By Maciek


2009-09-11 12:27:12 8 Comments

The meaning of both eludes me.

23 comments

@Brad Solomon 2019-08-16 13:53:08

There are some very clear definitions sprinkled throughout K&R (2nd edition); it helps to put them in one place and read them as one:

"Definition" refers to the place where the variable is created or assigned storage; "declaration" refers to the places where the nature of the variable is stated but no storage is allocated. [p. 33]

...

It is important to distinguish between the declaration of an external variable and its definition. A declaration announces the properties of a variable (primarily its type); a definition also causes storage to be set aside. If the lines

int sp;
double val[MAXVAL]

appear outside of any function, they define the external variables sp and val, cause storage to be set aside, and also serve as the declaration for the rest of that source file.

On the other hand, the lines

extern int sp;
extern double val[];

declare for the rest of the source file that sp is an int and that val is a double array (whose size is determined elsewhere), but they do not create the variables or reserve storage for them.

There must be only one definition of an external variable among all the files that make up the source program. ... Array sizes must be specified with the definition, but are optional with an extern declaration. [pp. 80-81]

...

Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions. [p. 210]

@Puneet Purohit 2013-01-03 06:54:23

Declaration means give name and type to a variable (in case of variable declaration), eg:

int i;

or give name,return type and parameter(s) type to a function without body(in case of function declaration), eg:

int max(int, int);

whereas definition means assign value to a variable (in case of variable definition), eg:

i = 20;

or provide/add body(functionality) to a function is called function definition, eg:

int max(int a, int b)
{
   if(a>b)   return a;
   return b;  
}

many time declaration and definition can be done together as:

int i=20;

and:

int max(int a, int b)
{
    if(a>b)   return a;
    return b;    
} 

In above cases we define and declare variable i and function max().

@Puneet Purohit 2013-01-03 06:57:17

the actual mean of definition if to assign value/body to a variable/function whereas declaration means provide name,type to a variable/function

@Lightness Races with Monica 2013-04-14 17:36:51

You can define something without assigning it a value.

@Lightness Races with Monica 2013-04-15 11:19:59

Just like this: int x;

@Puneet Purohit 2013-04-15 11:27:21

its a declaration of variable x not its defination

@Lightness Races with Monica 2013-04-15 14:10:45

No, it is both. You are confusing definition with initialisation.

@Michael Kristofik 2009-09-11 13:53:50

From the C++ standard section 3.1:

A declaration introduces names into a translation unit or redeclares names introduced by previous declarations. A declaration specifies the interpretation and attributes of these names.

The next paragraph states (emphasis mine) that a declaration is a definition unless...

... it declares a function without specifying the function’s body:

void sqrt(double);  // declares sqrt

... it declares a static member within a class definition:

struct X
{
    int a;         // defines a
    static int b;  // declares b
};

... it declares a class name:

class Y;

... it contains the extern keyword without an initializer or function body:

extern const int i = 0;  // defines i
extern int j;  // declares j
extern "C"
{
    void foo();  // declares foo
}

... or is a typedef or using statement.

typedef long LONG_32;  // declares LONG_32
using namespace std;   // declares std

Now for the big reason why it's important to understand the difference between a declaration and definition: the One Definition Rule. From section 3.2.1 of the C++ standard:

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.

@RJFalconer 2014-03-04 13:27:46

"declares a static member within a class definition" - This is true even if the static member is initialised, correct? Can we make the example struct x {static int b = 3; };?

@Kyle Strand 2014-08-14 17:08:47

@RJFalconer You're correct; initialization does not necessarily turn a declaration into a definition (contrary to what one might expect; certainly I found this surprising). Your modification to the example is actually illegal unless b is also declared const. See stackoverflow.com/a/3536513/1858225 and daniweb.com/software-development/cpp/threads/140739/… .

@Victor Zamanian 2014-10-07 13:52:17

This is interesting to me. According to your answer, it seems that in C++, a declaration is also a definition (with exceptions), whereas in the C standard it is phrased from the other perspective (C99, section 6.7, Declarations): "A definition of an identifier is a declaration for that identifier that: [followed by criteria for different cases]". Different ways to look at it, I suppose. :)

@Gab是好人 2016-02-11 14:45:27

Declaration is for the compiler to accept a name(to tell the compiler that the name is legal, the name is introduced with intention not a typo). Definition is where a name and its content is associated. The definition is used by the linker to link a name reference to the content of the name.

