Under what circumstances can an extern variable be used in definition?

Question

I am very very sorry. I didn\'t know my incomplete code attachment would create such a mess. I am very glad to see so many sincere helps.

This code will compile:

Answer 1

All variable declarations with an initializer are also definitions; that's an overriding rule. Regardless of extern. There are even cases where you need an extern on a definition: you can only instantiate a template using a variable which has external linkage. And const variables have internal linkage by default, so you need something like:

extern int const i = 42;

if you want to use it to instantiate a template.