Thread: Why I can not "extern" a variable from header file?

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.)

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 . . . .)

Originally Posted by meili100

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?

None whatsoever.

Originally Posted by dwks

Perhaps they like C, like me?

You really should give up C
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 . . . .

1 and 2 are correct.

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)

That might be more clearly and generally stated as:

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

Originally Posted by meili100

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

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)?

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)

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?

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Mats