Display the value of an entry

i am using the library Gtkmm with c++ but i have a problem to display the value of an entry. This is my code :

#include <gtkmm/box.h>
#include <gtkmm/button.h>
#include <gtkmm/main.h>
#include <gtkmm/window.h>
#include <gtkmm/entry.h>

#include <iostream>

int main(int argc, char* argv[]) {
    Gtk::Main app(argc, argv);
    Gtk::Window fenetre;
    Gtk::VBox *boiteV = Gtk::manage(new Gtk::VBox(false, 10)); 

    Gtk::Entry *param = Gtk::manage(new Gtk::Entry());
    boiteV->pack_start(*param);

    Gtk::Button *bouton = Gtk::manage(new Gtk::Button("Tester !"));
    boiteV->pack_start(*bouton); 

    fenetre.add(*boiteV); 
    std::string a = param->get_text();

    bouton->signal_clicked().connect([&a]() {std::cout << a << std::endl;});

    fenetre.show_all(); 
    Gtk::Main::run(fenetre);
    return EXIT_SUCCESS;
}

My problem is when i click on the button i have nothing whereas i wrote a value in the entry. Thank you a lot for your help !