The use of extern

[BlueSquirrelJQX]

Please tell me what's wrong with this period of code:

// code that goes into file "my.h":
extern int foo;

// code that goes into file "source.cpp":
#include "my.h"
int main()
{
int foo = 7;
}

[Golden Lizard]

something.cpp

int a=500;

source.cpp

/*DON'T INCLUDE my.h*/
#include<iostream>

int main(int argc, char ** argv)
{
 extern int a;
 std::cout<<a; //will output 500

 return EXIT_SUCCESS;
}

Include directive does nothing smart, it just copies the contents of the header. Extern on the other hand tells the compiler to look for the variable in an external file

[dhayden]

GoldenLizard has it backwards. Header files should contain declarations, but not definitions. The definition of a variable is basically where you allocate space.

BlueSquirrelJQX, the problem with your code is that you have declared the variable but you haven't defined it. The int foo at line 7 is a local variable, not the extern one declared in my.h.

Exactly one compilation unit must contain the definition, so just add that to source.cpp, or better yet, add it to my.cpp:

#include "my.h"
int foo=7;

[Golden Lizard]

GoldenLizard has it backwards. Header files should contain declarations, but not definitions. The definition of a variable is basically where you allocate space.

That was only to exemplify it. I changed it to use a cpp file too, to be more correct

[BlueSquirrelJQX]

dhayden and Golden Lizard, thank you very much. I've modified my code and it works as desired now.