C++ reference variable across files

Question

I have a project where I need to reference a variable declared in one CPP file in another, is that possible?

If so, how?

Answer 1

It is possible, if you declare it as global (top-level, above any function definition) and use "extern ;" in other files to make it known to the compiler.

// Main.cpp
#include <...>

int myNum;

int main(int argc, char** argv)
{
   // MAGIC BE HERE
   return 0;
}

and

// Second.cpp
#include <...>

extern int myNum;

int f()
{
   return myNum * 2;
}

extern prevents the compiler from allocating memory again when a variable was allocated in another file.

Answer 2

Create a .h file declaring the variable you need as extern (something like extern int X;), then include it in any file that need that variable. In one of the .cpp files you're linking, declare it without the extern.