Considering the example below:

header.h

extern int somevariable;
    
void somefunction();

source.cpp

#include "header.h"

somevariable = 0; // this declaration has no storage class or type specifier

void somefunction(){
    somevariable = 0; // works fine
}

I couldn't find the answer to the following questions:

1. Why can't I just forward declare int somevariable and initialize it in the .cpp file?

2. Why is the somefunction() function able to "see" somevariable when declared using extern int somevariable, and why is it not accessible outside of a function?

CodePudding user response：

extern int somevariable;

means definition of somevariable is located somewhere else. It is not forward declaration. Forward declaration is used in context of classes and structs.

somevariable = 0; 

is invalid since this is assignment and you can't run arbitrary code in global scope.
It should be:

int somevariable = 0; 

and this means define (instantiate) global variable somevariable in this translation unit and initialize it to 0. Without this statement linking of your program will fail with error something like: undefined reference to 'somevariable' referenced by ... .

Use of global variable in any langue is considered as bad practice, sine as project grows global variable makes code impossible to maintain and bug-prone.