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

problem:

i cannot code provided author link via clang++ in chapter 8 of programming principles , practice 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 << << 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 few different ways of compiling this:

one - compile driver app contains 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 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 architecture x86_64 clang: error: linker command failed exit code 1 (use -v see invocation) 

two - compile my.cpp file , not link. although successful in compiling i'm not sure how link use.cpp 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 .cpp files.

 ~/scratch/cpp/chp8 (09/10/2017-14:15:32[edt]) clang++ -std=c++14 -stdlib=libc++ my.cpp use.cpp undefined symbols architecture x86_64:   "_foo", referenced from:       print_foo() in my-2243d1.o       _main in use-796f91.o      (maybe meant: __z9print_foov) ld: symbol(s) not found architecture x86_64 clang: error: linker command failed exit code 1 (use -v see invocation) 

question:

besides obvious question of doing wrong, i'm more interested in missing or miss understanding here?

note:

i'm not looking personal opinions or preferences on should or should not doing. purely education adventure i'm taking book , keep 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 

the error has nothing difference between compiling , linking. rather there error in code. reason error because declaring global variable foo never define it. need add line

int foo = 0; 

to 1 of .cpp files. makes sense in my.cpp.

note using global variables considered coding horror. should not in real code.

regarding compiling , linking:

creating executable c++ takes several steps. here talk compiling versus linking , how each of these steps manually. there other steps, not them here.

first of all, can compile , link of source files @ once:

$ clang++ -std=c++14 -stdlib=libc++ my.cpp use.cpp 

i'm confused why not working in "attempt three". more shortly.

to compile , link source code manually, can 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 compile my.cpp my.o rather my.cpp.o. common convention this.

also, once comfortable these concepts, suggest use build tool such make or xcode project manages these steps automatically.