@Santosh 2014-03-12 18:01:50

Find similar answers here: Technical Interview Questions in C.

A declaration provides a name to the program; a definition provides a unique description of an entity (e.g. type, instance, and function) within the program. Declarations can be repeated in a given scope, it introduces a name in a given scope.

A declaration is a definition unless:

  • Declaration declares a function without specifying its body,
  • Declaration contains an extern specifier and no initializer or function body,
  • Declaration is the declaration of a static class data member without a class definition,
  • Declaration is a class name definition,

A definition is a declaration unless:

  • Definition defines a static class data member,
  • Definition defines a non-inline member function.

@Jeet Parikh 2018-08-08 04:06:13

Stages of an executable generation:

(1) pre-processor -> (2) translator/compiler -> (3) linker

In stage 2 (translator/compiler), declaration statements in our code tell to the compiler that these things we are going to use in future and you can find definition later, meaning is :

translator make sure that : what is what ? means declaration

and (3) stage (linker) needs definition to bind the things

Linker make sure that : where is what ? means definition

@LinuxBabe 2018-03-07 23:06:30

According to the GNU C library manual (http://www.gnu.org/software/libc/manual/html_node/Header-Files.html)

In C, a declaration merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.

@Karoly Nyisztor 2018-02-20 18:56:26

To understand the nouns, let's focus on the verbs first.

declare - to announce officially; proclaim

define - to show or describe (someone or something) clearly and completely

So, when you declare something, you just tell what it is.

// declaration
int sum(int, int);

This line declares a C function called sum that takes two arguments of type int and returns an int. However, you can't use it yet.

When you provide how it actually works, that's the definition of it.

// definition
int sum(int x, int y)
{
    return x + y;
}

@plinth 2009-09-11 18:20:35

Declaration: "Somewhere, there exists a foo."

Definition: "...and here it is!"

@Gab是好人 2016-02-11 14:41:46

Declaration is for the compiler to accept a name(to tell the compiler that the name is legal, the name is introduced with intention not a typo). Definition is where a name and its content is associated. The definition is used by the linker to link a name reference to the content of the name.

@princio 2017-10-03 15:30:37

To understand the difference between declaration and definition we need to see the assembly code:

uint8_t   ui8 = 5;  |   movb    $0x5,-0x45(%rbp)
int         i = 5;  |   movl    $0x5,-0x3c(%rbp)
uint32_t ui32 = 5;  |   movl    $0x5,-0x38(%rbp)
uint64_t ui64 = 5;  |   movq    $0x5,-0x10(%rbp)
double   doub = 5;  |   movsd   0x328(%rip),%xmm0        # 0x400a20
                        movsd   %xmm0,-0x8(%rbp)

and this is only definition:

ui8 = 5;   |   movb    $0x5,-0x45(%rbp)
i = 5;     |   movl    $0x5,-0x3c(%rbp)
ui32 = 5;  |   movl    $0x5,-0x38(%rbp)
ui64 = 5;  |   movq    $0x5,-0x10(%rbp)
doub = 5;  |   movsd   0x328(%rip),%xmm0        # 0x400a20
               movsd   %xmm0,-0x8(%rbp)

As you can see nothing change.

Declaration is different from definition because it gives information used only by the compiler. For example uint8_t tell the compiler to use asm function movb.

See that:

uint def;                  |  no instructions
printf("some stuff...");   |  [...] callq   0x400450 <[email protected]>
def=5;                     |  movb    $0x5,-0x45(%rbp)

Declaration haven't an equivalent instruction because it is no something to be executed.

Furthermore declaration tells the compiler the scope of the variable.

We can say that declaration is an information used by the compiler to establish the correct use of the variable and for how long some memory belongs to certain variable.

