如何在gtkmm上将Gtk::Widget转换为GtK::ScrolledWindow？

Question

在下面的代码中，我在网格容器上设置ScrolledWindows，然后在循环中设置它们的属性，因为我想将TextViews放在网格上。我不想使用TreeView，因为在那里放置较长或中等的文本将是不利的，这正是我的意图。不幸的是，用于获取附加小部件的函数get_child_at(int col，int row)返回Gtk::Widget，而对于我需要的属性，我必须对Gtk：：ScrolledWindow进行强制转换，但我不能这样做。

(some code)
descriptors->attach(* (new Gtk::ScrolledWindow ()), 0,0);
descriptors->attach(* (new Gtk::ScrolledWindow ()), 1,0);
descriptors->attach(* (new Gtk::ScrolledWindow ()), 2,0);

(some other code)

for(int i = 0; i < 3; i++)
{
    //Glib::RefPtr<Gtk::ScrolledWindow> * disposable_pointer = new Glib::RefPtr<Gtk::ScrolledWindow>();
    //disposable_pointer = descriptors->get_child_at (i,0);
    //Glib::RefPtr<Gtk::ScrolledWindow> disposable_pointer = Glib::RefPtr<Gtk::Widget>::cast_dynamic(*descriptors->get_child_at (i,0));
    //Glib::RefPtr<Gtk::ScrolledWindow>Glib::RefPtr<Gtk::ScrolledWindow>::cast_dynamic
    //current_grid_child_scrolledWin = Glib::RefPtr<Gtk::ScrolledWindow>cast_dynamic(descriptors->get_child_at (i,0));
    //current_grid_child_scrolledWin = disposable_pointer->get();
    
    Glib::RefPtr<Gtk::Widget> * something = new Glib::RefPtr<Gtk::Widget>(descriptors->get_child_at (i,0));
    
    Glib::RefPtr<Gtk::ScrolledWindow> disposable_pointer = *something;

    current_grid_child_scrolledWin->set_vadjustment (Glib::RefPtr<Gtk::Adjustment>());
    current_grid_child_scrolledWin->set_hadjustment (Glib::RefPtr<Gtk::Adjustment>());
    current_grid_child_scrolledWin->set_policy (Gtk::PolicyType::POLICY_AUTOMATIC, Gtk::PolicyType::POLICY_AUTOMATIC);
    current_grid_child_scrolledWin->set_vexpand (true);
    current_grid_child_scrolledWin->set_hexpand (true);
    current_grid_child_scrolledWin->set_margin_end (10);
    current_grid_child_scrolledWin->set_margin_bottom (10);
    current_grid_child_scrolledWin->set_visible (true);
}

我得到的最后一个错误是

/usr/include/glibmm-2.4/glibmm/refptr.h:309:31: error: invalid conversion from 'Gtk::Widget*' to 'Gtk::ScrolledWindow*' [-fpermissive]
309 |   pCppObject_(src.operator->())
|                               ^
|                               |
|                               Gtk::Widget*

我考虑过the gtkmm documentation on using casting with Glib::RefPtr

Answer 1

正如我们从Gtk::Grid documentation中看到的，get_child_at的返回类型是一个指向小部件的原始指针：

Widget* Gtk::Grid::get_child_at(int column,
                                int row 
                               ) 

而不是智能指针(在您的例子中是Glib::Refptr)。这是因为get_child_at的调用方不负责管理返回的指针所指向的对象的生存期。您只需获得一个句柄，在它上面工作，然后离开它。其他人负责在它上面调用delete。在你的情况下，我会这样做：

#include <gtkmm.h>
#include <iostream>

int main(int argc, char *argv[])
{
    // Initialize Gtkmm:
    auto app = Gtk::Application::create(argc, argv, "so.question.q66162108");

    // Create a grid and attach a scrolled window:
    Gtk::Grid container;
    Gtk::ScrolledWindow scrolledWindow;

    // Add the scrolled window to the grid. Make sure the container is responsible
    // of deleting it! Otherwise, remeber to call delete on it:
    container.attach(*Gtk::manage(new Gtk::ScrolledWindow()), 0, 0, 1, 1);

    // Here, you get a raw pointer to a widget:
    Gtk::Widget* pWidget = container.get_child_at(0, 0);

    // We cast it to its most derived type:
    Gtk::ScrolledWindow* pScrolledWindow = dynamic_cast<Gtk::ScrolledWindow*>(pWidget);

    // If the cast fail (i.e. not a ScrolledWindow at (0, 0), then the
    // casted pointer will be set to nullptr:
    if(pScrolledWindow)
    {
        std::cout << "Casting worked" << std::endl;

        // Handle properties here...
        pScrolledWindow->set_visible (true);

        // Set other properties here...
    }

    return 0;
} // At this point the container is destroyed and since Gtk::manage
  // was used, the scrolled window will be automatically deleted.

请注意，我使用的这段代码没有使用智能指针。这是因为当我new我的Gtk::ScrolledWindow时，我使用Gtk::manage*。此函数将对象(此处为Gtk::ScrolledWindow)标记为其父容器小部件(此处为Gtk::Grid)所有，因此无需手动删除它。我成为容器的责任，当它离开时，对它的子容器调用delete。

*如果使用较新的Gtkmm版本，则必须使用make_managed<T>。请参阅here。