Initialization and storage classes

This topic includes descriptions of the following:

Initialization of automatic variables
Initialization of static variables
Initialization of external variables
Initialization of register variables

Initialization of automatic variables

You can initialize any auto variable except function parameters. If you do not explicitly initialize an automatic object, its value is indeterminate. If you provide an initial value, the expression representing the initial value can be any valid C or C++ expression. The object is then set to that initial value each time the program block that contains the object's definition is entered.

Note that if you use the goto statement to jump into the middle of a block, automatic variables within that block are not initialized.

Note:

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). C++11

Initialization of static variables

You can initialize a static object with a constant expression, or an expression that reduces to the address of a previously declared extern or static object, possibly modified by a constant expression. If you do not explicitly initialize a static (or external) variable, it will have a value of zero of the appropriate type, unless it is a pointer, in which case it will be initialized to NULL.

C only A static variable in a block is initialized only one time, prior to program execution, whereas an auto variable that has an initializer is initialized every time it comes into existence. C only

C++ only A static variable in a block can be dynamically initialized when the flow of control passes through its definition in a block for the first time. Dynamic initialization of a static variable can occur with non-constant expressions. A static object of class type will use the default constructor if you do not initialize it. C++ only

Initialization of external variables

You can initialize any object with the extern storage class specifier at global scope in C or at namespace scope in C++. The initializer for an extern object must either:

Appear as part of the definition, and the initial value must be described by a constant expression;
Appear as part of the definition.
Reduce to the address of a previously declared object with static storage duration. You may modify this object with pointer arithmetic. (In other words, you may modify the object by adding or subtracting an integral constant expression.)

If you do not explicitly initialize an extern variable, its initial value is zero of the appropriate type. Initialization of an extern object is completed by the time the program starts running.

Initialization of register variables

You can initialize any register object except function parameters. If you do not initialize an automatic object, its value is indeterminate. If you provide an initial value, the expression representing the initial value can be any valid C or C++ expression. The object is then set to that initial value each time the program block that contains the object's definition is entered.