What is the difference between global and extern?

Answer 1

Answer

Global variables and extern variables are very similar, but with a major difference. Let's say you have a global variable in your header file, like this:
( Refer how u can add the variable declaration in the source code as extern, i think we cant add into header files as it said)
int x = 0;

Now, in each file that includes that header file, there will be a new variable called x. Modifying x in file1.cpp won't modify x in file2.cpp.

Now, for externs... Say you have something like this in your header:

extern int x;

Then, in file1.cpp (which includes your header):

int x = 0;

Then, in file2.cpp (also includes the header):

x = 46;

x in file1.cpp is also now equal to 46. You see, when you use an extern variable, you're telling the compiler that you want to use that variable across all of your source files. If you use a regular global, however, you create a new variable in each of your source files.

Anyway, that's just for variables, which I assume you're talking about - global and extern functions are a bit different, and I'm not entirely clear on them.