What is the difference between a static and const variable

Question

Can someone explain the difference between a static and const variable

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Static variables are common across all instances of a type.

constant variables are specific to each individual instance of a type but their values are known and fixed at compile time and it cannot be changed at runtime.

unlike constants, static variable values can be changed at runtime.

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Static variables in the context of a class are shared between all instances of a class   In a function  it remains a persistent variable  so you could for instance count the number of times a function has been called   When used outside of a function or class  it ensures the variable can only be used by code in that specific file  and nowhere else   Constant variables however are prevented from changing  A common use of const and static together is within a class definition to provide some sort of constant    class myClass   public       static const int TOTAL NUMBER   5          some public stuff private          some stuff

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Constant variables cannot be changed  Static variable are private to the file and only accessible within the program code and not to anyone else

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A constant value cannot change  A static variable exists to a function  or class  rather than an instance or object   These two concepts are not mutually exclusive  and can be used together

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static value may exists into a function and can be used in different forms and can have different value in the program   Also during program after increment of decrement their value may change but const in constant during the whole program

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static keyword defines the scope of variables whereas const keyword defines the value of variable that can t be changed during program execution

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Const means    cannot be changed      Static means    static instance  in memory  vs dynamic instance  on the stack      Static variables exist for the duration of the program   Dynamic ones are created and destroyed as needed   A variable can be one or both

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A static variable can get an initial value only one time  This means that if you have code such as  static int a 0  in a sample function  and this code is executed in a first call of this function  but not executed in a subsequent call of the function  variable  a  will still have its current value  for example  a current value of 5   because the static variable gets an initial value only one time   A constant variable has its value constant in whole of the code  For example  if you set the constant variable like  const int a 5   then this value for  a  will be constant in whole of your program

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static   is used for making the variable a class variable  You need not define a static variable while declaring   Example    include  lt iostream gt   class dummy           public                  static int dum       int dummy  dum   0    This is important for static variable  otherwise you d get a linking error  int main             dummy d          d dum   1           std  cout lt  lt  Printing dum from object    lt  lt d dum lt  lt std  endl          std  cout lt  lt  Printing dum from class    lt  lt dummy  dum lt  lt std  endl           return 0      This would print  Printing dum from object  1 Printing dum from class  1  The variable dum is a class variable  Trying to access it via an object just informs the compiler that it is a variable of which class   Consider a scenario where you could use a variable to count the number of objects created  static would come in handy there      const   is used to make it a read-only variable  You need to define and declare the const variable at once   In the same program mentioned above  let s make the dum a const as well   class dummy           public                  static const int dum     This would give an error  You need to define it as well                 static const int dum   1    this is correct                 const int dum   1    Correct  Just not making it a class variable       Suppose in the main  I am doing this   int main             dummy d          d dum   1    Illegal           std  cout lt  lt  Printing dum from object    lt  lt d dum lt  lt std  endl          std  cout lt  lt  Printing dum from class    lt  lt dummy  dum lt  lt std  endl           return 0      Though static has been manageable to understand  const is messed up in c    The following resource helps in understanding it better  http   duramecho com ComputerInformation WhyHowCppConst html

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const means constant and their values are defined at compile time rather than explicitly change it during run time also  the value of constant cannot be changed during runtime  However static variables are variables that can be initialised and changed at run time  However  static are different from the variables in the sense that static variables retain their values for the whole of the program ie their lifetime is of the program or until the memory is de allocated by the program by using dynamic allocation method  However  even though they retain their values for the whole lifetime of the program they are inaccessible outside the code block they are in  For more info on static variables refer here

