CODEWITHSUNDEEP
c++
Jeeves
Jeeves

Reputation: 21

Declaration or Definition

Is this, line of code that is presented below and under this sentence, considered a declaration or definition? 

extern const int &ri;

Upvotes: 2

Views: 430

Answers (6)

user2100815
user2100815

Reputation:

Apart from anything else, it is almost certainly a misuse of a reference. References are intended to be used as function parameters and and return values. THe fact that you can create "reference variables" (I don't know how else to describe them) seems to have been an accident of the C++ grammar. Yes, you can do tricks with them, but no, you shouldn't.

Bottom line - if you have a reference that isn't a parameter or a return value, you are probably doing something wrong. Unless you really, really know what you are doing, in which case you wouldn't be asking about it here, would you?

Upvotes: 1

AnT stands with Russia
AnT stands with Russia

Reputation: 320777

Firstly, declaration and definition are not mutually exclusive concepts. Every definition is a declaration at the same time (with few exceptions). Which means that the proper way to ask the question would be: "Is this a defining declaration or a non-defining declaration?".

Note that I'm not saying that the question the way it is now is meaningless. The question is fine, because everybody understands what is really meant by it. I just wanted to say this as an introductory note.

Secondly, what you have here is a non-defining declaration, meaning that this declaration is not a definition (a reference or object declaration with extern specifier but without an initializer is not a definition).

In case of

extern const int &ri = 5;

we'd also have a declaration, but this would be a defining declaration (i.e. a declaration that happens to be a definition).

Upvotes: 3

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361812

extern const int &ri;

Its a declaration ONLY with no definition, which indicates that the variable is defined elsewhere.

Such code is usually found in .h file, and its definition is found in .cpp file. This approach is used to avoid multiple-redefinition error if you include the header file in multiple files. Something like this:

//lib.h
extern Type object; //declaration Only;

//lib.cpp
#include "lib.h"
Type object = /*some initialization - optional*/; //definition

//A.cpp
#include "lib.h"
Type oA = object;

//B.cpp
#include "lib.h"
Type oB = object;

The usage of constat namespace level makes the variable being declared or defined to have internal-linkage, it becomes like an immutable static variable, which exists only in its own translation unit.

extern const makes the variable to have external-linkage, at the same time, the variable is constant.

namespace N
{
    const int i = 10;         //i has internal linkage
    extern const int j = 10;  //j has external linkage
}

In this case, extern is used to make the variable to have external-linkage!

§3.5/3 [basic.link]:

A name having namespace scope (3.3.5) has internal linkage if it is the name of
— an object, reference, function or function template that is explicitly declared static or,
— an object or reference that is explicitly declared const and neither explicitly declared extern nor previously declared to have external linkage; or
— a data member of an anonymous union.

Upvotes: 1

Robᵩ
Robᵩ

Reputation: 168876

C++03, §3.1, ¶2:

A declaration is a definition unless ... it contains the extern specifier or a linkage- specification and neither an initializer nor a function-body ...

So, extern const int &ri;, which contains the extern specifier and not an initializer is a declaration but not a definition.

Upvotes: 3

Bo Persson
Bo Persson

Reputation: 92391

Using extern would indicate that the object is not defined here, but somewhere else.

Upvotes: 0

Puppy
Puppy

Reputation: 147056

It's a declaration- the "extern" means that it lives somewhere else.

Upvotes: 4

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