如何在不同文件中声明变量？

Question

I'm new to C++ and just trying to separate function into files. Here is an example from one of my assignment. When I compile it, the message shows:

'b' is not declared

When I add #include "q1b.cpp" to q1a.cpp , it fail. How can it run by only changing q1a.cpp ? Glad to know the concept.

//q1a.cpp
#include <iostream>
#include "q1b.h"
using namespace std;
int main() {
    calC();
    cout << b << endl;
    return 0;
}

//q1b.cpp
#include "q1a.h"
#include "q1b.h"
int b=1;
void calB(int i) {
    b = calA(i) + 1;
}
void cal() {
    calB(calA(b));
}

//q1a.h
int calA(int i) {
    return i*10;
}

//q1b.h
void calC();

Answer 1

Declare the variable with extern specifier.

//q1a.cpp
#include <iostream>
#include "q1b.h"
using namespace std;
extern int b; // add this line
int main() {
calC();
cout << b << endl;
return 0;
}

If the constraint "by only changing q1a.cpp" weren't there, you should add the declaration of variable to q1b.h because the variable is defined in q1b.cpp .