@hdante 2017-05-10 04:54:23

A declaration presents a symbol name to the compiler. A definition is a declaration that allocates space for the symbol.

int f(int x); // function declaration (I know f exists)

int f(int x) { return 2*x; } // declaration and definition

@Sridharan 2017-01-04 12:13:02

Declaration :

int a; // this declares the variable 'a' which is of type 'int'

Thus declaration associates the variable with a type.

Following are some examples of declaration.

int a;
float b;
double c;

Now function declaration :

int fun(int a,int b); 

Note the semicolon at the end of function so it says it is only a declaration. Compiler knows that somewhere in the program that function will be defined with that prototype. Now if the compiler gets a function call something like this

int b=fun(x,y,z);

Compiler will throw an error saying that there is no such function. Because it doesn't has any prototype for that function.

Note the difference between two programs.

Program 1

#include <stdio.h>
void print(int a)
{
     printf("%d",a);
}
main()
{
    print(5);
}

In this, print function is declared and defined as well. Since function call is coming after the definition. Now see the next program.

Program 2

 #include <stdio.h>
 void print(int a); // In this case this is essential
 main()
 {
    print(5);
 }
 void print(int a)
 {
     printf("%d",a);
 }

It is essential because function call precedes definition so compiler must know whether there is any such function. So we declare the function which will inform the compiler.

Definition :

This part of defining a function is called Definition. It says what to do inside the function.

void print(int a)
{
    printf("%d",a);
}

Now with the variables.

int a; //declaration
a=10; //definition 

Some times declaration and definition are grouped into a single statement like this.

int a=10;

@Joey Pabalinas 2018-02-11 18:06:31

int a; //declaration; a=10; //definition This is completely wrong. When talking about automatic storage duration objects (objects declared inside a function definition that are not declared with another storage class specifier like extern) these are always definitions.

@Joey Pabalinas 2018-02-11 18:06:56

The main difference to grasp is that a declaration is saying "a thing exists somewhere that has these traits (type etc.)," whereas a definition is saying "I am declaring a thing with these traits, and I am also instantiating it here as well." Since you can't forward declare automatic storage duration objects like that, they will always be definitions.

@Joey Pabalinas 2018-02-11 18:07:03

Except for maybe some weird typedef corner cases that I always forget about, a rule of thumb is that All definitions are declarations. Think about it; when you are instantiating something, you also need to tell the compiler that that thing exists and what its traits are right?

@Jason K. 2016-10-09 23:15:02

My favorite example is "int Num = 5" here your variable is 1. defined as int 2. declared as Num and 3. instantiated with a value of five. We

  • Define the type of an object, which may be built-in or a class or struct.
  • Declare the name of an object, so anything with a name has been declared which includes Variables, Funtions, etc.

A class or struct allows you to change how objects will be defined when it is later used. For example

  • One may declare a heterogeneous variable or array which are not specifically defined.
  • Using an offset in C++ you may define an object which does not have a declared name.

When we learn programming these two terms are often confused because we often do both at the same time.

@Jason K. 2016-10-09 23:25:33

I do not understand why so many people upvoted sbi's answer. I did upvote the answer by bjhend, which was quite good, concise, accurate and much more timely than mine. I was sad to see that I was the first person to do so in 4 years.

@achoora 2014-11-13 11:44:27

The concept of Declaration and Definition will form a pitfall when you are using the extern storage class because your definition will be in some other location and you are declaring the variable in your local code file (page). One difference between C and C++ is that in C you the declarations are done normally at the beginning of a function or code page. In C++ it's not like that. You can declare at a place of your choice.

@sbi 2015-07-31 22:27:27

This confuses declaration with definition and is plain wrong.

@sbi 2009-09-11 12:43:03

A declaration introduces an identifier and describes its type, be it a type, object, or function. A declaration is what the compiler needs to accept references to that identifier. These are declarations:

extern int bar;
extern int g(int, int);
double f(int, double); // extern can be omitted for function declarations
class foo; // no extern allowed for type declarations

A definition actually instantiates/implements this identifier. It's what the linker needs in order to link references to those entities. These are definitions corresponding to the above declarations:

int bar;
int g(int lhs, int rhs) {return lhs*rhs;}
double f(int i, double d) {return i+d;}
class foo {};

A definition can be used in the place of a declaration.

