Variable question

[MarkoSan]

Hi to all, long time no see!!!

I have a question now:

In my project I will need variable that will hold the database connection. The variable will be initialized at application startup and all for objects in the application it is a must to see it. How and where do I put such a variable, that must be seen by all objects in an application? I want to follow c++ rules and I heard global variables are BAD idea. I also want to know why are global variables such a bad idea?

[vermarajeev]

Hi to all, long time no see!!!

I have a question now:

In my project I will need variable that will hold the database connection. The variable will be initialized at application startup and all for objects in the application it is a must to see it. How and where do I put such a variable, that must be seen by all objects in an application? I want to follow c++ rules and I heard global variables are BAD idea. I also want to know why are global variables such a bad idea?

You can make the variable static. Static variable applies to the class.

Global variables are dangerous coz a global variable can potentially be modified from anywhere, and any part of the program may depend on it.

[MarkoSan]

Yes, but I need a variable that will be seen to all objects in an application, not just one object/class.

[vermarajeev]

Yes, but I need a variable that will be seen to all objects in an application, not just one object/class.

Try something like this

//GlobalClass.h
 
// forward declaration
class GlobalClass;
// global GlobalClass object
// there must only be one of these per application (like qApp)
// defined and initialized in GlobalClass.cpp
extern GlobalClass *chemGlobals;
 
class GlobalClass : public QObject
{
   Q_OBJECT
 
   public:
         GlobalClass(int var = default){}
         virtual  ~GlobalClass(){}
 
   private:
         int m_val; 
};    
 
////GlobalClass.cpp
#include "GlobalClass.h"
 
// global ChemGlobals object
// there must only be one of these per application (like QApp)
ChemGlobals * chemGlobals = 0;

//GlobalClass.h

// forward declaration
class GlobalClass;
// global GlobalClass object
// there must only be one of these per application (like qApp)
// defined and initialized in GlobalClass.cpp
extern GlobalClass *chemGlobals;

class GlobalClass : public QObject
{
   Q_OBJECT
  
   public:
         GlobalClass(int var = default){}
         virtual  ~GlobalClass(){}

   private:
         int m_val; 
};    

////GlobalClass.cpp
#include "GlobalClass.h"

// global ChemGlobals object
// there must only be one of these per application (like QApp)
ChemGlobals * chemGlobals = 0;

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[sunil.thaha]

It would be better if you try not using global variable. The singleton pattern would be a better candidate here. The advantages are many ...