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Static (C++)

The static keyword can be used to declare variables, functions, class data members and class functions.

By default, an object or variable that is defined outside all blocks has static duration and external linkage. Static duration means that the object or variable is allocated when the program starts and is deallocated when the program ends. External linkage means that the name of the variable is visible from outside the file in which the variable is declared. Conversely, internal linkage means that the name is not visible outside the file in which the variable is declared.

The static keyword can be used in the following situations.

  • When you declare a variable or function at file scope (global and/or namespace scope), the static keyword specifies that the variable or function has internal linkage. When you declare a variable, the variable has static duration and the compiler initializes it to 0 unless you specify another value.

  • When you declare a variable in a function, the static keyword specifies that the variable retains its state between calls to that function.

  • When you declare a data member in a class declaration, the static keyword specifies that one copy of the member is shared by all instances of the class. A static data member must be defined at file scope. An integral data member that you declare as const static can have an initializer.

  • When you declare a member function in a class declaration, the static keyword specifies that the function is shared by all instances of the class. A static member function cannot access an instance member because the function does not have an implicit this pointer. To access an instance member, declare the function with a parameter that is an instance pointer or reference.

  • You cannot declare the members of a union as static. However, a globally declared anonymous union must be explicitly declared static.

For more information, see auto, extern, and register.

The following example shows how a variable declared static in a function retains its state between calls to that function.

// static1.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
void showstat( int curr ) {
   static int nStatic;    // Value of nStatic is retained
                          // between each function call
   nStatic += curr;
   cout << "nStatic is " << nStatic << endl;
}

int main() {
   for ( int i = 0; i < 5; i++ )
      showstat( i );
}
nStatic is 0
nStatic is 1
nStatic is 3
nStatic is 6
nStatic is 10

The following example shows the use of static in a class.

// static2.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
class CMyClass {
public:
   static int m_i;
};

int CMyClass::m_i = 0;
CMyClass myObject1;
CMyClass myObject2;

int main() {
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   myObject1.m_i = 1;
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   myObject2.m_i = 2;
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   CMyClass::m_i = 3;
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;
}
0
0
1
1
2
2
3
3

The following example shows a local variable declared static in a member function. The static variable is available to the whole program; all instances of the type share the same copy of the static variable.

NoteNote

Assigning a value to a static local variable in a multithreaded application is not thread safe and we do not recommend it as a programming practice.

// static3.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;
struct C {
   void Test(int value) {
      static int var = 0;
      if (var == value) 
         cout << "var == value" << endl;
      else
         cout << "var != value" << endl;

      var = value;
   }
}; 

int main() {
   C c1;
   C c2;
   c1.Test(100);
   c2.Test(100);
}
var != value
var == value

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