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

Everyone else has covered this pretty well, but just to show the variants in one place:

int x;                    // #1

is a declaration and definition. The initial value of x is zero.

int x = 3;                // #2

is a declaration and definition.

const int cx;             // #3

is illegal in C++.

const int cx = 3;         // #4

is a declaration and definition, but cx has internal linkage if this is its first declaration in the translation unit.

extern int x;             // #5

is a declaration but NOT a definition. There must be a definition of x somewhere else in the program.

extern int x = 3;         // #6

is a declaration and a definition. The extern is unnecessary, but makes things clear.

extern const int cx;      // #7

is a declaration but NOT a definition. There must be a definition of cx somewhere else in the program.

extern const int cx = 3;  // #8

is a declaration and a definition. The extern is needed unless the previous declaration above was already seen.