| The terms declaration and definition are often used interchangeably in common "programmer talk." In the context of the ODR, however, the exact meaning of these words is important. [1] [1] We also think it's a good habit to handle the terms carefully when exchanging ideas about C or C++. We do so throughout this book. A declaration is a C++ construct that introduces or reintroduces a name in your program. A declaration can also be a definition, depending on which entity it introduces and how it introduces it: -
Namespaces and namespace aliases: The declarations of namespaces and their aliases are always also definitions, although the term definition is unusual in this context because the list of members of a namespace can be "extended" at a later time (unlike classes and enumeration types for example). -
Classes, class templates, functions, function templates, member functions, and member function templates: The declaration is a definition if and only if the declaration includes a brace - enclosed body associated with the name. This rule includes unions, operators, member operators, static member functions, constructors and destructors, and explicit specializations of template versions of such things (that is, any class-like and function-like entity). -
Enumerations: The declaration is a definition if and only if it includes the brace-enclosed list of enumerators. -
Local variables and nonstatic data members: These entities can always be treated as definitions, although the distinction rarely matters. -
Global variables: If the declaration is not directly preceded by a keyword extern or if it has an initializer, the declaration of a global variable is also a definition of that variable. Otherwise, it is not a definition. -
Static data members: The declaration is a definition if and only if it appears outside the class or class template of which it is a member. -
Typedefs , using-declarations, and using-directives: These are never definitions, although typedefs can be combined with class or union definitions. -
Explicit instantiation directives: We can consider them to be definitions.  |
