How to return a function value when using a dropdown list widget

Question

I'm on Ubuntu. I need to return a function when I click in a dropdown list, but don't understand how to do that. For example:

...
dropdownList.set_active_text("Choose");
dropdownList.signal_changed().connect(sigc::mem_fun(*this, &usb_boot::showing));
std::cout << "var = " << var << std::endl;
...

void usb_boot::showing(){

Gtk::MessageDialog dialogue(*this, dropdownList.get_active_text());
dialog.set_secondary_text("Choose list");
dialog.run();
std::cout << "You choose :\n" << dropdownList.get_active_row_number() << " " << dropdownList.get_active_text() << std::endl;

add return here ?

How to return dropdownList.get_active_text() and dropdownList.get_active_row_number() to variables ?

Answer 1

If I understand your question correctly, you want to reuse some values you get from within your handler (i.e. showing) outside of its scope. To do this, I would suggest using a lambda expression and capture by reference your variables. For example:

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

int main(int argc, char **argv)
{
    auto app = Gtk::Application::create(argc, argv, "so.question.q63850579");
    
    Gtk::Window window;

    Gtk::ComboBoxText combo;
    combo.append("option 1");
    combo.append("option 2");
    combo.append("option 3");

    // These are your variables, which you want to set to the values the user will choose. They are defined
    // outise the handler:
    std::string choice;
    int row;

    // Here you set the handler. The variables 'choice', 'row' and 'combo' are all passed by reference to
    // the handler (notice the '&'):
    combo.signal_changed().connect([&choice, &row, &combo](){
        choice = combo.get_active_text();
        row = combo.get_active_row_number(); 

        // Inside the scope of the handler, we can see the variable content changing everytime the user
        // changes a value:
        std::cout << "Your current selection is item #" << row << ", which is: " << choice << std::endl;
    });

    window.add(combo);
    window.show_all();

    int returnCode = app->run(window);

    // When the window closes, the variables are read again, but this time from outside the handler's
    // scope. This is possible because they were references:
    std::cout << "Your final selection was item #" << row << ", which is: " << choice << std::endl;

    return returnCode;
}

Another option, since you use sigc::mem_fun, would be to save the contents inside a member variable of the class that represents the object pointed to by this. I personally prefer the lambda expression, since these values are propably unrelated to the class.