Question

In my code, due to efficiency consideration, I place a long function in it’s own file (let’s name it a.cpp). I have also created a second file named b.cpp which holds another function which uses the same variables names.

I have tried to create a header file for those variables but it didn’t work. Is there a way to do that (apart from placing the functions in the same file)?

A simple example:

a.cpp

double s;

void a(){
  s = 1.0;
  printf("%f\n",s);
}

b.cpp

double s;

void b(){
  s = 2.0;
  printf("%f\n",s);
}

Note
Each of those file is, in effect a c but the whole program is c++.

Answer 1

Write extern double s; in both (or in a header). This is a declaration without being a definition.

Then write double s; in just one .cpp file — this is where the double object will physically “live”.

More here.