'extern' construction and undefined reference

alan73

Hello to all! Respect to all :)

Please, help me with a question.
I'm trying to compile library from sources with 'extern' construction:

@A.cpp
int A = 5;
@

@B.cpp
extern int A;

void foo()
{ A=10; }@

But got an error: undefined reference to 'A' from 'foo()'. Both files are added to project. What it can be?
Thanks!

napajejenunedk0

Could you paste the contents of all files?

Jeroentjehome

Hi,
Adding extern int A in your B.cpp doesn't do anything for 'finding' A by the linker. It only tells the compiler that somewhere a variable exists named A. It is common practice to add the external definition to the header file corresponding to the source file where it is declared.
So:
@
//A.cpp:
int A = 5;
//A.hpp:
extern int A; // Make this variable known to the rest who includes this hpp file
// B.cpp:
#include "A.hpp"
void foo()
{
A = 10;
}
@
But to use a variable like this is more C programming and not C++/Object orientated programming.
C++ options would be to make a public variable in a class / Maybe even a singleton class. But still it is possible.

shreesh

Hi all,

I have been trying to use this "extern" variable for my project, but could not figure out the exact way of doing it. Here is what I have done,

I have a "login" form where I want to assign QString value to a "global_variable". And then use this saved value in a second form "account".

Code I wrote is,

@//login.h
extern QString global_variable //throws error that, storage space assigned

//login.cpp, in a signal slot function
QString global_variable = ui->lineEdit->text();

//account.cpp
#include "login.h"

QString exe = staff_username; // written under function
//: account::account(QWidget *parent) :
//QMainWindow(parent),
//ui(new Ui::staff)

ui->label_name->setText(exe); @

Can anybody help me with this please?

shreesh

Hi all,

I have been trying to use this "extern" variable for my project, but could not figure out the exact way of doing it. Here is what I have done,

I have a "login" form where I want to assign QString value to a "global_variable". And then use this saved value in a second form "account".

Code I wrote is,

@//login.h
extern QString global_variable //throws error that, storage space assigned

//login.cpp, in a signal slot function
QString global_variable = ui->lineEdit->text();

//account.cpp
#include "login.h"

QString exe = staff_username; // written under function
//: account::account(QWidget *parent) :
//QMainWindow(parent),
//ui(new Ui::staff)

ui->label_name->setText(exe); @

Can anybody help me with this please?

andreyc

If this construction is inside a function then it is not global variable.
@
//login.cpp, in a signal slot function
QString global_variable = ui->lineEdit->text();
@

"Global variable":http://www.learncpp.com/cpp-tutorial/42-global-variables/ are declared outside of any functions.

andreyc

If this construction is inside a function then it is not global variable.
@
//login.cpp, in a signal slot function
QString global_variable = ui->lineEdit->text();
@

"Global variable":http://www.learncpp.com/cpp-tutorial/42-global-variables/ are declared outside of any functions.

shreesh

Oh, yes. You were right.
But in my case I need to assign a value in a signal slot function.
How would you suggest me to go on this?

shreesh

Oh, yes. You were right.
But in my case I need to assign a value in a signal slot function.
How would you suggest me to go on this?

andreyc

You need to declare QString global_variable outside of slot function in login.cpp and assign it inside the function.
login.cpp
@
...
QString global_variable;

void slot_function()
{
global_variable = "whatever";
}
@

PS: Using global variables is not good practice.
I would suggest to rethink a design and use class member variables or pass it through signal/slot.

andreyc

You need to declare QString global_variable outside of slot function in login.cpp and assign it inside the function.
login.cpp
@
...
QString global_variable;

void slot_function()
{
global_variable = "whatever";
}
@

PS: Using global variables is not good practice.
I would suggest to rethink a design and use class member variables or pass it through signal/slot.

shreesh

Thanks a lot!!
I was stuck in this since hours.

It is working now..God bless!!

shreesh

Thanks a lot!!
I was stuck in this since hours.

It is working now..God bless!!