Drei Dateien:
main.cpp

#include "mywin.h"

int main(int argc, char *argv[])
{
	Gtk::Main kit(argc, argv);
	mywin testwin;
	Gtk::Main::run(testwin);
	return 0;
}

mywin.h

#ifndef MYWIN_H
#define MYWIN_H

#include <gtkmm.h>

class mywin : public Gtk::Window
{
	public:
		mywin();

	private:
		Gtk::Button btn_test;
		Gtk::VBox box_test;
};

#endif //ifndef MYWIN_H

mywin.cpp

#include "mywin.h"

mywin::mywin()
:	Gtk::Window(),
	btn_test("TEST"),
	box_test(true, 3)
{
	box_test.pack_start(btn_test);
	add(box_test);
}

Witzfrage: Warum lässt sich das nicht bauen?

Ausgabe:

--------------------Configuration: test - Debug--------------------
Compiling source file(s)...
main.cpp
In file included from main.cpp:1:
mywin.h:17:24: warning: no newline at end of file
main.cpp:9:2: warning: no newline at end of file
mywin.h
mywin.h:17:24: warning: no newline at end of file
mywin.cpp
In file included from mywin.cpp:1:
mywin.h:17:24: warning: no newline at end of file
mywin.cpp:10:2: warning: no newline at end of file
Linking...
C:\MinGWStudio\test\Debug\mywin.o: In function `ZN5mywinC2Ev':
C:\MinGWStudio\test\mywin.cpp:4: multiple definition of `mywin::mywin()'
C:\MinGWStudio\test\Debug\mywin.o:C:\MinGWStudio\test\mywin.cpp:4: first defined here
C:\MinGWStudio\test\Debug\mywin.o: In function `ZN5mywinC1Ev':
C:\MinGWStudio\test\mywin.cpp:4: multiple definition of `mywin::mywin()'
C:\MinGWStudio\test\Debug\mywin.o:C:\MinGWStudio\test\mywin.cpp:4: first defined here
collect2: ld returned 1 exit status

test.exe - 4 error(s), 5 warning(s)

Wenn ich alles in eine Datei packe, klappt es, also was mache ich falsch??

OMG bin ich duhm, ich hab die mywin.h unter Source Files gepackt anstatt Header Files.

Nochmal Danke Grober