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In some object-oriented languages like C++ and Objective-C, it is sometimes necessary to forward-declare classes. This is done in situations when it is necessary to know that the name of the class is a type, but where it is unnecessary to know the structure. In C++, classes and structs can be forward-declared like this:
In this example, class C can forward to any class that implements an interface I. Class C has a method to switch to another forwarder. Including the implements clauses improves type safety, because each class must implement the methods in the interface. The main tradeoff is more code.
In both C and C++, one can define nested struct types, but the scope is interpreted differently: in C++, a nested struct is defined only within the scope/namespace of the outer struct, whereas in C the inner struct is also defined outside the outer struct.
Nested classes are also a feature of the D programming language, Visual Basic .NET, Ruby, C++ and C#. In Python, it is possible to nest a class within another class, method or function. C++ has nested classes that are like Java's static member classes, except that they are not declared with "static".
A related concept is inner types, also known as inner data type or nested type, which is a generalization of the concept of inner classes. C++ is an example of a language that supports both inner classes and inner types (via typedef declarations). [30] [31] A local class is a class defined within a procedure or function. Such structure limits ...
^a Pascal requires "forward;" for forward declarations. ^b Eiffel allows the specification of an application's root class and feature. ^c In Fortran, function/subroutine parameters are called arguments (since PARAMETER is a language keyword); the CALL keyword is required for subroutines.
The C++ Core Guidelines [91] are an initiative led by Bjarne Stroustrup, the inventor of C++, and Herb Sutter, the convener and chair of the C++ ISO Working Group, to help programmers write 'Modern C++' by using best practices for the language standards C++11 and newer, and to help developers of compilers and static checking tools to create ...
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.