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Declarations and Definitions (C++)

Declarations tell the compiler that a program element or name exists. Definitions specify what code or data the name describes. A name must be declared before it can be used.

A declaration introduces one or more names into a program. Declarations can occur more than once in a program. Therefore, classes, structures, enumerated types, and other user-defined types can be declared for each compilation unit. The constraint on this multiple declaration is that all declarations must be identical. Declarations also serve as definitions, except when the declaration:

  1. Is a function prototype (a function declaration with no function body).

  2. Contains the extern specifier but no initializer (objects and variables) or function body (functions). This signifies that the definition is not necessarily in the current translation unit and gives the name external linkage.

  3. Is of a static data member inside a class declaration.

    Because static class data members are discrete variables shared by all objects of the class, they must be defined and initialized outside the class declaration. (For more information about classes and class members, see Classes.)

  4. Is a class name declaration with no following definition, such as class T;.

  5. Is a typedef statement.

Examples of declarations that are also definitions are:

// Declare and define int variables i and j.
int i;
int j = 10;

// Declare enumeration suits.
enum suits { Spades = 1, Clubs, Hearts, Diamonds };

// Declare class CheckBox.
class CheckBox : public Control
{
public:
            Boolean IsChecked();
    virtual int     ChangeState() = 0;
};

Some declarations that are not definitions are:

extern int i;
char *strchr( const char *Str, const char Target );

A definition is a unique specification of an object or variable, function, class, or enumerator. Because definitions must be unique, a program can contain only one definition for a given program element. There can be a many-to-one correspondence between declarations and definitions. There are two cases in which a program element can be declared and not defined:

  1. A function is declared but never referenced with a function call or with an expression that takes the function's address.

  2. A class is used only in a way that does not require its definition be known. However, the class must be declared. The following code illustrates such a case:

    // definitions.cpp
    class WindowCounter;   // Forward reference; no definition
    
    class Window
    {
       // Definition of WindowCounter not required
       static WindowCounter windowCounter;
    };
    
    int main()
    {
    }
    

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