Storage class specifiers

A storage class specifier is used to refine the declaration of a variable, a function, and parameters. Storage classes determine whether:

The object has internal, external, or no linkage
The object is to be stored in memory or in a register, if available
The object receives the default initial value of 0 or an indeterminate default initial value
The object can be referenced throughout a program or only within the function, block, or source file where the variable is defined
The storage duration for the object is maintained throughout program run time or only during the execution of the block where the object is defined

For a variable, its default storage duration, scope, and linkage depend on where it is declared: whether inside or outside a block statement or the body of a function. When these defaults are not satisfactory, you can use a storage class specifier to explicitly set its storage class. The storage class specifiers in C and C++ are:

auto
static
extern
mutable
register
__thread

In C++11, the keyword auto is no longer used as a storage class specifier. Instead, it is used as a type specifier. The compiler deduces the type of an auto variable from the type of its initializer expression. For more information, see The auto type specifier (C++11).

The keyword extern was previously used as a storage specifier or as part of a linkage specification. The C++11 standard adds a third usage to use this keyword to specify explicit instantiation declarations. For more information, see Explicit instantiation (C++ only).