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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:
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.
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".
The class will be accessible to other classes in the same package but will be inaccessible to classes outside the package. When we say that a class is inaccessible, it simply means that we cannot create an object of that class or declare a variable of that class type. The protected access specifier too cannot be applied to a class.
For these reasons, for C++ code to call a C function foo(), the C++ code must prototype foo() with extern "C". Likewise, for C code to call a C++ function bar(), the C++ code for bar() must be declared with extern "C". A common practice for header files to maintain both C and C++ compatibility is to make its declaration be extern "C" for the ...
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.
[98] [99] [100] However, the C++11 standard introduces new incompatibilities, such as disallowing assignment of a string literal to a character pointer, which remains valid C. To intermix C and C++ code, any function declaration or definition that is to be called from/used both in C and C++ must be declared with C linkage by placing it within ...