defining and using a global class

how do I create and use a global class? I have only created global variables, never a class; I tried to use the same formate but came across errors.

what I tried to do was declare the class at the top of my driver file:

Code:

 (driver file)
#include <normalStuff>

class MyGlobalClass gClass; // IS THIS HOW I DECLARE A GLOBAL CLASS 

class DriverClass{
private:
     ... // normal stuff
public:
     void start(void);
     ...// normal stuff
};

int main(void){
     DriverClass dc;
     dc.start();         // this is where I will put values in the global class
     ...blah blah blah
  }

Code:

(some other file who wants to access the global class)
#include <normalStuff>

extern class MyGlobalClass gClass; // IS THIS HOW I GET ACCESS TO THE GLOBAL CLASS? 

void class2::function(){
      double temp;
      temp = gClass.data;   // THIS IS WHAT GIVES ME AN ERROR 
  ...
}

What is "data"??? Is it a function? An attribute?

sorry I forgot to add that

data would just be a typical element of the data class (such as "double");

Code:

class MyGlobalClass{
private:
      ;
public:
     ... //typical functions

      double data;
};

i realize that I should make "data" a private member and just add get/set functions,but for this example I will just forgo that

Remove "class" from the following:

Code:

class MyGlobalClass gClass;

and

Code:

extern class MyGlobalClass gClass;

And also make sure you include the header file that defines MyGlobalClass in all files that use it. Otherwise, you're just making a forward declaration of an incomplete type.

Do not forget to put the extern keyword in the .h file, and the real definition at only one .cpp. Otherwise, you will get linker errors: multiple definition.