A class in C++ is a user-defined type or

data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the rel ...

declared with

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class that has data and functions (also called

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s and

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s) as its members whose access is governed by the three access specifiers ''private'', ''protected'' or ''public''. By default access to members of a C++ class is ''private''. The private members are not accessible outside the class; they can be accessed only through methods of the class. The public members form an

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to the class and are accessible outside the class. Instances of a class data type are known as

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and can contain member variables,

constant Constant or The Constant may refer to: Mathematics * Constant (mathematics), a non-varying value * Mathematical constant, a special number that arises naturally in mathematics, such as or Other concepts * Control variable or scientific const ...

s, member functions, and overloaded operators defined by the programmer.

Differences between a structure and a class in C++

In C++, a class defined with the class keyword has

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members and base classes by default. A structure is a class defined with the struct keyword.

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(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §9 Classes

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' para. 4 Its members and base classes are

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by default. In practice, structs are typically reserved for data without functions. When deriving a struct from a class/struct, default access-specifier for a base class/struct is public. And when deriving a class, default access specifier is private.

Aggregate classes

An aggregate class is a class with no user-declared constructors, no private or protected non-static data members, no base classes, and no virtual functions.

(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §8.5.1 Aggregates cl.init.aggr' para. 1 Such a class can be initialized with a brace-enclosed comma-separated list of initializer-clauses.

(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §8.5.1 Aggregates cl.init.aggr' para. 2 The following code has the same semantics in both C and C++. struct C ; struct D ; // initialize an object of type C with an initializer-list C c = ; // D has a sub-aggregate of type C. In such cases initializer-clauses can be nested D d = ;

POD-structs

A POD-struct (Plain Old Data Structure) is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined

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and no user-defined destructor. A POD-struct could be said to be the C++ equivalent of a C struct. In most cases, a POD-struct will have the same memory layout as a corresponding struct declared in C. For this reason, POD-structs are sometimes colloquially referred to as "C-style structs".

Properties shared between structs in C and POD-structs in C++

*Data members are allocated so that later members have higher addresses within an object, except where separated by an access-specifier.

(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §9.2 Class members lass.mem' para. 12 *Two POD-struct types are layout-compatible if they have the same number of nonstatic data members, and corresponding nonstatic data members (in order) have layout-compatible types.

(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §9.2 Class members lass.mem' para. 14 *A POD-struct may contain unnamed

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(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §9.2 Class members lass.mem' para. 17 *A pointer to a POD-struct object, suitably converted using a

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, points to its initial member and vice versa, implying that there is no padding at the beginning of a POD-struct. *A POD-struct may be used with the

offsetof C's macro is an ANSI C library feature found in . It evaluates to the offset (in bytes) of a given member within a struct or union type, an expression of type . The offsetof() macro takes two parameters, the first being a structure name, and the ...

macro.

(2003). '' ISO/IEC 14882:2003(E): Programming Languages - C++ §18.1 Types ib.support.types' para. 5

Declaration and usage

C++ classes have their own members. These members include variables (including other structures and classes), functions (specific identifiers or overloaded operators) known as methods, constructors and destructors. Members are declared to be either publicly or privately accessible using the public: and private: access specifiers respectively. Any member encountered after a specifier will have the associated access until another specifier is encountered. There is also inheritance between classes which can make use of the protected: specifier.

Global and local class

A class defined outside all methods is a global class because its objects can be created from anywhere in the program. If it is defined within a function body then it's a local class because objects of such a class are local to the function scope.

Basic declaration and member variables

Classes are declared with the class or struct

. Declaration of members are placed within this declaration. The above definitions are functi