Storage Classes and Namespaces

Storage Classes Of Objects

Availability of Objects

C++ program

Storage Class Specifiers

The storage class of an object is determined by

• the position of its declaration in the source file
• the storage class specifier, which can be supplied optionally.

The following storage class specifiers can be used

extern   static   auto   register 

When an object is declared, not only are the object's type and name defined but also its storage class. The storage class specifies the lifetime of the object, that is, the period of time from the construction of the object until its destruction. In addition, the storage class delimits the part of the program in which the object can be accessed directly by its name, the so-called object scope.

Essentially, an object is only available after you have declared it within a translation unit. A translation unit, also referred to as module, comprises the source file you are compiling and any header files you have included.

As a programmer, you can define an object with:

block scope	The object is only available in the code block in which it was defined. The object is no longer visible once you have left the code block.
file scope	The object can be used within a single module. Only the functions within this module can reference the object. Other modules cannot access the object directly.
program scope	The object is available throughout the program, providing a common space in memory that can be referenced by any program function. For this reason, these objects are often referred to as global.

Access to an object as defined by the object's storage class is independent of any access controls for the elements of a class. Namespaces that subdivide program scope and classes will be introduced at a later stage.

Lifetime

Objects with block scope are normally created automatically within the code block that defines them. Such objects can only be accessed by statements within that block and are called local to that block. The memory used for these objects is freed after leaving the code block. In this case, the lifetime of the objects is said to be automatic.

However, it is possible to define objects with block scope that are available throughout the runtime of a program. The lifetime of these objects is said to be static. When the program flow re-enters a code block, any pre-existing conditions will apply.

Objects with program and file scope are always static. These objects are created when a program is launched and are available until the program is terminated.

Four storage classes are available for creating objects with the scope and lifetime you need. These storage classes will be discussed individually in the following sections.