Why doesn't this "undefined extern variable" result in a linker error in C++17?

Question

I have compiled and ran the following program in a C++17 compiler (Coliru). In the program, I declared an extern variable, but did not define it. However, the compiler doesn't give a linker error.

#include <iostream>  extern int i; // Only declaration  int func()  {     if constexpr (true)         return 0;     else if (i)         return i;     else         return -1; }  int main()  {     int ret = func();     std::cout<<"Ret : "<<ret<<std::endl; } 

Why doesn't the compiler give a linker error?

Answer 1

Because the variable isn't odr-used. You have a constexpr if there that always discards the branch that could use it.

One of the points of constexpr if is that the discarded branch need not even compile, only be well-formed. That's how we can place calls to non-existing member functions in a discarded branch.

Answer 2

In your case the variable is used in discarded statements only. However, even if we ignore that fact, C++ language specification still explicitly states that no diagnostic is required for missing definitions

3.2 One-definition rule

4 Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (6.4.1); no diagnostic required.

The language specification understands that an optimizing compiler might be smart enough to eliminate all odr-uses of a variable. In that case it would be excessive and unnecessary to require the implementation to detect and report the potential ODR violations.