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Forward declaration of a class is not sufficient if you need to use the actual class type, for example, if you have a member whose type is that class directly (not a pointer), or if you need to use it as a base class, or if you need to use the methods of the class in a method. In Objective-C, classes and protocols can be forward-declared like this:
The curiously recurring template pattern (CRTP) is an idiom, originally in C++, in which a class X derives from a class template instantiation using X itself as a template argument. [1] More generally it is known as F-bound polymorphism , and it is a form of F -bounded quantification .
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".
UML class diagram that illustrates forwarding. In this Java example, the Printer class has a print method. This print method, rather than performing the print itself, forwards to an object of class RealPrinter. To the outside world it appears that the Printer object is doing the print, but the RealPrinter object is the one actually doing the work.
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 ...
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,
Forward-declaring enums is also possible in C++11. Formerly, enum types could not be forward-declared because the size of the enumeration depends on the definition of its members. As long as the size of the enumeration is specified either implicitly or explicitly, it can be forward-declared: