What is the difference between a static and const variable?

Can someone explain the difference between a static and const variable?

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.

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 <fenv.h>
#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.

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.

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.

What is the difference between a static and const variable?

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