'extern' construction and undefined reference
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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! -
Could you paste the contents of all files?
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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. -
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?
-
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?
-
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.
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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.
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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. -
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.