An identifier can be declared as often as you want. Thus, the following is legal in C and C++:

double f(int, double);
double f(int, double);
extern double f(int, double); // the same as the two above
extern double f(int, double);

However, it must be defined exactly once. If you forget to define something that's been declared and referenced somewhere, then the linker doesn't know what to link references to and complains about a missing symbols. If you define something more than once, then the linker doesn't know which of the definitions to link references to and complains about duplicated symbols.


Since the debate what is a class declaration vs. a class definition in C++ keeps coming up (in answers and comments to other questions) , I'll paste a quote from the C++ standard here.
At 3.1/2, C++03 says:

A declaration is a definition unless it [...] is a class name declaration [...].

3.1/3 then gives a few examples. Amongst them:

[Example: [...]
struct S { int a; int b; }; // defines S, S::a, and S::b [...]
struct S; // declares S
—end example

To sum it up: The C++ standard considers struct x; to be a declaration and struct x {}; a definition. (In other words, "forward declaration" a misnomer, since there are no other forms of class declarations in C++.)

Thanks to litb (Johannes Schaub) who dug out the actual chapter and verse in one of his answers.

@San Jacinto 2009-09-11 12:56:33

Is that multiple-declaration legal according to the STANDARD, or your compiler, or what? I cannot do that multiple times in the same scope.

@Steve Jessop 2009-09-11 13:09:38

@unknown: either your compiler is broken of you have mis-copied sbi's code. For example, 6.7.2(2) in N1124: "All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined."

@Steve Jessop 2009-09-11 13:10:51

@unknown: or possibly it's issuing a warning, which you're promoting to an error.

@San Jacinto 2009-09-11 13:13:18

yes, it was a bad compiler. i tried it again on GCC and it workd. hint: don't use imagecraft's c compiler.

@Steve Jessop 2009-09-11 13:13:35

"If you define something more than once, then the linker doesn't know which of the definitions to link references to". Although you can get away with that if the definitions are in different translation units, are equivalent, and and have appropriate modifiers telling the linker it's OK to fold them (principally "inline")

@sbi 2009-09-11 13:39:19

@onebyone: Yes, there are exceptions, notably inlined functions that the compiler won't inline for whatever reason. However, I didn't want to add exceptions to the answer if the questioner doesn't know the difference between declaration and definition. I was quite surprised about all the misleading answers (that have since disappeared). I hadn't thought such misconceptions are so wide-spread.

@Brian Postow 2009-09-11 13:59:55

I would say that "int i;" is also a declaration, and you never actually DEFINE an int variable... but other than that, +1

@David Thornley 2009-09-11 14:05:06

@Brian: "extern int i;" says that i is an int somewhere, don't worry about it. "int i;" means that i is an int, and its address and scope is determined here.

@sbi 2009-09-11 14:09:23

@Brian: You're wrong. extern int i is a declaration, since it just introduces/specifies i. You can have as many extern int i in each compilation unit as you want. int i, however, is a definition. It denotes the space for the integer to be in this translation unit and advices the linker to link all references to i against this entity. If you have more or less than exactly one of these definitions, the linker will complain.

@Steve Jessop 2009-09-11 14:14:47

@Brian int i; in file/global scope or function scope is a definition both in C and C++. In C because it allocates storage, and in C++ because it does not have the extern specifier or a linkage-specification. These amount to the same thing, which is what sbi says: in both cases this declaration specifies the object to which all references to "i" in that scope must be linked.

@Johannes Schaub - litb 2009-09-11 16:54:04

@unknown, beware you cannot redeclare members in class scope: struct A { double f(int, double); double f(int, double); }; invalid, of course. It's allowed elsewhere though. There are some places where you can declare things, but not define, too: void f() { void g(); } valid, but not the following: void f() { void g() { } };. What is a definition and what a declaration has subtle rules when it comes to templates - beware! +1 for a good answer though.

@Marc van Leeuwen 2014-05-26 22:44:04

My main issue with this answer is that the initial explanation that a definition is what the linker needs is unhelpful in the case of a class definition. The linker never needs to link to a class itself (which like a typedef is really of declarative nature only); it may however link to a (static) class instance, to class methods, or to the class vtable, for none of which the class definition is used. But the compiler does not to see more than a class declaration for many things. So calling it a class definition is really just a matter of convention, not justified by the given explanation.

