Use of 'extern' keyword while defining the variable

Question

After seeing this answer I have this doubt. In my project, I have seen some extern variables declared and defined like below:

file1.h

Answer 1

It depends. In this case, it makes no difference, but it can.

There are two issues here: definition vs. just declaration, and linkage. And C++ doesn't handle them in an orthogonal manner. In C++, the declaration of a variable or a reference is a definition if and only if neither the extern keyword nor an initialization are present. (Note that the rules are different for functions, types and templates.) So:

extern int a;       //  not a definition
int a;              //  definition
extern int a = 42;  //  definition
int a = 42;         //  definition

The rules say you must have exactly one definition, so you put a definition in a source file, and the declaration in a header.

With regards to linkage, a symbol declared as a variable or a reference has external linkage if it is declared at namespace scope, is not declared static, and is either not const (nor constexpr in C++11) or has been declared extern. The fact that const can give a variable internal linkage occasionally means that the extern is necessary:

int const a = 42;           //  internal linkage
extern int const a = 42;    //  external linkage

Note that the extern doesn't have to be on the same declaration:

extern int const a;         //  declaration, in header...
int const a = 42;           //  external linkage, because of
                            //  previous extern

Still, I've occasionally needed the extern; typically because I want to use a local constant to instantiate a template. (This is only an issue if the template parameter takes a pointer or a reference. You can instantiate a template with an int parameter with an int const a = 42;, because the template is instantiated with the value 42, and not the variable a.)

Answer 2

It does not change the meaning. extern only makes sense when you declare a variable. Defining a variable with extern is the same because all global variables that are not marked static are symbols visible to the linker by default.

Note that if you didn't want to initialise the variable, that is, not having the part = 10, the compiler will assume that extern int a is always a declaration and not a definition. In the same sense, having int a globally is always a definition and not just a declaration.