One Definition Rule - Multiple definition of inline functions

Question

I was reading ODR and as the rule says "In the entire program, an object or non-inline function cannot have more than one definition" and I tried the following...

file1.cpp

#include <iostream>
using namespace std;

inline int func1(void){ return 5; }
inline int func2(void){ return 6; }
inline int func3(void){ return 7; }
int sum(void);

int main(int argc, char *argv[])
{
    cout << func1() << endl;
    cout << func2() << endl;
    cout << func3() << endl;
    cout << sum() << endl;
    return 0;
}

file2.cpp

inline int func1(void) { return 5; }
inline int func2(void) { return 6; }
inline int func3(void) { return 7; }
int sum(void) { return func1() + func2() + func3(); }

It worked as the rule says. I can have multiple definition of inline functions.

• What is the difference between non-inline function linkage and inline function linkage?
• How the linker differentiate between these two?

Answer 1

Making a function inline does two things (the second point is more relevant to your question):

1. It is a suggestion by the programmer to the compiler, to make calls to this function fast, possibly by doing inline expansion. Roughly, inline expansion is similar to treating the inline function like a macro, expanding each call to it, by the code of its body. This is a suggestion - the compiler may not (and sometimes cannot) perform various optimizations like that.

2. It specifies the scope of the function to be that of a translation unit. So, if an inline function appears in foo.cpp (either because it was written in it, or because it #includes a header in which it was written, in which case the preprocessor basically makes it so). Now you compile foo.cpp, and possibly also some other bar.cpp which also contains an inline function with the same signature (possibly the exact same one; probably due to both #includeing the same header). When the linker links the two object files, it will not be considered a violation of the ODR, as the inline directive made each copy of the file local to its translation unit (the object file created by compiling it, effectively). This is not a suggestion, it is binding.

It is not coincidental that these two things go together. The most common case is for an inline function to appear in a header #included by several source files, probably because the programmer wanted to request fast inline expansion. This requires the translation-unit locality rule, though, so that linker errors shouldn't arise.