Storage classes (C++)

  • Article

 

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A storage class in the context of C++ variable declarations is a type specifier that governs the lifetime, linkage, and memory location of objects and. A given object can have only one storage class. Variables defined within a block have automatic storage unless otherwise specified using the extern, static, or thread_local specifiers. Automatic objects and variables have no linkage; they are not visible to code outside the block.

Notes

  1. The mutable keyword may be considered a storage class specifier. However, it is only available in the member list of a class definition.

  2. Starting with Visual C++ 2010, the auto keyword is no longer a C++ storage-class specifier, and the register keyword is deprecated.

  • Static

  • extern

  • thread_local

static

The static keyword can be used to declare variables and functions at global scope, namespace scope, and class scope. Static variables can also be declared at local scope.

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. By default, an object or variable that is defined in the global namespace has static duration and external linkage. The static keyword can be used in the following situations.

  1. 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.

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

  3. 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.

  4. 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.

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

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.

// 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

Starting in C++11, a static local variable initialization is guaranteed to be thread-safe. This feature is sometimes called magic statics. However, in a multithreaded application all subsequent assignments must be synchronized. The thread-safe statics feature can be disabled by using the /Zc:threadSafeInit- flag to avoid taking a dependency on the CRT.

extern

Objects and variables declared as extern declare an object that is defined in another translation unit or in an enclosing scope as having external linkage.

Declaration of const variables with the extern storage class forces the variable to have external linkage. An initialization of an extern const variable is allowed in the defining translation unit. Initializations in translation units other than the defining translation unit produce undefined results. For more information, see Using extern to Specify Linkage

The following code shows two extern declarations, DefinedElsewhere (which refers to a name defined in a different translation unit) and DefinedHere (which refers to a name defined in an enclosing scope):

// external.cpp  
// defined in another translation unit  
extern int DefinedElsewhere;     
int main() {  
   int DefinedHere;   
   {  
      // refers to DefinedHere in the enclosing scope  
      extern int DefinedHere;  
    }  
}

thread_local (C++11)

A variable declared with the thread_local specifier is accessible only on the thread on which it is created. The variable is created when the thread is created, and destroyed when the thread is destroyed. Each thread has its own copy of the variable. On Windows, thread_local is functionally equivalent to the Microsoft-specific __declspec( thread ) attribute.

thread_local float f = 42.0; //global namespace  
  
struct C // cannot be applied to type definition  
{  
thread_local int i; //local  
thread_local static char buf[10]; // local and static  
};  
  
void DoSomething()  
{  
thread_local C my_struct; // Apply  thread_local to a variable  
}

  1. The thread_local specifier may be combined with static or extern.

  2. You can apply thread_local only to data declarations and definitions; thread_local cannot be used on function declarations or definitions.

  3. The use of thread_local may interfere with delay loading of DLL imports.

  4. On XP systems, thread_local may not function correctly if a DLL uses thread_local data and it is loaded dynamically via LoadLibrary.

  5. You can specify thread_local only on data items with static storage duration. This includes global data objects (both static and extern), local static objects, and static data members of classes. You cannot declare automatic data objects with thread_local.

  6. You must specify thread_local for both the declaration and the definition of a thread local object, whether the declaration and definition occur in the same file or separate files.

On Windows, thread_local is functionally equivalent to __declspec(thread) except that __declspec(thread) can be applied to a type definition and is valid in C code. Whenever possible, use thread_local because it is part of the C++ standard and is therefore more portable.

For more information, see Thread Local Storage (TLS).

register

In C++11, the register keyword is deprecated. It specifies that the variable is to be stored in a machine register, if possible. Only function arguments and local variables can be declared with the register storage class.

register int num;

Like automatic variables, register variables persist only until the end of the block in which they are declared.

The compiler does not honor user requests for register variables; instead, it makes its own register choices when global optimizations are on. However, all other semantics associated with the register keyword are honored by the compiler.

If the address-of operator (&) is used on an object that is declared with register, the compiler must put the object in memory rather than a register.

Example: automatic vs. static initialization

A local automatic object or variable is initialized every time the flow of control reaches its definition. A local static object or variable is initialized the first time the flow of control reaches its definition.

Consider the following example, which defines a class that logs initialization and destruction of objects and then defines three objects, I1, I2, and I3:

// initialization_of_objects.cpp  
// compile with: /EHsc  
#include <iostream>  
#include <string.h>  
using namespace std;  
  
// Define a class that logs initializations and destructions.  
class InitDemo {  
public:  
   InitDemo( const char *szWhat );  
   ~InitDemo();  
  
private:  
   char *szObjName;  
   size_t sizeofObjName;  
};  
  
// Constructor for class InitDemo  
InitDemo::InitDemo( const char *szWhat ) :  
   szObjName(NULL), sizeofObjName(0) {  
   if( szWhat != 0 && strlen( szWhat ) > 0 ) {  
      // Allocate storage for szObjName, then copy  
      // initializer szWhat into szObjName, using  
      // secured CRT functions.  
      sizeofObjName = strlen( szWhat ) + 1;  
  
      szObjName = new char[ sizeofObjName ];  
      strcpy_s( szObjName, sizeofObjName, szWhat );  
  
      cout << "Initializing: " << szObjName << "\n";  
   }  
   else  
      szObjName = 0;  
}  
  
// Destructor for InitDemo  
InitDemo::~InitDemo() {  
   if( szObjName != 0 ) {  
      cout << "Destroying: " << szObjName << "\n";  
      delete szObjName;  
   }  
}  
  
// Enter main function  
int main() {  
   InitDemo I1( "Auto I1" ); {  
      cout << "In block.\n";  
      InitDemo I2( "Auto I2" );  
      static InitDemo I3( "Static I3" );  
   }  
   cout << "Exited block.\n";  
}  

Initializing: Auto I1  
In block.  
Initializing: Auto I2  
Initializing: Static I3  
Destroying: Auto I2  
Exited block.  
Destroying: Auto I1  
Destroying: Static I3

The preceding code demonstrates how and when the objects I1, I2, and I3 are initialized and when they are destroyed.

There are several points to note about the program.

First, I1 and I2 are automatically destroyed when the flow of control exits the block in which they are defined.

Second, in C++, it is not necessary to declare objects or variables at the beginning of a block. Furthermore, these objects are initialized only when the flow of control reaches their definitions. (I2 and I3 are examples of such definitions.) The output shows exactly when they are initialized.

Finally, static local variables such as I3 retain their values for the duration of the program, but are destroyed as the program terminates.

See Also

Declarations and Definitions