How to Include-guard when using multiple headers on a main.cpp? - c++

Question

I'm learning C++ in a course, using Visual Studio 2013, and I have an issue with include-guards on my main.cpp. I can't use either class or #pragma once (though they work) due to conditions my professor said.
If I only use Coordinates and Line, and I use in main.cpp the #include Line.h (which get code from both Line and Coordinates), this works ok, but when I add Rectangle and Triangle (both have #include "Line.h"), then it throws the "already defined" error LNK2005 several times.
Is there something missing?
This is my code:
Coordinates.h
#ifndef Coordinates
#define Coordinates
//Code declaration. Other headers have similar declaration
struct CoordinatesType { double x, y; } coordinates;
void setCoordinates(double x, double y);
CoordinatesType getCoordinates();
#endif
Coordinates.cpp
#include "Coordinates.h"
//Code implementation
Line.h
#ifndef Line
#define Line
#include "Coordinates.h"
//Code declaration
#endif
Line.cpp
#include "Line.h"
#include <math.h>
//Code implementation
Rectangle.h
#ifndef Rectangle
#define Rectangle
#include "Line.h"
//Code declaration
#endif
Rectangle.cpp
#include "Rectangle.h"
//Code implementation
Triangle.h
#ifndef Triangle
#define Triangle
#include "Line.h"
//Code declaration
#endif
main.cpp
#include "Triangle.h"
#include "Rectangle.h"
int main(){
//Do stuff here.
}
If you need me to add the implementation and declaration codes, let me know, but I feel like it has to do with the include-guard.
EDIT: I'll add code in the Coordinate header so you can get an idea of what I'm doing and to avoid consuming a lot of space in the post. Remember, I can't use class Coordinates{} due to my professor's restriction to not use it.

Answer 1

This error is a linker error, not a compiler error.
Header guards do nothing for the linker.
You have successfully guarded against multiple declarations of things within the same translation unit, but you have not guarded against multiple definitions of things across your whole program.
The way to guard against that is to, well, not do that. There should be no non-inline, non-template definitions of anything in your header if you want to include it into multiple source files.
Here, you declare a type CoordinatesType, and create an object of that type:
struct CoordinatesType { double x, y; } coordinates;
Don't do that! Create an instance of CoordinatesType only in a source file, not in a header. Otherwise, every source file that includes this header (directly or indirectly) will get its own coordinates object and your linker complains about the name collision.
The code should be:
struct CoordinatesType { double x, y; };
Then, either coordinates in the one source file in which you wish to use the object… or extern on its declaration in a header. There are better approaches but I shan't enumerate them all here since this topic has been covered to death on SO already. Furthermore, your C++ book will have an explanation.
But the long and short of it is that this has nothing to do with header guards.

Answer 2

As already pointed out, your problem has nothing to do with include-guards.
Your problem is in this line:
struct CoordinatesType { double x, y; } coordinates;
While the declaration of the struct
struct CoordinatesType { double x, y; };
may appear in multiple .cpp files, as long as it is identical (it is, since you include the same header-file), you also define a variable
CoordinatesType coordinates;
in the same line. Since a definition is only allowed once, your linker complains.
If you really need a global variable of that type (check if you really need one, as it might not be necessary), change your header to a declaration:
struct CoordinatesType { double x, y; };
extern CoordinatesType coordinates;
and use a definition
CoordinatesType coordinates;
in exactly one cpp-file.
Note that the error didn't happen when only including Line and Coordinates, because your include-guards work and you only include the headers in a single .cpp file

Answer 3

Coordinates.h doesn't have any include guard.
When you include it multiple times, it defines Coordinates only once, but the rest of the file is included multiple times.
Put the #endif in the right place.
PS. The identifiers used in the include guards should be something that is never, ever used elsewhere by coincidence. I wouldn't be surprised if someone tried to use a variable named Line or Triangle. And since you #define those, that will lead to very unexpected errors. Use something like #define LineHeader_Included__ .