When to use extern in C

0 votes

I'm now reading "Think in C++," which recently introduced the extern declaration. 

As an example:

extern int x;
extern float y;

I believe I get the meaning (statement without explanation), but I'm curious when it will be beneficial.

Could someone provide me an example?

Jun 21, 2022 in C++ by Nicholas
• 7,760 points
2,305 views

1 answer to this question.

0 votes

This is useful when dealing with global variables. 

Global variables are declared in a header so that any source file that contains the header is aware of them, but you only need to "define" them once in one of your source files.

To explain, using extern int x; informs the compiler that an int object named x exists elsewhere. 

It is not the compiler's responsibility to know where it exists; it just needs to know the type and name so that it may utilise it. 

After compiling all of the source files, the linker will resolve all x references to the one definition found in one of the generated source files.

For it to operate, the x variable's declaration must have "external linkage," which simply means that it must be defined outside of a function (at what's known as "the file scope") and without the static keyword.

header:

#ifndef HEADER_H
#define HEADER_H

// any source file that includes this will be able to use "global_x"
extern int global_x;

void print_global_x();

#endif

source 1:

#include "header.h"

// since global_x still needs to be defined somewhere,
// we define it (for example) in this source file
int global_x;

int main()
{
    //set global_x here:
    global_x = 5;

    print_global_x();
}

source 2:

#include <iostream>
#include "header.h"

void print_global_x()
{
    //print global_x here:
    std::cout << global_x << std::endl;
}
answered Jun 21, 2022 by Damon
• 4,960 points

Related Questions In C++

0 votes
0 answers

When and why do I need to use cin.ignore() in C++?

In C++, I developed a simple application that requested the user to enter a number and then a string.  Surprisingly, when I ran the application, it never paused to ask for the string.  It simply ignored it.  After conducting some research on StackOverflow, I discovered that I needed to include the following line: cin.ignore(256, '\n'); before the line with the string input  That addressed the problem and allowed the software to run.  My issue is why C++ need the cin.ignore() line, and how can I forecast when I will need to use it. Here's the software I created: #include <iostream> #include <string> using namespace std; int main() { ...READ MORE

Jul 4, 2022 in C++ by Nicholas
• 7,760 points
739 views
0 votes
0 answers

What is the difference between cout, cerr, clog of iostream header in c++? When to use which one?

I looked up the differences between cout, ...READ MORE

Jul 27, 2022 in C++ by Nicholas
• 7,760 points
686 views
0 votes
1 answer

How to use new[ ] and delete[ ] operator in C++

int main(){ char *str; ...READ MORE

answered Jun 20, 2022 in C++ by Damon
• 4,960 points
548 views
0 votes
1 answer

How to use enums in C++

This will be sufficient to declare your ...READ MORE

answered Jun 20, 2022 in C++ by Damon
• 4,960 points
594 views
0 votes
1 answer

setuptools: build shared libary from C++ code, then build Cython wrapper linked to shared libary

There is a seemingly undocumented feature of setup that ...READ MORE

answered Sep 11, 2018 in Python by Priyaj
• 58,100 points
666 views
0 votes
1 answer

setuptools: build shared libary from C++ code, then build Cython wrapper linked to shared libary

There is a seemingly undocumented feature of setup that ...READ MORE

answered Sep 21, 2018 in Python by Priyaj
• 58,100 points
2,353 views
0 votes
1 answer

How to pass large records to map/reduce tasks?

Hadoop is not designed for records about ...READ MORE

answered Sep 25, 2018 in Big Data Hadoop by Frankie
• 9,830 points
1,422 views
0 votes
1 answer

Invalid method parameters for eth_sendTransaction

params needs to be an array, try {"jsonrpc":"2.0","method":"eth_se ...READ MORE

answered Sep 28, 2018 in Blockchain by digger
• 26,740 points
1,823 views
0 votes
1 answer

How to use std::sort to sort an array in C++

We receive std::begin and std::end in C++0x/11, which are overloaded for arrays: #include <algorithm> int main(){ int v[2000]; ...READ MORE

answered Jun 1, 2022 in C++ by Damon
• 4,960 points
1,110 views
0 votes
1 answer

What is the best way to use a HashMap in C++?

The ordered and unordered map containers (std::map and std::unordered map) are included in the standard library.  The items in an ordered map are sorted by key, and insert and access are in O (log n).  For ordered maps, the standard library often use red black trees.  However, this is only an implementation detail.  Insert and access are in O in an unordered map (1).  It is simply another term for a hashtable. An illustration using (ordered) std::map: #include <map> #include <iostream> #include <cassert> int main(int argc, char ...READ MORE

answered Jun 10, 2022 in C++ by Damon
• 4,960 points
786 views
webinar REGISTER FOR FREE WEBINAR X
REGISTER NOW
webinar_success Thank you for registering Join Edureka Meetup community for 100+ Free Webinars each month JOIN MEETUP GROUP