declarations and definitions

[theuser]

Hello guys.

I'm very new with C++ and I'm trying to understand the difference between a declaration and a definition.

As far as I understand, a declaration tells the compiler that an identifier exists and a definition assigns memory (not a value) to it. Am I right? :).

I'd like to know if this statement is a declaration or a definition:
static int x;

and why a static variable inside a class is a declaration and not definition:
struct x {
static int y;
};

Thanks for advance and I hope you can help me, I have been searching information in google but I really want to be sure.
Kind regards.

[Cubbi]

You're close, you're only off when you say "not a value" -- a definition of an object usually provides the initial value as well as the storage. The automatic scalar variables are an exception -- their initial value may be left indeterminate.

To quote the standard,

all but one of the following are definitions:

int a; // defines a
extern const int c = 1; // defines c
int f(int x) { return x+a; } // defines f and defines x
struct S { int a; int b; }; // defines S, S::a, and S::b
struct X { // defines X
    int x; // defines non-static data member x
    static int y; // declares static data member y
    X(): x(0) { } // defines a constructor of X
};
int X::y = 1; // defines X::y
enum { up, down }; // defines up and down
namespace N { int d; } // defines N and N::d
namespace N1 = N; // defines N1
X anX; // defines anX 

whereas these are just declarations:

extern int a; // declares a
extern const int c; // declares c
int f(int); // declares f
struct S; // declares S
typedef int Int; // declares Int
extern X anotherX; // declares anotherX
using N::d; // declares d 

Your non-member static int x; is a definition of an object of type int, with static storage duration, and the initial value zero.

A static data member is declared inside the class body: you're telling the compiler that somewhere out there exists this static int x::y, which every single instance of your struct x will access.

And then in that one file where you want to actually place it, you define it, outside the class body.

[theuser]

You're close, you're only off when you say "not a value" -- a definition of an object usually provides the initial value as well as the storage. The automatic scalar variables are an exception -- their initial value may be left indeterminate.

I didn't mean that it is not a definition if an assignment is made, I meant that if an assignment is not made, it's a definition anyway, because what defines a definition is the assignment of memory, not the value.

Your non-member static int x; is a definition of an object of type int, with static storage duration, and the initial value zero.

That is what I thought, it's a definition :).

A static data member is declared inside the class body: you're telling the compiler that somewhere out there exists this static int x::y, which every single instance of your struct x will access.

And then in that one file where you want to actually place it, you define it, outside the class body.

Great.