria, puede usar los widgets automáticos en el alcance de clase. Una ventaja de los widgets automáticos en el alcance de clase es que la gestión de la memoria se agrupa en un único lugar. El programador no se arriesga a fugas de memoria por no eliminar un widget.

La principal desventaja de usar widgets de alcance de clase es revelar la implementación de la clase en lugar de la interfaz en su cabecera.

#include <gtkmm/button.h>
#include <gtkmm/window.h>
class Foo : public Gtk::Window
{
private:
  Gtk::Button theButton;
  // will be destroyed when the Foo object is destroyed
};

23.1.1.2. Widgets de alcance de función

If a programmer does not need a class scope widget, a function scope widget may also be used. The advantages to function scope over class scope are the increased data hiding and reduced dependencies.

{
  Gtk::Button aButton;
  aButton.show();
  ...
  app->run();
}

23.1.1.3. Asignación dinámica con new y delete

Usually, the programmer will prefer to allow containers to automatically destroy their children by creating them using Gtk::make_managed() (see below). This is not strictly required, as the new and delete operators may also be used, but modern C++ style discourages those in favour of safer models of memory management, so it is better to create widgets using Gtk::make_managed() and let their parent destroy them, than to manually perform dynamic allocation.

auto pButton = new Gtk::Button("Test");

// do something useful with pButton

delete pButton;
Here, the programmer deletes pButton to prevent a memory leak.

23.1.2. Widgets gestionados

Alternatively, you can let a widget's container control when the widget is destroyed. In most cases, you want a widget to last only as long as the container it is in. To delegate the management of a widget's lifetime to its container, create it with Gtk::make_managed() and then pack it into its container with Gtk::Container::add() or a similar method. Now the widget will be destroyed whenever its container is destroyed.

23.1.2.1. Dynamic allocation with make_managed() and add()

gtkmm provides ways including the make_managed() function and Gtk::Container::add() method to simplify creation and destruction of widgets whose lifetime can be managed by a parent.

Every widget except a top-level window must be added to a parent container in order to be displayed. The manage() function marks a widget so that when that widget is added to a parent container, said container becomes responsible for deleting the widget, meaning the user no longer needs to do so. The original way to create widgets whose lifetime is managed by their parent in this way was to call manage(), passing in the result of a new expression that created a dynamically allocated widget.

However, usually, when you create such a widget, you will already know that its parent container should be responsible for destroying it, In addition, modern C++ style discourages use of the new operator, which was required when passing a newly created widget to manage(). Therefore, gtkmm has added make_managed(), which combines creation and marking with manage() into a single step. This avoids you having to write new, which is discouraged in modern C++ style, and more clearly expresses intent to create a managed widget.

MyContainer::MyContainer()
{
  auto pButton = Gtk::make_managed<Gtk::Button>("Test");
  add(*pButton); //add *pButton to MyContainer
}
Now, when objects of type MyContainer are destroyed, the button will also be deleted. It is no longer necessary to delete pButton to free the button's memory; its deletion has been delegated to the MyContainer object.

Note that if you never added the widget to any parent container, or you did but later Gtk::Container::remove()d it from said parent, gtkmm restores the widget’s lifetime management to whatever state it had before manage() was called, which typically means that the responsibility for deleteing the widget returns to the user.

Por supuesto, un contenedor de nivel superior no se añadirá a otro contenedor. El programador es responsable de la destrucción del contenedor de nivel superior usando una de las técnicas tradicionales de C++. Por ejemplo, una ventana de nivel superior podría ser sólo una instancia en su función main().