C++ Global variable declaration

Question

What I want to do is just define a variable in a header file and use it in two different cpp files without redefining that variable each time I include that header.

Here is how I tried:

Variables.h

#ifndef VARIABLES_H // header guards
#define VARIABLES_H

static bool bShouldRegister;

#endif

(I also tried extern but nothing changed)

And in a cpp file I give it a value ::bShouldRegister = true or bShouldRegister = true;

In my other cpp file I check it's value by creating a thread and checking its value in a loop (and yes my thread function works well)

 while (true)
 {
     if (::bShouldRegister) // Or if (bShouldRegister)
        {
            MessageBox(NULL,"Value Changed","Done",MB_OK|MB_ICONINFORMATION);
        }
  Sleep(100);
 }

And yes, that MessageBox never appears (bShouldRegister never gets set to true :/)

C++ FAQ 27.15: What's a good coding standard for using global variables? — Dmitry Ledentsov
– Dmitry Ledentsov, Commented Nov 12, 2013 at 14:31
@DmitryLedentsov the information you linked is currently dead. Is it the same as What's a good coding standard for using global variables?, C++ FAQ? — Wolf
– Wolf, Commented Sep 29, 2016 at 13:46

CPlus · Accepted Answer · 2025-01-13 03:20:04Z

101

You should use extern, otherwise you will have separated bShouldRegister variables in each translation unit with probably different values.

Put this in a header file (.h):

extern bool bShouldRegister;

Put this in one of the implementation files (.cpp):

bool bShouldRegister;

Another way which is simpler is to use inline keyword. Put your variable in a header file as below:

inline bool bShouldRegister;

edited Jan 13 at 3:20

CPlus

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answered Nov 12, 2013 at 12:52

masoud

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5 Comments

Shahriyar Over a year ago

As I said I did as you say, but nothing changed

masoud Over a year ago

So, your code doesn't show how you're using this global value. But the obvious thing is you should use extern.

Shahriyar Over a year ago

I should include my header file in both cpp files, right ? if yes I'm doing this already

masoud Over a year ago

You can include the header file everywhere you want. but you should not include the cpp file anywhere. Notice, you should just have one definition for bShouldRegister just in a single cpp file.

drescherjm Over a year ago

@Shahriyar Did you remove "static bool bShouldRegister;" from the header and change it to "extern bool bShouldRegister;"?

bcd · Accepted Answer · 2018-08-27 18:58:40Z

36

If you can use C++17, consider using an inline variable:

// in a header file
inline bool bShouldRegister = true;

See How do inline variables work? for more information.

answered Aug 27, 2018 at 18:58

bcd

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1 Comment

Konrad Over a year ago

this is really a life saver... really annoying to use extern in 2019

Wolf · Accepted Answer · 2020-05-03 17:19:59Z

22

A more C++-like way would be using a class member, syntactically indicated by the static keyword. Class member variables have implicit external linkage.

#ifndef VARIABLES_H
#define VARIABLES_H

class RegUtil {
public:

    static bool bShouldRegister;

};

#endif

in one of your cpp files (maybe variables.cpp), you have to define this class member:

#include "variables.h"

bool RegUtil::bShouldRegister;

edited May 3, 2020 at 17:19

answered Nov 12, 2013 at 14:10

Wolf

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2 Comments

463035818_is_not_an_ai Over a year ago

imho this is almost more java-like than c++like, i would rather put it inside a namespace and drop the class

Wolf Over a year ago

@user463035818 Both options make sense, but I think classes tend to provide narrower contexts than namespaces. I generally try to avoid exposing "naked" simple variables, especially bools. PS: yes you're right: I learned about static member variables from Java, but it's also a reasonable option in C++ (I never used Java for production code).

Fred Foo · Accepted Answer · 2013-11-12 12:52:11Z

11

You need to define the variable in one of the modules:

bool bShouldRegister;

Then declare it extern (not static!) in the header:

extern bool bShouldRegister;

answered Nov 12, 2013 at 12:52

Fred Foo

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Comments

Jayesh Vaghasiya · Accepted Answer · 2015-06-13 19:18:50Z

-3

Here you need to define bool bShouldRegister in one class. Your header file should like this.

 #ifndef VARIABLES_H // header guards
#define VARIABLES_H

class abc{

public:
      bool bShouldRegister;
      abc();
#endif

Now initialize bShouldRegister variable in cpp file in constructor of abc class and then use this variable in your second cpp file using object of class. You will get your message box.

answered Jun 13, 2015 at 19:18

Jayesh Vaghasiya

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2 Comments

Wolf Over a year ago

Please have another look on your "answer", it's syntactically wrong.

Wolf Over a year ago

initialize bShouldRegister variable in cpp file in constructor of abc class -- this is a bad advice for static members.

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