@Thomson 2014-08-01 06:12:11

"A definition can be used in the place of a declaration." This may be incorrect. If is only legal if no duplicated definition will be introduced.

@sbi 2014-08-02 07:21:53

@Thomson: "An identifier can be declared as often as you want. [...] However, it must be defined exactly once."

@Thomson 2014-08-02 14:10:54

@sbi thanks for the clarification. I know the statement I quoted is correct with some condition. Just the condition is a little far from the quoted sentence.

@Koray Tugay 2015-05-21 15:53:40

Quoting from this page: pubs.opengroup.org/onlinepubs/7908799/xsh/unistd.h.html "The following are declared as functions and may also be defined as macros. Function prototypes must be provided for use with an ISO C compiler." What is meant by 'Function prototypes must be provided for use with an ISO C compiler.' ?

@Koray Tugay 2015-05-21 15:55:49

quoting from your answer: "then the linker doesn't know what to link references to" do you mean "then the linker doesn't know what the link references to"?

@sbi 2015-05-23 07:00:48

@Koray: I didn't even mention C's macros, because they are so strange breasts. Basically, you cannot declare a macro, you can only define it. But the preprocessor is not a real compiler, but a simple text processor anyway... In K&R C, functions didn't declare their parameters, so all you needed as a declaration was their name. In ISO C, you need proper declarations. I suppose this is what the comment you quoted refers to. And, no, I meant that sentence to be the way I wrote it.

@Destructor 2015-08-25 08:34:37

@sbi: why ideone.com/wIBBTi fails in compilation. there can be as many as I want declarations of identifier but must be exactly 1 definition as I think. I haven't defined & called any of the function. Why an error in the program? what is the reason?

@Destructor 2015-08-25 08:35:48

@JohannesSchaub-litb: you says that double f(int, double); double f(int, double); } is allowed elsewhere but not in class scope. then why it isn't allowed at global scope. Why ideone.com/wIBBTi fails in compilation?

@sbi 2015-08-25 13:39:08

@Pravasi: You declare two functions which have the same name, but differ in their return types. In C++, this is not allowed, you can only overload functions when their parameters differ.

@Zebrafish 2016-12-24 01:04:02

@sbi I wanted to edit your answer, but I'm not confident enough to. I've tried putting extern before a struct or class and it seems to be in the category with the functions, ie., adding extern doesn't seem to make a difference. I'm talking about the bit: // no extern allowed for type declarations - at the top of your answer.

@YuZ 2017-02-28 09:10:26

does int x; implicitly initiate x to zero whereas extern int x does not?

@sbi 2017-02-28 17:55:37

@user3921720: Nope.

@YuZ 2017-03-02 16:15:54

then why is int x; a definition and extern int x a declaration?

@sbi 2017-03-27 14:28:06

Because one defines a variable x of type int, while the other declares x to be a variable of type int which is to be defined elsewhere? shrug I really don't know what to say here. (Have you tried reading my answer? It explains this.)

@陳 力 2017-12-05 10:42:08

May mislead newbie: int x also includes a declaration. Because definition is a subset of declaration

@sbi 2017-12-09 09:43:15

@czxyl: "A definition can be used in the place of a declaration."

@rehctawrats 2018-02-08 08:46:54

Would be great if someone could extend this answer to include the meaning of initialization. Such as stackoverflow.com/q/23345554/6060872

@It'sPete 2013-07-02 22:46:52

This is going to sound really cheesy, but it's the best way I've been able to keep the terms straight in my head:

Declaration: Picture Thomas Jefferson giving a speech... "I HEREBY DECLARE THAT THIS FOO EXISTS IN THIS SOURCE CODE!!!"

Definition: picture a dictionary, you are looking up Foo and what it actually means.

@legends2k 2013-06-26 19:43:06

C++11 Update

Since I don't see an answer pertinent to C++11 here's one.

