What is the difference between a definition and a declaration?

c declaration terminology definition c++-faq

Declaration of a variable is for informing to the compiler the following information: name of the variable, type of value it holds and the initial value if any it takes. i.e., declaration gives details about the properties of a variable. Whereas, Definition of a variable says where the variable gets stored. i.e., memory for the variable is allocated during the definition of the variable.

Declaration says "this thing exists somewhere"

int sampleFunc(); // function
extern int car;  // variable

Definition says "this thing exists here; make memory for it"

int sampleFunc() {} // function
int car; // variable

Initialization is optional at the point of definition for objects, and says "here is the initial value for this thing":

int car = 0; // variable

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.

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

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

Adding definition and declaration examples from the C++ standard document(from the section 3.1 Declarations and definitions)

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

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

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.

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.

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]

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.

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.

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.

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.

Declaration: "Somewhere, there exists a foo."

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

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

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

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.

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 <printf@plt>
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.

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.

Definition :

extern int a;      // Declaration 
int a;             // Definition
a = 10             // Initialization
int b = 10;        // Definition & Initialization

Definition associates the variable with a type and allocates memory, whereas declaration just specifies the type but doesn't allocate memory. Declaration is more useful when you want to refer the variable before definition.

*Don't confuse definition with initialization. Both are different, initialization gives value to the variable. See the above example.

Following are some examples of definition.

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);
}

The meaning of both eludes me.