Is extern keyword really necessary?

Question

...
#include \"test1.h\"

int main(..)
{
    count << aaa <

aaa is defined in test1.h,and I didn\'t u

Answer 1

In that case, extern is not necessary. Extern is needed when the symbol is declared in another compilation unit.

When you use the #include preprocessing directive, the included file is copied out in place of the directive. In this case you don't need extern because the compiler already know aaa.