C++ call inline function from another cpp file

Question

I'm trying to understand a few basics about extern, static, etc. and tried the following example, but I don't get why I can't call the function "just" because it's (possibly) inlined.

My first file : F1.cpp

 #include <iostream>
 
 void Modify();
 int i;

 int main() {
      i = 1;

      std::cout << "i = " << i << std::endl;
      Modify();
      std::cout << "i = " << i << std::endl;
 
      return 0;
 }

The second file : F2.cpp

#include <iostream>

extern int i;

inline void Modify() {
    i = 99;

    std::cout << "i = " << i << std::endl;

}

With inline keyword in F2.cpp, I get : undefined reference to Modify() in my F1.cpp file. Removing it, the code compiles and works fine.

I assume that the inline keyword in C++ has some sort of behaviour like the static keyword ?

I had a look at this topic aswell, but apart from the fact that the documentation says that an inline function should always be in the header file, I don't get it : C++ inline member function in .cpp file

Thanks for your help !

Answer 1

According to the C++ Standard (7.1.2 Function specifiers)

4....If a function with external linkage is declared inline in one translation unit, it shall be declared inline in all translation units in which it appears; no diagnostic is required.

and

4 An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case

In C ( section 6.7.4 Function specifiers of the C Standard ) function specifier inline for external functions has different semantic

7....An inline definition does not provide an external definition for the function, and does not forbid an external definition in another translation unit. An inline definition provides an alternative to an external definition, which a translator may use to implement any call to the function in the same translation unit. It is unspecified whether a call to the function uses the inline definition or the external definition