How to make all-files-global variable?

[hakermania]

Hello, I have a project that has a lot of dialogs.
In the constructor of the MainWindow I check a file in order to set the language. If, e.g. the language is Spanish the program turns to Spanish, if it is English it turns into English etc. The thing is that I have to do this in every single constructor of every dialog in order to check the language and do the appropriate changes in every dialog.
Is it possible to use a all-files-global variable ? I don't want to perform the same check several times.
(e.g.

//this will be at the constructor of the MainWindow (mainwindow.cpp)
if(the.file.says.the.program.to.be.english) english=1 //from here the other constructors of the other dialogs will check if english==1 and if so they'll do the changes

//this will be at the constructor of the MainWindow (mainwindow.cpp)
if(the.file.says.the.program.to.be.english) english=1 //from here the other constructors of the other dialogs will check if english==1 and if so they'll do the changes

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)
Thx in advance

[Lykurg]

If it is only for translation you should have a look at Qt internationalization functions. Otherwise have a look at [WIKI]Singleton Pattern[/WIKI].

EDIT: Or use global functions/variables... But I am fine with the singleton pattern

[hakermania]

Ok, I simply want a fully global variale. I don't want to give singleton a chance right now, so how do I set a "fully" global value?

[Lykurg]

Well there are different approaches one is described here: http://www.learncpp.com/cpp-tutorial...bal-variables/. Or you can use global functions in a separate file with static variables and then do something like

int languageCode()
{
  static int code = -1;
  if (-1 == code)
  {
    // calculate code
  }
  return code;
}

int languageCode()
{
  static int code = -1;
  if (-1 == code)
  {
    // calculate code
  }
  return code;
}

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so the calculation will also only be done once.

[wysota]

There is no concept of "fully global" variable. The variable is either global or not.

http://www.cplusplus.com/doc/tutorial/variables/

[hakermania]

Ok, I just thought than in 2010 somebody would have though about variables than can be used everywhere and can be created in a simple way....
Anyway, I'll have a try at the singleton. Thanx both

[wysota]

Ok, I just thought than in 2010 somebody would have though about variables than can be used everywhere and can be created in a simple way....

Hmm?? Did you follow the link I gave you?

[hakermania]

Hmm?? Did you follow the link I gave you?

Ofcourse, but I know how to create a global variable! The problem is that I want to has access to this variable from all the .cpp files I use (as I said I have a lot of dialogs, every of which has its one .cpp file). That's why I named it "fully global" and not just global.

[wysota]

Ofcourse, but I know how to create a global variable! The problem is that I want to has access to this variable from all the .cpp files I use (as I said I have a lot of dialogs, every of which has its one .cpp file). That's why I named it "fully global" and not just global.

What's the difference between "global" and "fully global"? Do you know the "extern" keyword from C?

[Lykurg]

Do you know the "extern" keyword from C?

...and that is perfectly described at the above mentioned site: http://www.learncpp.com/cpp-tutorial...bal-variables/.

[squidge]

The problem is that I want to has access to this variable from all the .cpp files I use (as I said I have a lot of dialogs, every of which has its one .cpp file). That's why I named it "fully global" and not just global.

There's no such thing as a "fully global" variable. It's either global (Accessible by all files) or not. Maybe you are confusing global (accessible to all files in a project) and static (file-local) variables.

[Timoteo]

Do you know the "extern" keyword from C?

Do you know the "extern" keyword from asm? Sorry, but I couldn't resist. Know your roots, people!

[squidge]

No, as "asm" is meaningless without a processor architecture and manufacturer.

For example, some packages used XDEF to define a symbol which could be exported, and XREF to import those same symbols.

[Timoteo]

MASM/Wintel/ALL (Intel and AMD share mnemonics).

[wysota]

Do you know the "extern" keyword from asm? Sorry, but I couldn't resist. Know your roots, people!

My roots is Atari BASIC, there is no "extern" keyword there

[Timoteo]

Hah, my BASIC was the Commodore flavor. How old are you?

Atari had a cartridge based assembler didn't it? Crazy stuff.

[squidge]

Ok, I just thought than in 2010 somebody would have though about variables than can be used everywhere and can be created in a simple way....

Think about the future. Sure, you only want one variable now, but in a week, month or so you may two variables. Do you change your strategy (and the code already written for the first variable) or do something that is scalable? Thats what people are trying to teach you.

If you are not interested in that, fine, wysota gave you a perfectly usable link for creating a simple global variable.

Just note that simple global variables are fine for "bedroom coding", but are typically rejected for "workplace coding" and most multi-author and large-scale open-source projects.

[SixDegrees]

Singletons are vastly overrated and overused; in reality, they are nothing more than global variables in disguise, and they carry all of the shortcomings of globals along with them while hiding behind an OO veneer. And they bring their own problems to the table, as well.

[wysota]

Singletons are vastly overrated and overused; in reality, they are nothing more than global variables in disguise, and they carry all of the shortcomings of globals along with them while hiding behind an OO veneer. And they bring their own problems to the table, as well.

I would disagree. With a singleton you have much greater control over the object than with a regular global variable. A global variable is created before the main() function is executed and is destroyed after main() returns which can cause many problems (try declaring a global pixmap in Qt). With singleton this is not the case. Furthermore it lets you safely use C++ paradigms such as inheritance. The article you mention is also irrelevant here - it describes use of the singleton pattern against passing data as arguments, not against using global variables. All solutions have their pros and cons, you can't say "singletons are bad" just because they don't solve a particular problem.

[nroberts]

Ok, I just thought than in 2010 somebody would have though about variables than can be used everywhere and can be created in a simple way....
Anyway, I'll have a try at the singleton. Thanx both

This being 2010, many decades of studying design techniques and measuring the success of the projects that use them...people have pretty much universally decided that global variables are to be avoided at nearly all cost.

Actually, people knew this a long time ago. Most people discover it on their own if they don't learn from those ahead of them in the field.

You might have a case for using one. I'd probably look for a different way but I'm not working on your project so...wtf do I know. At any rate, there's no reason to go through the incredibly expensive effort of altering a language for something like this. Declaring and defining global variables is already fairly easy, though it could be admitted that someone brand new could find it unintuitive.

I followed the link you were given, didn't see what you need to know there. Here's how to do it:

In a header file:

extern type my_var;

extern type my_var;

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This is a declaration.

In ONE .cpp file:

type my_var = a_value;

type my_var = a_value;

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This is a definition AND an initialization. I suggest initializing your variable, but if you don't it will be "default initialized" (set to 0 in case of an int). You could also initialize through a function call, and in fact I use this a LOT for factory registration (a bigger topic). Just remember that anything you use to initialize your variable will happen before main is called and you have *absolutely no way of knowing in which order* except within specific cpp files.

Once you've done the above two things you can use that variable anywhere that includes the .h you declared it in.

Technically you don't have to have a header, but the reasons behind that would just confuse the hell out of you. Just do it this way...if you really must use a global variable.