Classes: A Deeper Look, Part 2

‘const’ (Constant) Objects and ‘const’ Member Functions

The keyword ‘const‘ can be used to specify that an object is not modifiable and that any attempt to modify the object should result in a compilation error.
C++ compilers disallow non-const member function calls on const objects.
An attempt by a const member function to modify an object of its class is a compilation error.
A member function (behavior) is specified as const both in its prototype and in its definition.
A const object must be initialized.
Constructors & destructors cannot be declared const.
const data member & reference data members must be initialized using member initializers.

Composition: Objects as Members of Classes

A class can have objects of other classes as members–this concept is called composition.
Member objects are constructed in the order in which they’re declared in the class definition, & before their enclosing class objects are constructed.
If a member initializer is not provided for a member object, the member object’s default constructor will be called implicitly.

A ‘friend‘ function of a class is defined outside that class’s scope, yet has the right to access all of the class’s members.
Stand-alone functions or entire classes may be declared to be friends.
A friend declaration can appear anywhere in the class.
The friendship relation is neither symmetric nor transitive.

Every object has access to its own address through the ‘this‘ pointer.
An object’s ‘this‘ pointer is not part of the object itself–i.e., the size of the memory occupied by the ‘this‘ pointer is not reflected in the result of a ‘sizeof‘ operation on the object.
The ‘this‘ pointer is passed as an implicit argument to each non-‘static’ member function.
Objects use the ‘this‘ pointer implicitly or explicitly to reference their data members (attributes) & member functions (behaviors).
The ‘this‘ pointer enables cascaded member-function calls in which multiple functions are invoked in the same statement.

A ‘static‘ data member represents “class-wide” information (i.e., a property of the class shared by all instances, not a property of a specific object of the class).
‘static‘ data members have ‘class‘ scope & can be declared ‘public‘, ‘private‘, or ‘protected‘.
A class’s ‘static‘ members exist even when no objects of that class exist.
To access a ‘public static‘ class member when no objects of the class exist, simply prefix the class name & the binary scope resolution operator ( :: ) to the name of the data member.
A member function should be declared ‘static‘ if it does not access ‘non-static‘ data members or non-‘static‘ member functions of the class. Unlike non-‘static‘ member functions, a ‘static‘ member function does not have a ‘this‘ pointer, because ‘static‘ data members & ‘static‘ member functions exist independently of any objects of a class.

Abstract data types are ways of representing real-world & conceptual notions to some satisfactory level of precision within a computer system.
An abstract data type captures two notions: a data representation & the operations that can be performed on that data.