Problem with global variables

Bare with me :/

I have two class definitions in file MyClass.h:

Code:

#define MYCLASS_H

class MyClass
{
    int i;

    public:
        MyClass(int n):i(n){}
        int GetI(void){return i;}
};

class MyClass2
{
    float x;

    public:
        MyClass2(float n):x(n){}
        float GetX(void){return x;}
        void DoStuffToX(void);
};

I then have a file named globals.h with a global object of type MyClass in a namespace:

Code:

#define GLOBALS_H

#ifndef MYCLASS_H
    #include "myClass.h"
#endif

namespace my_name
{
    MyClass myObject(234);
}

I then have a file called MyMain.cpp which includes both header files and refers to my global MyClass object:

Code:

#include <iostream>

#ifndef GLOBALS_H
    #include "globals.h"
#endif

#ifndef MYCLASS_H
    #include "myClass.h"
#endif



int main()
{
    cout << my_name::myObject.GetI();
    cin.get();

  return 0;
}

Now, at this point in the operation everything compiles and works fine; we have an output on the screen of "234". Hoorah. The error occurrs when I try adding another file wanting to include globals.h. This one is called MyClass2.cpp:

Code:

#ifndef MYCLASS_H
    #include "myClass.h"
#endif
#ifndef GLOBALS_H
    #include "globals.h"
#endif

void MyClass2::DoStuffToX()
{
    x *= static_cast<float>(my_name::myObject.GetI());
}

I try to compile this (all four files) and I get the following linker errors:

d:/dom/my documents/developing tools/temp programming/globalobjects/myclass2.o: In function `MyClass2:oStuffToX(void)':
//d/dom/my documents/developing tools/temp programming/globalobjects/myclass2.cpp:9: multiple definition of `my_name::myObject'
d:/dom/my documents/developing tools/temp programming/globalobjects/mymain.o(.bss+0x0)://D/DEV-C_~1/Include/G__~1/streambuf.h: first defined here

This is of course a simplified version of a much more complicated program, where I would really like to be able to have a global variable that can be included by more than one file. Any light on this would be much appreciated.

dt

Dont declare globals in a header file. Instead declare them in the .cpp file. Then if you want to use them in another .cpp file, you can redeclare them as 'extern'.

Ahhahhh, thankyou. So would this be right?

Code:

#file1
...
namespace myNameSpace
{
   MyClass myObject(34);
}

#file2
...
extern myNameSpace::myObject(34);

Thanks again

dt

Having problems referring to the global in it's namespace. If I use:

Code:

using namespace myNameSpace;

The compiler quite rightly doesn't recognise the namespace (as it's in the .cpp and not included in any headers). And then I am baffled as to how to refer to my global. Have tried randomly:

Code:

extern MyClass myNameSpace::MyObject;

...and a few other combinations, to no avail. Anyone know how it's done??!



Another very related question: I would like the global to be an array. At the moment, I have set it as a pointer; then, when I want to initialize it I do something like:

Code:

extern MyClass myNameSpace::*MyObject;  // but obvioously not that!!!

myNameSpace::MyObject = new MyClass[100];
// myNameSpace::MyObject[0] = blah etc....

Once this is done, will the global be initialzed globally, so that whenever I rfer to it in another file (using extern) it will still contain the values I gave it in intitializing it. (I assume so, but this extern thing is confusing me!)

I did something similar in a project long ago... try something like this in your header file:

Code:

namespace myNameSpace
{
    extern MyClass MyObject;
}

Originally Posted by hk_mp5kpdw

I did something similar in a project long ago... try something like this in your header file:

Code:

namespace myNameSpace
{
    extern MyClass MyObject;
}

That will work, but you still need to declare the variable in one of the .cpp files as well (declare it normally, not as 'extern').


To make a global array, you can do:

[CODE]
MyObject* mynamespace::MyObjectname;
// Dont forget to declare this as extern in your header file

MyObjectname = new MyObject[100];
[CODE]

Once this is done, will the global be initialzed globally, so that whenever I rfer to it in another file (using extern) it will still contain the values I gave it in intitializing it. (I assume so, but this extern thing is confusing me!)

Yes, the variable will be initialized globally.
The extern keyword just tells the compiler that this variable exists elsewhere globally. Therefore it tells the compiler not to include the variable in the object file, but it still lets the object file use the variable.

Thanks guys