Multiple file compilation error

I am starting to create my GUI toolkit project for the X Window System, and I created these classes, each of which has a .h and .cpp file:

Dimension (actually contains 3 classes, Dimension, FloatDimension, and BoolDimension): An object to contain an x, y pair.
Application: Initializes the resources used by the widgets, manages the event loop.
TopLevelWindow: A window.

Each of these individually compile into a .o file just fine, but my test program that uses them gives an error:

Code:

#include "Dimension.h"
#include "Application.h"
#include "TopLevelWindow.h"

int main() {
	Application app;
	TopLevelWindow win(app, "Test", Dimension(3, 3));
	win.pack();
	win.show();
	app.eventLoop();
	return 0;
}

In case you don't know the g++ command, this is an explanation in english:

g++ compile-only-no-linker include-library-X11 file:src/main.cpp

Code:

$ g++ -c -lX11 src/main.cpp
In file included from src/TopLevelWindow.h:5,
                 from src/main.cpp:4:
src/Application.h:3: error: redefinition of ‘class Application’
src/Application.h:3: error: previous definition of ‘class Application’
In file included from src/TopLevelWindow.h:7,
                 from src/main.cpp:4:
src/Dimension.h:1: error: redefinition of ‘class Dimension’
src/Dimension.h:1: error: previous definition of ‘class Dimension’
src/Dimension.h:10: error: redefinition of ‘class FloatDimension’
src/Dimension.h:10: error: previous definition of ‘class FloatDimension’
src/Dimension.h:18: error: redefinition of ‘class BoolDimension’
src/Dimension.h:18: error: previous definition of ‘class BoolDimension’
src/main.cpp: In function ‘int main()’:
src/main.cpp:7: error: no matching function for call to ‘TopLevelWindow::TopLevelWindow(Application&, const char [10], Dimension)’
src/TopLevelWindow.h:16: note: candidates are: TopLevelWindow::TopLevelWindow(Application*, std::string, Dimension)
src/TopLevelWindow.h:8: note:                 TopLevelWindow::TopLevelWindow(const TopLevelWindow&)

I have no idea what's going on. Can anyone help me fix this?

Your headers probably include the other headers yes? Do you have appropriate header inclusion guards? What do the header files look like?

I think your headers are missing header guards.

Code:

#ifndef TOPLEVEL_WINDOW_H
#define TOPLEVEL_WINDOW_H

//header contents

#endif

What is a header guard?

I figured out the header guard thing, it compiled fine, and gave a segfault. Probably that big mess of pointers that lets all the objects refer to each other.

I figured out the problem using the GDB debugger and it works, displaying a blank window with the correct title!

Originally Posted by MTK

What is a header guard?

the 'technical' term is 'Conditional Compilation'.