A declaration is a definition unless it declares a/n:

  • opaque enum - enum X : int;
  • template parameter - T in template<typename T> class MyArray;
  • parameter declaration - x and y in int add(int x, int y);
  • alias declaration - using IntVector = std::vector<int>;
  • static assert declaration - static_assert(sizeof(int) == 4, "Yikes!")
  • attribute declaration (implementation-defined)
  • empty declaration ;

Additional clauses inherited from C++03 by the above list:

  • function declaration - add in int add(int x, int y);
  • extern specifier containing declaration or a linkage specifier - extern int a; or extern "C" { ... };
  • static data member in a class - x in class C { static int x; };
  • class/struct declaration - struct Point;
  • typedef declaration - typedef int Int;
  • using declaration - using std::cout;
  • using directive - using namespace NS;

A template-declaration is a declaration. A template-declaration is also a definition if its declaration defines a function, a class, or a static data member.

Examples from the standard which differentiates between declaration and definition that I found helpful in understanding the nuances between them:

// except one all these 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


// all these are 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 N::d


// specific to C++11 - these are not from the standard
enum X : int;                           // declares X with int as the underlying type
using IntVector = std::vector<int>;     // declares IntVector as an alias to std::vector<int>
static_assert(X::y == 1, "Oops!");      // declares a static_assert which can render the program ill-formed or have no effect like an empty declaration, depending on the result of expr
template <class T> class C;             // declares template class C
;                                       // declares nothing

@Marcin Gil 2009-09-11 12:30:09

From wiki.answers.com:

The term declaration means (in C) that you are telling the compiler about type, size and in case of function declaration, type and size of its parameters of any variable, or user defined type or function in your program. No space is reserved in memory for any variable in case of declaration. However compiler knows how much space to reserve in case a variable of this type is created.

for example, following are all declarations:

extern int a; 
struct _tagExample { int a; int b; }; 
int myFunc (int a, int b);

Definition on the other hand means that in additions to all the things that declaration does, space is also reserved in memory. You can say "DEFINITION = DECLARATION + SPACE RESERVATION" following are examples of definition:

int a; 
int b = 0; 
int myFunc (int a, int b) { return a + b; } 
struct _tagExample example; 

see Answers.

@sbi 2009-09-11 12:37:35

This, too, is wrong (although much closer than the others): struct foo {}; is a definition, not a declaration. A declaration of foo would be struct foo;. From that, the compiler doesn't know how much space to reserve for foo objects.

@San Jacinto 2009-09-11 12:43:32

sbi, my answer reflects what you are saying. in your example, you define a foo as an empty structure. I fail to understand how our examples are technically different.

@Steve Jessop 2009-09-11 13:01:44

@Marcin: sbi is saying that "compiler knows how much space to reserve in case a variable of this type is created" is not always true. struct foo; is a declaration, but it does not tell the compiler the size of foo. I'd add that struct _tagExample { int a; int b; }; is a definition. So in this context it is misleading to call it a declaration. Of course it is one, since all definitions are declarations, but you seem to be suggesting that it is not a definition. It is a definition, of _tagExample.

@David Thornley 2009-09-11 14:07:56

@Marcin Gil: Which means that "Answers" wiki is not always accurate. I have to downvote for misinformation here.

@sbi 2009-09-11 14:14:43

So we have an answer copied straight from MSDN (adatapost's) and one from answers.com and both are misleading or even plain wrong. What do we learn from this?

@Steve Jessop 2009-09-11 14:18:27

We learn that what adatapost quoted is true but does not (IMO) really answer the question. What Marcin quoted is false. Quoting the standards is true and answers the question, but is very difficult to make head or tail of.

@sbi 2009-09-11 14:23:20

@onebyone: A very nice summary indeed! (I, however, had reinforced what I learned as a student and later tried to hammer into my students: Copying without thinking might lead to a disaster. :^>)

@Marcin Gil 2009-09-11 18:15:31

@David Thornley - not a problem :) This is what this site is about. We select and verify info.

@Johannes Schaub - litb 2009-09-11 18:25:45

It should be noted that struct foo { int a; }; in C is not a definition. C doesn't know struct definitions. In reverse, a typedef is a definition in C.

