Is my understanding of declaration and definition correct?

Question

As per my understanding when you say ;

int var;          // it is a declaration because no value stored in var

But when you do ;

int var = 90;     // it is a definition because var got its value

Reason to ask this question is that I am following book and internet for programming. But, I see everywhere different meaning of definition and declaration. Please clarify , If you know better ?

Some people are saying that both are definitions , if that's the case why program below gives " redeclaration error " instead of "redifination error" in gcc.

int main(){
  int var = 100;
  int var;

  return 0;
}

Error while compiling :

No, your understanding is incorrect. Both are definitions, one with an initializer and one without. See also: http://en.cppreference.com/w/cpp/language/definition — Igor Tandetnik, Mar 02 '17 at 04:37
@SurajJain, Both of them are declarations, but they're also definitions. The two aren't mutually exclusive. — chris, Mar 02 '17 at 04:50
@chris Yes Definition itself contains declaration , am i right — Suraj Jain, Mar 02 '17 at 04:52
@SurajJain, I recommend taking a look at Igor's link for more detail :) *Definitions are declarations that...* — chris, Mar 02 '17 at 04:53
The correct complete answer differs between C and C++. Which **one** do you want? — chux - Reinstate Monica, Mar 02 '17 at 05:15
C and C++ are different languages. C has a feature called "tentative declaration" which allows you to declare a variable multiple times provided no more than once it is initialised (which should be the last one). — Nawaz, Mar 02 '17 at 05:25
I thought answer for all the programming language will be same. That's why I tagged c and c++ — WeAreRight, Mar 02 '17 at 05:30
@AnttiHaapala Frankly speaking I was thinking to add PHP because I write PHP codes ! Tags modified by the way........ — WeAreRight, Mar 02 '17 at 06:13
Again hammered to a C/C++ question that doesn't answer the **C** :( — Antti Haapala -- Слава Україні, Mar 02 '17 at 07:37
`int var;` later followed by `extern int var;` in file scope is not a definition. — Antti Haapala -- Слава Україні, Mar 02 '17 at 08:02
http://stackoverflow.com/questions/3095861/about-tentative-definition — Antti Haapala -- Слава Україні, Mar 02 '17 at 08:22

venky513 · Answer 1 · 2017-03-02T04:54:06.283

1

Weather it is declaring or defining that all depends upon where you are using it . weather it is a global variable for same program or an extern variable of other program . case 1

int var; 
main ()

The line int var; both declares and defines the variable; it effectively says, "create a variable named var, of type int. Also, the storage for the variable is that it is a global variable defined in the object file associated with this source file .

case 2

extern int var;
main()

but here it is declaring the variable but not defining it . so it all depends on where you are using this

edited Mar 02 '17 at 04:54

answered Mar 02 '17 at 04:43

venky513

321
2
15

2

`int var;` is not a definition. It is a "tentative definition", which means that it might not be definition after all.. – Antti Haapala -- Слава Україні Mar 02 '17 at 07:48
@AnttiHaapala A tentative definition is a definition. "tentative" is an adjective – M.M Mar 02 '17 at 08:45

score 0 · Answer 2 · answered Mar 02 '17 at 08:48

Inside a function, int var; and int var = 90; are both definitions. You get a compilation error for having two definitions of var in the same scope. Note that they are also both declarations (all definitions are declarations).

The same syntax has different meaning outside of a function, which might be why you got confused.

score 0 · Answer 3 · answered Mar 02 '17 at 09:29

No, your understanding of declarations and definitions is incorrect.

The line

int var;

is a declaration. It may ALSO be a definition depending on context (since all definitions are declarations). For example, in;

int var;
int main()
{

}

the declaration of var is a tentative definition of a variable var that has external linkage. In the context above, it is actually a definition. However, if we modify the code to be;

int var = 42;
int var;

int main()
{

}

then int var = 42 is a definition with initialiser (of 42) and the following tentative definition int var becomes a declaration but not a definition. This is also true if the two lines int var and int var = 42 are swapped.

Even better, the meaning changes if int var is placed inside a function. For example;

int main()
{
    int var;
}

In this context, int var is a definition of a variable, var, of automatic storage duration. Placing a second definition of var in the same scope is illegal.

int main()
{
    int var;          /* definition */
    int var = 42;     /*  second definition - illegal */
}

Is my understanding of declaration and definition correct?

3 Answers3