a.cpp

#include "foo.h"

int get_foo_a() { return foo; }

a.h

#ifndef A_H_
#define A_H_

int get_foo_a();

#endif  // A_H_

b.cpp

#include "foo.h"

int get_foo_b() { return foo; }

b.h

#ifndef B_H_
#define B_H_

int get_foo_b();

#endif  // B_H_

foo.cpp

#include "foo.h"

#ifdef FOO_VALUE_IN_IMPL
#ifdef FOO_CONST
const
#endif
int foo = 100;
#endif

foo.h

#ifndef FOO_H_
#define FOO_H_

#ifdef FOO_EXTERN
extern
#endif
#ifdef FOO_CONST
const
#endif
int foo
#ifdef FOO_VALUE_IN_HEADER
= 100
#endif
;

#endif  // FOO_H_

main.cpp

#include <iostream>

#include "a.h"
#include "b.h"

int main(int argc, char** argv) {
  std::cout << "a: " << get_foo_a() << ", B: " << get_foo_b() << std::endl;
  return 0;
}

test.log

$ bash test.sh
======= WITH VALUE IN HEADER =======
duplicate symbol _foo in:
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/a-dfaee4.o
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/foo-6afa08.o
duplicate symbol _foo in:
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/a-dfaee4.o
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/b-210a48.o
ld: 2 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
======= CONST WITH VALUE IN HEADER =======
Result: a: 100, B: 100
======= WITHOUT VALUE IN HEADER =======
duplicate symbol _foo in:
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/a-15bab8.o
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/b-b7a30b.o
duplicate symbol _foo in:
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/a-15bab8.o
    /var/folders/hl/l1bbbjnn5rndfvt9v5t73br82fc5dd/T/foo-8b4caf.o
ld: 2 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
======= CONST WITHOUT VALUE IN HEADER =======
In file included from a.cpp:1:
./foo.h:10:5: error: default initialization of an object of const type 'const int'
int foo
    ^
        = 0
1 error generated.
In file included from b.cpp:1:
./foo.h:10:5: error: default initialization of an object of const type 'const int'
int foo
    ^
        = 0
1 error generated.
In file included from foo.cpp:1:
./foo.h:10:5: error: default initialization of an object of const type 'const int'
int foo
    ^
        = 0
1 error generated.
======= EXTERN WITHOUT VALUE IN HEADER AND VALUE IN IMPL =======
Result: a: 100, B: 100
======= EXTERN CONST WITHOUT VALUE IN HEADER AND VALUE IN IMPL =======
Result: a: 100, B: 100

test.sh

#!/bin/bash
set -eu

: ${BIN:=./a.out}

rm -f "$BIN"

echo '======= WITH VALUE IN HEADER ======='
if clang++ -std=c++11 -stdlib=libc++ a.cpp b.cpp foo.cpp main.cpp -DFOO_VALUE_IN_HEADER; then
  echo -n 'Result: '
  "$BIN"
  rm -f "$BIN"
fi

echo '======= CONST WITH VALUE IN HEADER ======='
if clang++ -std=c++11 -stdlib=libc++ a.cpp b.cpp foo.cpp main.cpp -DFOO_CONST -DFOO_VALUE_IN_HEADER; then
  echo -n 'Result: '
  "$BIN"
  rm -f "$BIN"
fi

echo '======= WITHOUT VALUE IN HEADER ======='
if clang++ -std=c++11 -stdlib=libc++ a.cpp b.cpp foo.cpp main.cpp; then
  echo -n 'Result: '
  "$BIN"
  rm -f "$BIN"
fi

echo '======= CONST WITHOUT VALUE IN HEADER ======='
if clang++ -std=c++11 -stdlib=libc++ a.cpp b.cpp foo.cpp main.cpp -DFOO_CONST; then
  echo -n 'Result: '
  "$BIN"
  rm -f "$BIN"
fi

echo '======= EXTERN WITHOUT VALUE IN HEADER AND VALUE IN IMPL ======='
if clang++ -std=c++11 -stdlib=libc++ a.cpp b.cpp foo.cpp main.cpp -DFOO_EXTERN -DFOO_VALUE_IN_IMPL; then
  echo -n 'Result: '
  "$BIN"
  rm -f "$BIN"
fi

echo '======= EXTERN CONST WITHOUT VALUE IN HEADER AND VALUE IN IMPL ======='
if clang++ -std=c++11 -stdlib=libc++ a.cpp b.cpp foo.cpp main.cpp -DFOO_EXTERN -DFOO_VALUE_IN_IMPL -DFOO_CONST; then
  echo -n 'Result: '
  "$BIN"
  rm -f "$BIN"
fi