Chapter 3. Basics

# Chapter 3. Basics

This chapter will introduce some of the most important aspects of gtkmm coding. These will be demonstrated with simple working example code. However, this is just a taster, so you need to look at the other chapters for more substantial information.

Your existing knowledge of C++ will help you with gtkmm as it would with any library. Unless we state otherwise, you can expect gtkmm classes to behave like any other C++ class, and you can expect to use your existing C++ techniques with gtkmm classes.

## Simple Example

To begin our introduction to gtkmm, we'll start with the simplest program possible. This program will create an empty 200 x 200 pixel window.

Source Code

File: `base.cc` (For use with gtkmm 3, not gtkmm 2)

    #include <gtkmm.h>

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

      Gtk::Window window;
      window.set_default_size(200, 200);

      return app->run(window);
    }

We will now explain each line of the example

    #include <gtkmm.h>

All gtkmm programs must include certain gtkmm headers; `gtkmm.h` includes the entire gtkmm kit. This is usually not a good idea, because it includes a megabyte or so of headers, but for simple programs, it suffices.

The next statement:

    Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv, "org.gtkmm.examples.base");

creates a `Gtk::Application` object, stored in a `RefPtr` smartpointer. This is needed in all gtkmm applications. The `create()` method for this object initializes gtkmm, and checks the arguments passed to your application on the command line, looking for standard options such as `--display`. It takes these from the argument list, leaving anything it does not recognize for your application to parse or ignore. This ensures that all gtkmm applications accept the same set of standard arguments.

The next two lines of code create a window and set its default (initial) size:

    Gtk::Window window;
    window.set_default_size(200, 200);

The last line shows the window and enters the gtkmm main processing loop, which will finish when the window is closed. Your `main()` function will then return with an appropriate success or error code.

    return app->run(window);

After putting the source code in `simple.cc` you can compile the above program with gcc using:

    g++ simple.cc -o simple `pkg-config gtkmm-3.0 --cflags --libs`

Note that you must surround the `pkg-config` invocation with backquotes. Backquotes cause the shell to execute the command inside them, and to use the command's output as part of the command line. Note also that `simple.cc` must come before the `pkg-config` invocation on the command line.

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