@bjhend 2012-04-17 18:15:50

Rule of thumb:

  • A declaration tells the compiler how to interpret the variable's data in memory. This is needed for every access.

  • A definition reserves the memory to make the variable existing. This has to happen exactly once before first access.

@Lightness Races with Monica 2013-04-14 17:38:53

This only holds for objects. What about types and functions?

@user565367 2011-01-07 04:42:05

definition means actual function written & declaration means simple declare function for e.g.

void  myfunction(); //this is simple declaration

and

void myfunction()
{
 some statement;    
}

this is definition of function myfunction

@sbi 2013-04-24 12:57:08

And what about types and objects?

@Johannes Schaub - litb 2009-09-11 18:15:37

There are interesting edge cases in C++ (some of them in C too). Consider

T t;

That can be a definition or a declaration, depending on what type T is:

typedef void T();
T t; // declaration of function "t"

struct X { 
  T t; // declaration of function "t".
};

typedef int T;
T t; // definition of object "t".

In C++, when using templates, there is another edge case.

template <typename T>
struct X { 
  static int member; // declaration
};

template<typename T>
int X<T>::member; // definition

template<>
int X<bool>::member; // declaration!

The last declaration was not a definition. It's the declaration of an explicit specialization of the static member of X<bool>. It tells the compiler: "If it comes to instantiating X<bool>::member, then don't instantiate the definition of the member from the primary template, but use the definition found elsewhere". To make it a definition, you have to supply an initializer

template<>
int X<bool>::member = 1; // definition, belongs into a .cpp file.

@user154171 2009-09-11 14:46:45

Couldnt you state in the most general terms possible, that a declaration is an identifier in which no storage is allocated and a definition actually allocates storage from a declared identifier?

One interesting thought - a template cannot allocate storage until the class or function is linked with the type information. So is the template identifier a declaration or definition? It should be a declaration since no storage is allocated, and you are simply 'prototyping' the template class or function.

@sbi 2009-09-11 15:09:58

Your definition isn't per se wrong, but "storage definition" always seems awkward when it comes to function definitions. Regarding templates: This template<class T> struct foo; is a template declaration, and so is this template<class T> void f();. Template definitions mirror class/function definitions in the same way. (Note that a template name is not a type or function name. One place where you can see this is when you cannot pass a template as another template's type parameter. If you want to pass templates instead of types, you need template template parameters.)

@user154171 2009-09-11 15:59:27

Agreed that 'storage definition' is awkward, especially regarding function definitions. The declaration is int foo() and definition is int foo() {//some code here..}. I usually need to wrap my small brain with concepts I am familiar - 'storage' is one such way to keep it straight to me at least... :)

@Steve Jessop 2009-09-11 14:03:38

From the C99 standard, 6.7(5):

A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:

  • for an object, causes storage to be reserved for that object;
  • for a function, includes the function body;
  • for an enumeration constant or typedef name, is the (only) declaration of the identifier.

From the C++ standard, 3.1(2):

A declaration is a definition unless it declares a function without specifying the function's body, it contains the extern specifier or a linkage-specification and neither an initializer nor a function-body, it declares a static data member in a class declaration, it is a class name declaration, or it is a typedef declaration, a using-declaration, or a using-directive.

Then there are some examples.

So interestingly (or not, but I'm slightly surprised by it), typedef int myint; is a definition in C99, but only a declaration in C++.

@sbi 2009-09-11 14:20:11

@onebyone: Regarding the typedef, wouldn't that mean that it could be repeated in C++, but not in C99?

@Steve Jessop 2009-09-11 14:35:26

That's what surprised me, and as far as a single translation unit is concerned, yes there is that difference. But clearly a typedef can be repeated in C99 in different translation units. C doesn't have an explicit "one definition rule" like C++, so the rules it does have just allow it. C++ chose to change it to a declaration, but also the one definition rule lists what kinds of things it applies to, and typedefs isn't one of them. So repeats would be allowed in C++ under the ODR as it's worded, even if a typedef was a definition. Seems unnecessarily picky.

@Steve Jessop 2009-09-11 14:35:57

... but I'd guess that list in the ODR actually lists all the things it's possible to have definitions of. If so, then the list is actually redundant, and is just there to be helpful.

