CODEWITHSUNDEEP
c++
trrrrrrm
trrrrrrm

Reputation: 11812

How to declare a global variable that could be used in the entire program

I have a variable that I would like to use in all my classes without needing to pass it to the class constructor every time I would like to use it. How would I accomplish this in C++?

Thanks.

Upvotes: 26

Views: 105284

Answers (9)

Darshan b
Darshan b

Reputation: 157

The below solution should be simple enough as per the title "How to declare a global variable that could be used in the entire program " . If you want to use it in a different file then make use of extern keyword.

Please let me know if there is any issue with the solution

#include<iostream>

using namespace std;

int global = 5;

class base {
    public:
        int a;
        float b;
        base (int x) {
            a = global;
            cout << "base class value =" << a << endl;
        }
};

class derived : public base {
    public:
        int c;
        float d;
        derived (float x, int y) : base (y)
        {
            d = global;
            cout << "derived class value =" << d << endl;
        }
};


int main ()
{
    derived d(0,0);
    cout << "main finished" << endl;
    return 0;
}

Upvotes: 0

Emile Cormier
Emile Cormier

Reputation: 29209

If you're not going to use the Singleton pattern as Lyndsey suggests, then at least use a global function (inside a namespace) to access the variable. This will give you more flexibily in how you manage that global entity.

// mymodule.h
namespace mynamespace // prevents polluting the global namespace
{
   extern int getGlobalVariable();
}

// mymodule.cpp
namespace mynamespace
{
   int myGlobalVariable = 42;

   int getGlobalVariable()
   {
      return myGlobalVariable;
   }
}

Upvotes: 5

Lyndsey Ferguson
Lyndsey Ferguson

Reputation: 5384

While I would like to avoid global variables like the plague as our software cannot be multithreaded effectively due to the high reliance on global variables, I do have some suggestions:

Use a Singleton. It will allow you to keep the code and access clean. Part of the problem with a global variable is you don't know what code has modified it. You could set the value of global somewhere in your function relying on the hope that no one else will change it, but function your code calls, fooA, changes it and now your code is a) broken, and b) hard to debug.

If you have to use a global variable without touching the singleton pattern, look at fupsduck's response.

Upvotes: 4

justin
justin

Reputation: 104698

// L.hpp
struct L { static int a; };

// L.cpp
int L::a(0);

Upvotes: 0

rtpg
rtpg

Reputation: 2439

if you want to declare it in different header files/cpp files, just declare it extern outside of other files

//file1.c
int x=1;
int f(){}
//file2.c
extern int x;

Upvotes: -1

Paul Nathan
Paul Nathan

Reputation: 40309

keyword extern

//file1.cpp

int x = 0;

//file1 continues and ends.


//file2.cpp

extern int x; //this gets tied into file1.cpp's x at link time.

//file2.cpp goes on and ends

Upvotes: 1

fupsduck
fupsduck

Reputation: 3189

global.h
extern int myVar;

global.cpp
#include "global.h"
int myVar = 0;  // initialize

class1.cpp
#include "global.h"
...

class2.cpp
#include "global.h"
...

class3.cpp
#include "global.h"
...

MyVar will be known and usable in every module as a global variable. You do not have to have global.cpp. You could initialize myVar in any of the class .cpp's but I think this is cleaner for larger programs.

Upvotes: 33

Drew Dormann
Drew Dormann

Reputation: 63775

Declare the variable as extern in a common header.

Define it in any source file.

Upvotes: 0

Alex Brown
Alex Brown

Reputation: 42872

Just declare it outside the class:

Header file:

extern int x;

class A {
  int z;
  public:
  A() : z(x++) {}
};

One source file:

int x = 0;

Upvotes: 3

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