Re: extern variables
On May 1, 3:23 pm, Comp1...@yahoo.co.uk wrote:
I know this is a standard topic and I've done a fair amount of
googling but I'm still a bit confused by extern variables.
My main text is C++ Primer, 4th edition (Lippman, Lajoie and Moo).
For example, I ran the following program:
// Code below in a source file
#include "SomeHeader.h"
#include <iostream>
int main ()
{
int j = doropi;
std::cout << j;
}
//Code in SomeHeader.h
const int doropi = 9;
The result of the program was that 9 was output. However, what I
expected was a compile error based on the rule (or what I thought was
the rule) that const definitions have file scope and that the way to
use consts from other files was via extern.
My code had no extern and yet I was able to use the const anyway.
This made me question the whole purpose of externs. I didn't need the
extern to fetch a member from another file and yet I thought that was
exactly what extern is for.
Two points:
A) You are not actually "fetching" anything from "external file" here.
The "SomeHeader.h" has been "included" in the source file, which means
that the preprocessor will add the entire code from SomeHeader.h
inside your c++ source file, even before the "compiler proper" gets to
see the code. So what "compiler proper" sees is:
const int doropi = 9; //This is added by preprocessor by replacing
#include "SomeHeader.h"
//Preprocessor also replaces #include <iostream> by all its content
int main ()
{
int j = doropi;
std::cout << j;
}
So observe that this file has "doropi" defined as a global variable in
this file itself. There is no need of "extern" anywhere here.
More importantly the following point must be noted:
(B) The keyword "extern" is (typically but not necessarily) used for
declaring variables that are defined in (usually) different
"compilation unit". For example, you may have 1.cc and 2.cc files as:
//1.cc
int i = 10;
//2.cc
extern int i;
int main()
{
int j = i;
}
Now you compile 1.cc and 2.cc separately to create object code. On a
typical linux machine using g++ command line option, one would say:
sh$> g++ -c 1.cc 2.cc
This produces 1.o and 2.o. They can now be linked properly because "i"
used in 2.cc gets its definition from 1.cc
Try doing this:
sh$> g++ 2.cc
Observe what error you get. This error says that you have declared int
i as extern, but haven't given any definition.
Technically, the keyword "extern" indicates that the variable will
have external linkage.