@sbi 2009-09-11 15:03:36

What does the std's ODR definition say about class definitions? They have to be repeated.

@Johannes Schaub - litb 2009-09-11 20:52:59

I suspect the rationale for it is because a typedef declares just a name, without producing something beyond (like, a type, object or something). So it's just a declaration, much like a using-declaration.

@Johannes Schaub - litb 2009-09-11 20:55:08

However, note that you can have multiple definitions of the same namespace, although it sounds odd: namespace A { } namespace A { }

@Steve Jessop 2009-09-12 12:21:47

@sbi: ODR says "(1) No translation unit shall contain more than one definition of any ... class type" and "(5) There can be more than one definition of a class type ... in a program provided that each definition appears in a different translation unit" and then some extra requirements which amount to "the definitions are the same".

@Steve Jessop 2009-09-12 12:24:00

@litb: yep, namespaces aren't mentioned in the first clause of the ODR. Indeed I often use that in header files, where I have groups of functions I close and re-open the namespaces between them, so that each "section" of the header file stands alone.

@sbi 2009-09-12 13:35:18

@onebyone: I always thought ODR deals with definitions across translation units. I'm surprised it's limited to TUs.

@Steve Jessop 2009-09-12 13:52:45

@sbi: it deal with both. Clause 1 is about what can't be duplicated in a single translation unit. Clause 5 lists some things which can be duplicated in the program provided they're in different units. There are other wondrous clauses too large to fit in this margin^Hcomment.

@sbi 2009-09-12 21:58:38

@onebyone: I tried to dig through the Holy Paper several times, but my understanding of English seems to simply lack too much in order to understand standardeze. <sigh> But then again, maybe I'm just not the kind of person who reads legaleze, not matter what language it comes in...

@Steve Jessop 2009-09-13 10:32:49

@sbi: I've certainly never read the standard cover-to-cover, but the index is pretty good :-)

@Destructor 2016-02-03 16:40:08

@SteveJessop: update your answer according to C11 standard because as you know C11 allows repeated typedef also.

@adatapost 2009-09-11 12:35:30

Declaration

Declarations tell the compiler that a program element or name exists. A declaration introduces one or more names into a program. Declarations can occur more than once in a program. Therefore, classes, structures, enumerated types, and other user-defined types can be declared for each compilation unit.

Definition

Definitions specify what code or data the name describes. A name must be declared before it can be used.

@sbi 2009-09-11 13:03:17

Um, isn't it that you can even define classes and enums in each compilation unit? At least I put class definitions into my headers and include them all over. Er, class foo {}; is a class definition, isn't it?

@David Thornley 2009-09-11 14:01:49

Yes. However, "class foo;" is a declaration. It tells the compiler that foo is a class. "class foo {};" is a definition. It tells the compiler exactly what sort of class foo is.

@sbi 2009-09-11 14:17:17

@David: Right. And since we all put class and enum definitions into our headers files, it's class and enum definitions, not declarations, that can be repeated for each compilation unit. That makes adatapost's answer, um, misleading.

@Johannes Schaub - litb 2009-09-11 16:56:42

The exception are class member names which may be used before they're declared.

@sbi 2009-09-11 17:41:35

@litb: I don't think that's true: class blah { foo bar(); typedef int foo; }; gives a compile-time error. What you mean is that member function definitions are, even when they are defined within the class' definition, parsed as if they were defined right behind the class' definition.

@Johannes Schaub - litb 2009-09-11 18:00:16

Yeah, that's what i meant. So you can do the following: struct foo { void b() { f(); } void f(); }, f is visible even though not declared yet. The following works too: struct foo { void b(int = bar()); typedef int bar; };. It's visible before its declaration in "all function bodies, default arguments, constructor ctor-initializers". Not in the return type :(

@sbi 2009-09-11 19:27:36

@litb: It isn't visible before it's declaration, it's only that the use of the identifier is moved behind the declaration. Yeah, I know, the effect is the same for many cases. But not for all cases, which is why I think we should use the precise explanation. -- Oops, wait. It is visible in default arguments? Well, that surely wreaks havoc with my understanding. Dammit! <pouts>

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