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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.
In C++, struct is essentially the same as for C. Further, a class is the same as a struct but with different default visibility: class members are private by default, whereas struct members are public by default.
The relevant field of a union variable is typically determined by the state of other variables, possibly in an enclosing struct. One common C programming idiom uses unions to perform what C++ calls a reinterpret_cast , by assigning to one field of a union and reading from another, as is done in code which depends on the raw representation of ...
This article, however, focuses on differences that cause conforming C code to be ill-formed C++ code, or to be conforming/well-formed in both languages but to behave differently in C and C++. Bjarne Stroustrup , the creator of C++, has suggested [ 4 ] that the incompatibilities between C and C++ should be reduced as much as possible in order to ...
^a Only classes are supported. ^b structs in C++ are actually classes, but have default public visibility and are also POD objects. C++11 extended this further, to make classes act identically to POD objects in many more cases. ^c pair only
The fact that structs in C++ are classes/objects with members that are public by default should not be of any real interest, unless you're writing code to parse C++ source. There's a clear conceptual difference between a struct and a class, and C++'s "structs" are not really structs at all. --StuartBrady 20:35, 4 March 2006 (UTC)
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 ...
Therefore, a reference to the structure field my_addr->sin_family (where my_addr is of type struct sockaddr*) will actually refer to the field sa.sin_family (where sa is of type struct sockaddr_in). In other words, the sockets library uses type punning to implement a rudimentary form of polymorphism or inheritance.