Re: extern variables
On May 1, 12:23 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.
There is no "filescope" in C++. And even in C, filescope refers
to the result after preprocessing---translation unit scope would
be a better name.
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.
What I'd be grateful for is a very simple example of a
necessary use of extern variables. Googling gave me lots of
definitions but no examples.
Extern is used when the same entity (variable or function) must
be used in more than one translation unit. Also, a variable
declared extern, but with no initializer, is only a declaration,
not a definition. (You need a declaration everywhere you use
the variable, but there should only be one definition in the
entire program.) Thus, if you want all of your translation
units to access a single "bool shutdownRequested" (to use
something that might reasonably be a "global" variable), you'd
declare:
extern bool shutdownRequested ;
in a header, including it where ever needed, and in a single
source file (which also includes this header):
bool shutdownRequested = false ;
or
extern bool shutdownRequested = false ;
The difference between const and non-const is when neither
extern nor static are specified:
int const c = 42 ;
is the same as:
static int const c = 42 ;
whereas:
int c = 42 ;
is the same as:
extern int c = 42 ;
and
int const c ;
is illegal, whereas:
int c ;
defines an uninitialized variable.
When I tried to create my own examples, I always found that
extern was unnecessary, and that the program ran fine without
this concept.
Try taking the address of the variable in several different
translation units. For const variables, if the extern is
absent, you have a different variable in each translation unit.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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