Introduction to declaration syntax

All six interrelated attributes (storage classes, types, scope, visibility, duration, and linkage) are determined in diverse ways by declarations.

Declarations can be defining declarations (also known as definitions) or referencing declarations (sometimes known as nondefining declarations). A defining declaration, as the name implies, performs both the duties of declaring and defining; the nondefining declarations require a definition to be added somewhere in the program. A referencing declaration introduces one or more identifier names into a program. A definition actually allocates memory to an object and associates an identifier with that object.
Tentative definitions
The ANSI C standard supports the concept of the tentative definition. Any external data declaration that has no storage class specifier and no initializer is considered a tentative definition. If the identifier declared appears in a later definition, then the tentative definition is treated as if the extern storage class specifier were present. In other words, the tentative definition becomes a simple referencing declaration.

If the end of the translation unit is reached and no definition has appeared with an initializer for the identifier, then the tentative definition becomes a full definition, and the object defined has uninitialized (zero-filled) space reserved for it. For example,

int x;

int x; /*legal, one copy of x is reserved */
int y;
int y = 4; /* legal, y is initialized to 4 */
int z = 5;

int z = 6; /* not legal, both are initialized definitions */

Unlike ANSI C, C++ doesn't have the concept of a tentative declaration; an external data declaration without a storage class specifier is always a definition.
Possible declarations
The range of objects that can be declared includes

Variables
Functions
Classes and class members (C++)
Types
Structure, union, and enumeration tags
Structure members
Union members
Arrays of other types
Enumeration constants
Statement labels
Preprocessor macros

The full syntax for declarations is shown in Tables 2.1 through 2.3. The recursive nature of the declarator syntax allows complex declarators. You'll probably want to use typedefs to improve legibility.

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