The differences between initialize define declare a variable

Question

After reading the question  I know the differences between declaration and definition  So does it mean definition equals declaration plus initialization

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Declaration  Declaration  generally  refers to the introduction of a new name in the program  For example  you can declare a new function by describing it s  signature    void xyz      or declare an incomplete type   class klass  struct ztruct    and last but not least  to declare an object   int x    It is described  in the C   standard  at   3 1 1 as      A declaration  Clause 7  may introduce one or more names into a translation unit or redeclare names introduced by previous declarations    Definition  A definition is a definition of a previously declared name  or it can be both definition and declaration   For example   int x  void xyz         class klass        struct ztruct        enum   x  y  z      Specifically the C   standard defines it  at   3 1 1  as      A declaration is a definition unless it declares a function without specifying the function   s body  8 4   it contains the extern specifier  7 1 1  or a linkage-specification25  7 5  and neither an initializer nor a function- body  it declares a static data member in a class definition  9 2  9 4   it is a class name declaration  9 1   it is an opaque-enum-declaration  7 2   it is a template-parameter  14 1   it is a parameter-declaration  8 3 5  in a function declarator that is not the declarator of a function-definition  or it is a typedef declaration  7 1 3   an alias-declaration  7 1 3   a using-declaration  7 3 3   a static assert-declaration  Clause 7   an attribute- declaration  Clause 7   an empty-declaration  Clause 7   or a using-directive  7 3 4     Initialization  Initialization refers to the  assignment  of a value  at construction time  For a generic object of type T  it s often in the form   T x   i    but in C   it can be   T x i     or even   T x  i     with C  11   Conclusion     So does it mean definition equals declaration plus initialization    It depends  On what you are talking about  If you are talking about an object  for example   int x    This is a definition without initialization  The following  instead  is a definition with initialization   int x   0    In certain context  it doesn t make sense to talk about  initialization    definition  and  declaration   If you are talking about a function  for example  initialization does not mean much   So  the answer is no  definition does not automatically mean declaration plus initialization

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For C  at least  per C11 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  is the  only  declaration of the identifier    for a typedef name  is the first  or only  declaration of the identifier       Per C11 6 7 9 8-10      An initializer specifies the initial value stored in an object     if   an object that has automatic storage is not initialized explicitly    its value is indeterminate    So  broadly speaking  a declaration introduces an identifier and provides information about it  For a variable  a definition is a declaration which allocates storage for that variable   Initialization is the specification of the initial value to be stored in an object  which is not necessarily the same as the first time you explicitly assign a value to it  A variable has a value when you define it  whether or not you explicitly give it a value  If you don t explicitly give it a value  and the variable has automatic storage  it will have an initial value  but that value will be indeterminate  If it has static storage  it will be initialized implicitly depending on the type  e g  pointer types get initialized to null pointers  arithmetic types get initialized to zero  and so on    So  if you define an automatic variable without specifying an initial value for it  such as   int myfunc void        int myvar            You are defining it  and therefore also declaring it  since definitions are declarations   but not initializing it  Therefore  definition does not equal declaration plus initialization

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Declaration says  this thing exists somewhere    int foo             function extern int bar      variable struct T      static int baz      static member variable      Definition says  this thing exists here  make memory for it    int foo             function int bar             variable int T  baz          static member variable   Initialisation is optional at the point of definition for objects  and says  here is the initial value for this thing    int bar   0         variable int T  baz   42     static member variable   Sometimes it s possible at the point of declaration instead   struct T      static int baz   42        hellip but that s getting into more complex features

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So does it mean definition equals declaration plus initialization    Not necessarily  your declaration might be without any variable being initialized like    void helloWorld      declaration or Prototype    void helloWorld          std  cout  lt  lt   Hello World n

[c++] The differences between initialize, define, declare a variable

Declaration

Definition

Initialization

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