CODEWITHSUNDEEP
c++variablesglobal-variables
Shahriyar
Shahriyar

Reputation: 1583

C++ Global variable declaration

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 :/)

Upvotes: 67

Views: 119893

Answers (5)

masoud
masoud

Reputation: 56479

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;

Upvotes: 99

Wolf
Wolf

Reputation: 10238

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;

Upvotes: 22

bcd
bcd

Reputation: 450

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.

Upvotes: 35

Jayesh Vaghasiya
Jayesh Vaghasiya

Reputation: 216

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.

Upvotes: -3

Fred Foo
Fred Foo

Reputation: 363487

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;

Upvotes: 11

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