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The enclosing class can be instantiated, rather a new derived class, which provides the definition of the method, would need to be created in order to create an instance of the class. Starting with Java 8 , the lambda expression was included in the language, which could be viewed as a function declaration.
In object-oriented programming, a class defines the shared aspects of objects created from the class. The capabilities of a class differ between programming languages, but generally the shared aspects consist of state and behavior that are each either associated with a particular object or with all objects of that class. [1] [2]
A class in C++ is a user-defined type or data structure declared with any of the keywords class, struct or union (the first two are collectively referred to as non-union classes) that has data and functions (also called member variables and member functions) as its members whose access is governed by the three access specifiers private, protected or public.
Using this structure, a class definition file containing the declaration of the class and its members is also created. If the class definition has been included and the implementation file for its methods is available, the user can instantiate an object of the class. The purpose of this structure is to keep the implementation code hidden, but ...
Terminology: class declaration vs definition [ edit ] According to the C++ standard section 9.2 (I'm reading a draft), class is declared immediately after the class-name is seen, and is considered defined after the closing brace of its class-specifier has been seen even though its member functions are in general not yet defined.
This is especially useful inside class definitions, e.g. if a class contains a member that is a pointer (or a reference) to another class. Forward-declaration is used to avoid unnecessary coupling which help reducing compilation time by reducing the number of header inclusion. This has a triple advantage:
A definition provides an instance. In the entire program, an object or non-inline function cannot have more than one definition; if an object or function is used, it must have exactly one definition. You can declare an object or function that is never used, in which case you don't have to provide a definition.
A class or abstract class may implement one or more interfaces. An interface can only extend other interfaces. An abstract class may have non-public methods and properties (also abstract ones). An interface can only have public members. An abstract class may have constants, static methods and static members. An interface cannot.