Linking Error due to symbol(s) not found in Bjarne Stroustrup "Programming and Practices using c++"

Question

I'm new to c++ (and compiled languages in general) and am doing the drill at the end of chapter 8 in Bjarne Stroustrup "Programming and Practices using c++" but I'm getting the following error when I try to compile the code

➜  Desktop g++ -std=c++11 *.cpp -o use
Undefined symbols for architecture x86_64:
  "_foo", referenced from:
      print_foo() in my-4f7853.o
      _main in use-46cb26.o
     (maybe you meant: __Z9print_foov)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I've also tried using g++ -c my.cpp use.cpp followed by g++ -o use.exe my.o use.o but this gave the same error. The other approach I tried was g++ -c use.cpp -o use.exe, however use.exe produced no output when it ran. The source code files are

my.h

extern int foo;
void print_foo();
void print_int(int);

my.cpp

#include "my.h"
#include <iostream>

void print_foo() {
  std::cout << foo << '\n';
}

void print_int(int num) {
  std::cout << num << '\n';
}

use.cpp

#include "my.h"
#include <iostream>

int main() {

  std::cout<<"DSGFSGFSG"<< '\n';
  foo = 7;
  print_foo();

  int i = 99;
  print_int(i);

}

I've looked at other questions that are similar (if not seemingly the same is in Link-time errors in VS 2013 while compiling the C++ program - B. Stroustrup's PPP using C++: Ch. 8 - Q1 Drill?) but the solutions haven't worked for me. Is the problem to do with my compilation using g++ or have I made a more fundamental error?

Answer 1

The global variable foo is only declared in your header file.
extern int foo;

You also need to define it in my.cpp
int foo;

The declaration is a promise: "it exists somewhere".
The definition actually reserves some storage for this variable.

So your linker complains because some code relying on this promise needs to access this missing storage.