Question

I have the following source files:

//test1.cpp
#include <iostream>
using namespace std;

inline void foo()
{
  cout << "test1's foo" << endl;
}

void bar();

int main(int argc, char *argv[])
{
  foo();
  bar();
}

and

//test2.cpp
#include <iostream>

using namespace std;

inline void foo()
{
    cout << "test2's foo" << endl;
}

void bar()
{
    foo();
}

The output:

test1's foo
test1's foo

Huh??? Ok, so I should have declared the foos static… but shouldn’t this kind of thing generate a linker error, or at least a warning? And how does the compiler “see” the inline functions from across compilation units?

EDIT: This is using gcc 4.4.1.

Answer 1

You are running into the one-definition-rule. You are not seeing any error because:

[Some] violations, particularly those that span translation units, are not required to be diagnosed

What going on under the covers is that the compiler is not inlining those functions (many compilers will not inline a function unless the code is compiled with the optimizer). Since the function is inline and can appear in multiple translation units, the compiler will mark the function as link-once which tells the linker that it not treat multiple definitions as an error but just use one of them.

If you really want them to be different, you want a static function.