Programming Principles and Practice 2nd ed chapter 8 drill 1 - c++

Question

Problem:
I cannot get the code provided by the author to link via clang++ in chapter 8 of Programming Principles and Practice by Bjarne Stroustrup.
Code:
~/scratch/cpp/chp8 (09/10/2017-13:51:43[EDT]) cat my.cpp
#include <iostream>
#include "my.h"
void print_foo() { std::cout << foo << std::endl; }
void print(int i) { std::cout << i << std::endl; }
~/scratch/cpp/chp8 (09/10/2017-13:52:29[EDT]) cat my.h
#ifndef MY_H
#define MY_H
extern int foo;
void print_foo();
void print(int);
#endif
~/scratch/cpp/chp8 (09/10/2017-13:52:33[EDT]) cat use.cpp
#include "my.h"
int main() {
foo = 7;
print_foo();
print(99);
return 0;
}
Attempts:
I have tried a few different ways of compiling this:
One - Compile the driver app that contains the main function.
~/scratch/cpp/chp8 (09/10/2017-13:52:39[EDT]) clang++ -std=c++14 -stdlib=libc++ use.cpp -o use.cpp.o
Undefined symbols for architecture x86_64:
"print(int)", referenced from:
_main in use-2864c4.o
"print_foo()", referenced from:
_main in use-2864c4.o
"_foo", referenced from:
_main in use-2864c4.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Two - Specifically compile just the my.cpp file and not link. Although I was successful in compiling I'm not sure how to link use.cpp with this object file now.
~/scratch/cpp/chp8 (09/10/2017-14:09:40[EDT]) clang++ -std=c++14 -stdlib=libc++ -c my.cpp -o my.cpp.o
Three - Use all .cpp files.
~/scratch/cpp/chp8 (09/10/2017-14:15:32[EDT]) clang++ -std=c++14 -stdlib=libc++ my.cpp use.cpp
Undefined symbols for architecture x86_64:
"_foo", referenced from:
print_foo() in my-2243d1.o
_main in use-796f91.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)
Question:
Besides the obvious question of what am I doing wrong, I'm more interested in what am I missing or miss understanding here?
Note:
I'm not looking for personal opinions or preferences on what I should or should not be doing. This is a purely education adventure that I'm taking with this book and I would like to keep it as such.
Misc:
~/scratch/cpp/chp8 (09/10/2017-14:15:25[EDT]) uname -a
Darwin abes-MacBook-Pro.local 16.7.0 Darwin Kernel Version 16.7.0: Thu Jun 15 17:36:27 PDT 2017; root:xnu-3789.70.16~2/RELEASE_X86_64 x86_64
~/scratch/cpp/chp8 (09/10/2017-14:15:28[EDT]) clang++ -v
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Answer 1

The error you get has nothing to do with the difference between compiling and linking. Rather there is an error in your code. The reason for the error is because you are declaring the global variable foo but you never define it. You need to add the line
int foo = 0;
to one of your .cpp files. This probably makes the most sense in my.cpp.
Note that using global variables like this is considered a Coding Horror. You should not do this in real code.
Regarding compiling and linking:
Creating an executable from your C++ takes several steps. Here we will talk about compiling versus linking and how to do each of these steps manually. There are also other steps, but I will not get into them here.
First of all, you can compile and link all of your source files all at once:
$ clang++ -std=c++14 -stdlib=libc++ my.cpp use.cpp
I'm confused why this is not working in your "Attempt three". I will look into this more shortly.
To compile and link your source code manually, you can do the following:
$ clang++ -std=c++14 -stdlib=libc++ -c my.cpp -o my.o
$ clang++ -std=c++14 -stdlib=libc++ -c use.cpp -o use.o
$ clang++ -std=c++14 -stdlib=libc++ my.o use.o -o use
Note that I compile my.cpp to my.o rather than my.cpp.o. This is the common convention for this.
Also, once you are comfortable with these concepts, I strongly suggest that you use a build tool such as make or an XCode project which manages these steps for you automatically.