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Here is the code:
Code://file main.cc
//NOTE: main.cc does NOT include any file
extern int a1;
extern int a2;
int main(){
cout<<a1<<a2<<endl;
}
Code://file test.cpp
int a1=5;
It seems that a1 can be externed to main.cc , but a2 can not. Why is that?Code://file test.h
int a2=6;
Because header files are not compiled and do not generate any code. There is no definition of a2 anywhere in your program. (IOW: Just because you have a file called test.h, that doesn't mean that it magically appears when you use test.cpp.)
Caution: do not define variables in header files. It will cause linking errors or other weird behavior.
Define all global variables in source files.
Because a2, which is "declared" in test.h, is never "discovered" by the compiler. The compiler doesn't know that test.h exists; it only compiles source files. main.cc is referring to an "extern int a2", which never exists. The linker is complaining because you're referring to a variable that, as far as it knows, does not exist.
Note that declaring variables (without extern) in header files is a bad idea for the same reason. If you did this, and included the header file twice, the variable would be defined twice. The linker would complain about a multiple declaration, instead of an unresolved one. :)
"extern" means you assert that the variable is declared elsewhere. If it isn't, it's not going to work.
[edit] Beaten. But at least I made a more elaborate response. :) [/edit]
Thanks. But I don't agree with you about "Don't declare any variable in header files"
You can include the header file, so everything is OK.
In the above example, just let main.cc include "test.h"
Quote:
Originally Posted by dwks
That will just lead to a mess. A really, really big mess.
[edit] If you did as you described, you will in effect have created a header file that can only be included in one place. That's not very useful, and it's counter-intuitive. [/edit]
Here's how you can do it. Put the extern declaration in the header file. Put a non-extern declaration in only one source file. Include the header file everywhere you need to use the variable.
This works because this is valid code:
It's not a re-declaration or anything. In fact, this is valid code too:Code:extern int x;
int x;
Just for global variables, of course. :)Code:int x, x, x, x, x;
example.h:
int x;
a.cpp:
#include "example.h"
b.cpp:
#include "example.h"
Compile that and tell me if it works.
I still dont understand why "you will in effect have created a header file that can only be included in one place" ?
Why the code won't work :
Code://main.cc
#include "test.h"
...
Code://test.cc
#include "test.h"
...
Code://test.h
int a;
Quote:
Originally Posted by dwks
Hey, that's the question I was just about to ask. Why is that?
Quote:
Originally Posted by Elysia
Because the global variable "x" will end up in TWO source files, and you cannot have two global variables with the same name (unless they are only visible in the source file it's defined).
So in effect, the linker will find two global variables named "x" and give a linking error.
That's why you shouldn't define variables in headers.
dwks hints at because you ARE doing this, you can only include the header from one source file. Try to include from more than one file and you will get a linking error.
Because you have two different int x's now, one in a.cpp and one in b.cpp.
Basically, you can only have one definition of a variable (like "int x"). Since header files are by their very nature copied into each source file that includes the header file, putting variable definitions in header files won't work if the header file is included multiple times -- because you will have created multiple variables with the same name.
"extern" says that you're not creating a variable, you're referencing a variable by this name which is created somewhere else.
(I realize I'm repeating what everyone else has just said, but the OP is still online and has been quiet for over 15 minutes, which probably means [s]he's confused. No offence. :) )
What if I want to define constants for many cpp files to use, can I do this:
Code://common.h
static const int VAR = 100;
static const int LENGTH = 30;
Sure. This is sort of going back to your other thread again . . . . A global variable that is declared as "static" is limited in scope to the file in which it is declared. In other words, a static global variable is only visible inside one file. Consequently, each file can have its own set of static global variables, and even call them the same thing as static variables in other files, with no linker errors.
So having a static variable declaration in a header file is just fine. Every source file that includes this header file will get its own copy of the static variable.
Note that this isn't really useful unless the variables are constant.
In C++, you can just define them as constants:
const int VAR = 100;
const int LENGTH = 30;
So long as you do not try to take the address to any of these constants, it will compile.
The reason for this is that the compiler is smart enough to just replace the variables with its value instead or writing code for accessing the variable.
So as long as you don't force its hand - ie force the compiler to create those variables by taking the address of them or something else, it should compile.
Otherwise you would need to use static or define them in one source file and use extern.
Yes, it seems rather strange to me. Add "const", and a variable is implicitly static, in C++. Of course it makes sense, because then the compiler can more easily do optimisations on such a variable, but I'm just used to C, I guess. :)
(In C, the process that Elysia described does not work. You have to put "static" in.)
Thanks all~
BTW, what is the goodness to use static const against const ?
I have seen my friends define static const in header files and included by cpp files (just like my example above). Why they don't just const? Any advantages?
Perhaps they like C, like me? :)
That's the only thing that I can think of. By using "static", you make the code compatible with C. (Assuming you don't use classes and other C++ stuff . . . .)
None whatsoever.Quote:
Originally Posted by meili100
You really should give up C ;)Quote:
Originally Posted by dwks
It's... deprecated today ;)
There are currently 14 pages in the C++ programming forum here. The C one has 16. (I think this is the last 2 weeks or something, but it doesn't matter.)
Yes, I should. We should probably all learn Scheme and write everything in it. :) They doesn't mean we will . . . .
So let me summarize:
1. Never define a variable in a header, unless you want a global constant to be used by cpp files that include this header file.
2. If you do define a variable in a header, this makes this header only be included by one cpp file, or a linker error
3. If you do define a variable in a header but don't include the header, you can not use extern to refer to the variable. (You can, if it's in a separated cpp file, not a header file)
1 and 2 are correct.
That might be more clearly and generally stated as:Quote:
3. If you do define a variable in a header but don't include the header, you can not use extern to refer to the variable. (You can, if it's in a separated cpp file, not a header file)
You can only define a global variable once; all other references to the variable must be with "extern".
It's basically the same thing. as #2. Two sides of the same coin :)
Any why would you prefer to do this over putting said variable in the source file that includes the header file (and if necessary making a extern declaration in the header)?Quote:
Originally Posted by meili100
As above, why not define the variable in ONE source file (.c or .cpp file), then put an extern declaration in a suitable header file?Quote:
3. If you do define a variable in a header but don't include the header, you can not use extern to refer to the variable. (You can, if it's in a separated cpp file, not a header file)
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Mats