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Weird Unintelligible Error: `app` was not declared in this scope

Karanaka Kun (6)
OK, now I am experiencing compilation errors. Again. But the guide I'm following right now is the guide for gtkmm version 3.20 and 3.18. Both examples from both version documentation gives out the same error.

I was trying to compile one of the examples.

examplewindow.cc

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#include "examplewindow.h"
#include <iostream>


ExampleWindow::ExampleWindow()
: m_ButtonBox(Gtk::ORIENTATION_VERTICAL),
  m_Button_File("Choose File"),
  m_Button_Folder("Choose Folder")
{
  set_title("Gtk::FileSelection example");

  add(m_ButtonBox);

  m_ButtonBox.pack_start(m_Button_File);
  m_Button_File.signal_clicked().connect(sigc::mem_fun(*this,
              &ExampleWindow::on_button_file_clicked) );

  m_ButtonBox.pack_start(m_Button_Folder);
  m_Button_Folder.signal_clicked().connect(sigc::mem_fun(*this,
              &ExampleWindow::on_button_folder_clicked) );

  show_all_children();
}

ExampleWindow::~ExampleWindow()
{
}

void ExampleWindow::on_button_folder_clicked()
{
  Gtk::FileChooserDialog dialog("Please choose a folder",
          Gtk::FILE_CHOOSER_ACTION_SELECT_FOLDER);
  dialog.set_transient_for(*this);

  //Add response buttons the the dialog:
  dialog.add_button("_Cancel", Gtk::RESPONSE_CANCEL);
  dialog.add_button("Select", Gtk::RESPONSE_OK);

  int result = dialog.run();

  //Handle the response:
  switch(result)
  {
    case(Gtk::RESPONSE_OK):
    {
      std::cout << "Select clicked." << std::endl;
      std::cout << "Folder selected: " << dialog.get_filename()
          << std::endl;
      break;
    }
    case(Gtk::RESPONSE_CANCEL):
    {
      std::cout << "Cancel clicked." << std::endl;
      break;
    }
    default:
    {
      std::cout << "Unexpected button clicked." << std::endl;
      break;
    }
  }
}

void ExampleWindow::on_button_file_clicked()
{
  Gtk::FileChooserDialog dialog("Please choose a file",
          Gtk::FILE_CHOOSER_ACTION_OPEN);
  dialog.set_transient_for(*this);

  //Add response buttons the the dialog:
  dialog.add_button("_Cancel", Gtk::RESPONSE_CANCEL);
  dialog.add_button("_Open", Gtk::RESPONSE_OK);

  //Add filters, so that only certain file types can be selected:

  auto filter_text = Gtk::FileFilter::create();
  filter_text->set_name("Text files");
  filter_text->add_mime_type("text/plain");
  dialog.add_filter(filter_text);

  auto filter_cpp = Gtk::FileFilter::create();
  filter_cpp->set_name("C/C++ files");
  filter_cpp->add_mime_type("text/x-c");
  filter_cpp->add_mime_type("text/x-c++");
  filter_cpp->add_mime_type("text/x-c-header");
  dialog.add_filter(filter_cpp);

  auto filter_any = Gtk::FileFilter::create();
  filter_any->set_name("Any files");
  filter_any->add_pattern("*");
  dialog.add_filter(filter_any);

  //Show the dialog and wait for a user response:
  int result = dialog.run();

  //Handle the response:
  switch(result)
  {
    case(Gtk::RESPONSE_OK):
    {
      std::cout << "Open clicked." << std::endl;

      //Notice that this is a std::string, not a Glib::ustring.
      std::string filename = dialog.get_filename();
      std::cout << "File selected: " <<  filename << std::endl;
      break;
    }
    case(Gtk::RESPONSE_CANCEL):
    {
      std::cout << "Cancel clicked." << std::endl;
      break;
    }
    default:
    {
      std::cout << "Unexpected button clicked." << std::endl;
      break;
    }
  }
}


examplewindow.h

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#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H

#include <gtkmm.h>

class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();

protected:
  //Signal handlers:
  void on_button_file_clicked();
  void on_button_folder_clicked();

  //Child widgets:
  Gtk::ButtonBox m_ButtonBox;
  Gtk::Button m_Button_File, m_Button_Folder;
};

#endif //GTKMM_EXAMPLEWINDOW_H


main.cc

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#include "examplewindow.h"
#include <gtkmm/application.h>

int main(int argc, char *argv[])
{
  auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");

  ExampleWindow window;

  //Shows the window and returns when it is closed.
  return app->run(window);
}


This is the error I get from g++ and gcc

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ander@Ander:~/C++_Programming/test$ g++ main.cc -o simple2 `pkg-config gtkmm-3.0 --cflags --libs`
main.cc: In function ‘int main(int, char**)’: 
main.cc:6:8: error: ‘app’ does not name a type 
auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example"); 
       ^ 
main.cc:11:10: error: ‘app’ was not declared in this scope 
return app->run(window);
          ^
gunnerfunner (2127)
What if you spell out the return type of Gtk::Application::create() explicitly instead of using auto in the declaration of variable app?
helios (17575)
Are you getting that error only in that instance of auto? What happens if you try to compile examplewindow.cc?
JLBorges (13770)
> This is the error I get from g++ and gcc

Compile with C++11 enabled.

g++ main.cc -o simple2 `pkg-config gtkmm-3.0 --cflags --libs`
g++ -std=c++11 main.cc -o simple2 `pkg-config gtkmm-3.0 --cflags --libs
`

Or ideally:
g++ -std=c++11 -Wall -Wextra -pedantic-errors main.cc -o simple2 `pkg-config gtkmm-3.0 --cflags --libs`
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