What is the difference between a definition and a declaration

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The meaning of both eludes me

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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

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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

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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

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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

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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 lt typename T gt  class MyArray  parameter declaration - x and y in int add int x  int y   alias declaration - using IntVector   std  vector lt int gt   static assert declaration - static assert sizeof int     4   quot Yikes  quot   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  quot C quot           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 lt int gt          declares IntVector as an alias to std  vector lt int gt  static assert X  y    1   quot Oops  quot            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  lt class T gt  class C                 declares template class C                                            declares nothing

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Definition     extern int a          Declaration  int a                 Definition a   10                Initialization int b   10            Definition  amp  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  lt stdio h gt  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  lt stdio h gt   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

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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 gt 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 gt b    return a      return b           In above cases we define and declare variable i and function max

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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

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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

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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

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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

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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

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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

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definition means actual function written  amp  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

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Declaration   Somewhere  there exists a foo    Definition      and here it is

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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  lt printf plt gt  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

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There are some very clear definitions sprinkled throughout K amp 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

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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

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The declaration is when a primitive or object reference variable or method is created without assigning value or object  int a  final int a  The definition means assigning the value or object respectively int a  10  Initialization means allocating memory for a respective variable or object

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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

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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

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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  lt typename T gt  struct X      static int member     declaration     template lt typename T gt  int X lt T gt   member     definition  template lt  gt  int X lt bool gt   member     declaration    The last declaration was not a definition  It s the declaration of an explicit specialization of the static member of X lt bool gt   It tells the compiler   If it comes to instantiating X lt bool gt   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 lt  gt  int X lt bool gt   member   1     definition  belongs into a  cpp file

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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

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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

[c] What is the difference between a definition and a declaration?

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