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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".
Like raw strings, there can be any number of equals signs between the square brackets, provided both the opening and closing tags have a matching number of equals signs; this allows nesting as long as nested block comments/raw strings use a different number of equals signs than their enclosing comment: --[[comment --[=[ nested comment ...
In languages such as Pascal that require declaration before use, mutually recursive functions require forward declaration, as a forward reference cannot be avoided when defining them. As with directly recursive functions, a wrapper function may be useful, with the mutually recursive functions defined as nested functions within its scope if this ...
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.
C++ doesn't have: PROC – first class nested functions (emulation due to local definitions of class types, which could be functors; also, C++11 has lambda functions), OP and PRIO – definable operator symbols and priorities, garbage collection (could be emulated with help of smart pointers), use before define,