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The short answer   A const is a promise that you will not try to modify the value once set    A static variable means that the object s lifetime is the entire execution of the program and it s value is initialized only once before the program startup  All statics are initialized if you do not explicitly set a value to them The manner and timing of static initialization is unspecified   C99 borrowed the use of const from C    On the other hand  static has been the source of many debates  in both languages  because of its often confusing semantics   Also  with C  0x until C  11 the use of the static keyword was deprecated for declaring objects in namespace scope  This deprecation was removed in C  11 for various reasons  see here    The longer answer  More on the keywords than you wanted to know  right from the standards    C99   include  lt fenv h gt   pragma STDC FENV ACCESS ON     file scope  static storage  internal linkage    static int i1     tentative definition  internal linkage extern int i1     tentative definition  internal linkage  int i2     external linkage  automatic duration  effectively lifetime of program   int  p    int     2  4      unnamed array has static storage     effect on string literals    char  s     tmp fileXXXXXX      static storage always  may not be modifiable char  p    char       tmp fileXXXXXX       static  modifiable const char  cp    const char       tmp fileXXXXXX       static  non-modifiable   void f int m        static int vla  m       err      float w       0 0 0 0       raises an exception         block scope  static storage  no-linkage        static float x   0 0 0 0     does not raise an exception                       effect on string literals        char  s     tmp fileXXXXXX      static storage always  may not be modifiable     char  p    char       tmp fileXXXXXX       automatic storage  modifiable     const char  cp    const char       tmp fileXXXXXX       automatic storage  non-modifiable     inline void bar void         const static int x   42     ok         Note  Since an inline definition is distinct from the          corresponding external definition and from any other         corresponding inline definitions in other translation          units  all corresponding objects with static storage         duration are also distinct in each of the definitions      static int y   -42     error  inline function definition       the last declaration also specifies that the argument     corresponding to a in any call to f must be a non-null     pointer to the first of at least three arrays of 5 doubles void f double a static 3  5     static void g void      internal linkage   C    Has the same semantics mostly except as noted in the short answer  Also  there are no parameter qualifying statics     extern  C    static void f4       the name of the function f4 has                      internal linkage  not C language                      linkage  and the function   s type                      has C language linkage     class S      mutable static int i     err    mutable static int j     err    static int k     ok  all instances share the same member     inline void bar void         const static int x   42     ok      static int y   -42     ok     There are a few more nuances of C   s static that I leave out here  Have a look at a book or the standard

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const is equivalent to  define but only for value statements e g   define myvalue   2   The value declared replaces the name of the variable before compilation   static is a variable  The value can change  but the variable will persist throughout the execution of the program even if the variable is declared in a function  It is equivalent to a global variable who s usage scope is the scope of the block they have been declared in  but their value s scope is global   As such  static variables are only initialized once  This is especially important if the variable is declared in a function  since it guarantees the initialization will only take place at the first call to the function   Another usage of statics involves objects  Declaring a static variable in an object has the effect that this value is the same for all instances of the object  As such  it cannot be called with the object s name  but only with the class s name   public class Test         public static int test    Test myTestObject new Test    myTestObject test 2   ERROR Test test 2   Correct   In languages like C and C    it is meaningless to declare static global variables  but they are very useful in functions and classes  In managed languages  the only way to have the effect of a global variable is to declare it as static

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Static Variables    Initialized only once  Static variables are for the class  not per object   i e memory is allocated only once per class and every instance uses it  So if one object modifies its value then the modified value is visible to other objects as well    A simple thought   To know the number of objects created for a class we can put a static variable and do    in constructor  Value persists between different function calls   Const Variables    Const variables are a promise that you are not going to change its value anywhere in the program  If you do it  it will complain

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Simple and short answer is memory is allocated for static and const only once  But in const that is for only one value where as in static values may change but the memory area remains the same until the end of the program

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static is a storage specifier  const is a type qualifier

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static means local for compilation unit  i e  a single C   source code file   or in other words it means it is not added to a global namespace  you can have multiple static variables in different c   source code files with the same name and no name conflicts   const is just constant  meaning can t be modified

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Constants can t be changed  static variables have more to do with how they are allocated and where they are accessible   Check out this site

[c++] What is the difference between a static and const variable?

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