C++ Training Datascope Lawrence D’Antonio Lecture 4 An Overview of C++: What is a Class/Object?

What is a class?  A class is a set of objects sharing common features.  A class defines an object’s attributes and behavior. Methods are provided to act on an object and to pass messages between objects.  A class is the basic unit of abstraction.  A class is the basic unit of modularity.  A class can be concrete or abstract.

Class Design as Type Design   Scott Meyers, Item #19   How should objects of your new type be created and destroyed?   How should object initialization differ from object assignment?   What does it mean for objects of your new type to be passed by value?

Class Design as Type Design 2   What are the restrictions on legal values for your new type?   Does your new type fit into an inheritance graph?   What kind of type conversions are allowed for your new type?   What operators and functions make sense for the new type?

Class Design as Type Design 3   What standard functions should be disallowed?   Who should have access to members of your new type?   What is the “undeclared interface” of your new type?   How general is your new type?   Is a new type really what you need?

What is an object?  An object is an instance of a class.  An object has state, behavior, identity.

What is an object? Coad-Yourdon An abstraction of something in a problem domain, reflecting the capabilities of the system to keep information about it, interact with it, or both; an encapsulation of attribute values and their exclusive services.

What is an object? OMG An object is a thing. It is created as the instance of an object type. Each object has a unique identity that is distinct from and independent of any of its characteristics. Each object offers one or more operations.

What is an object? Firesmith An object is defined as a software abstraction that models all relevant aspects of a single tangible or conceptual entity or thing from the application domain or solution space. An object is one of the primary entities in an object-oriented application, typically corresponds to a software module, and consists of a set of related attribute types, messages, exceptions, operations, and optional component objects.

What is an object? Booch From the perspective of human cognition, an object is any of the following:   A tangible and/or visible thing.   Something that may be apprehended intellectually.   Something toward which thought or action is directed.

What is an object? Booch continued. An object has state, behavior, and identity; the structure and behavior of similar objects are defined in their common class; the terms instance and object are interchangeable.

What is an object? Shlaer-Mellor An object is an abstraction of a set of real- world things such that:   All the things in the set have the same characteristic.   All instances are subject to and conform to the same set of rules and policies.

What is an object? Jacobson An object is characterized by a number of operations and a state which remembers the effect of these operations.

What is encapsulation?  Internal details of objects are concealed from the objects users (information hiding).  Both data and implementation may be hidden. The object is a black box.  Access to members is controlled through the class definition.  The accessible part of a class is called its interface.

Data encapsulation example class Clock { private: int hours; // 1-12 private int minutes; // 0-59 public: Clock(int h, int m) { if (h 12) { throw(”Hours must be between 1 and 12″); } if (m 59) { throw(”Minutes must be between 0 and 59″); } h = hours; m = minutes; } //... };

Class Invariants   The above is an example of “Programming by Contract.”   The class guarantees that   These are called class invariants

Data Members Data members can be declared as:   const – a declaration that an object is read only. The object may be stored in a CPU register.   volatile – a declaration that an object’s value may be changed asynchronously. The object may not be stored in a CPU register.

Data Members 2   static – A data member shared by all objects of a class. There is only one copy of a static member, it is not part of the object memory layout.   mutable – A data member that is allowed to be modified, even if it a member of a const object.

Is the following code legal? struct X { static int a = 2; }; main() { X my_x; X::a = 4; my_x::a = 5; }

Not legal! Cannot initialize static data member a within class. static variables are similar to extern variables.

Is the following code legal? struct X { static int a; }; main() { X my_x; X::a = 4; my_x::a = 5; }

Not legal! This is a linker error. X::a was used but never defined.

Is the following code legal? struct X { static int a; }; int X::a; main() { X my_x; X::a = 4; my_x::a = 5; }

Legal. X::a was defined before being used. Note: it is okay that X::a was not initialized.

Is the following code legal? struct X { static int a; static const int b = 3; }; int X::a = 2; main() { X my_x; X::a = 4; my_x::a = X::b; }

Legal. static const members can be declared and defined at the same time.

Is the following code legal? void func(volatile std::list li) { int n = li.front(); } Not legal! Only volatile member functions can be called on a volatile object. list::front() not a volatile function

What is a method?  A method is a member function that acts upon an object. A method is generally called for one object (exception: static members).  Commonly found methods are constructors, destructors, assignment, mutators, accessors.

Static Members #include using namespace std; class X { public: int a; void f(int b) { cout << “X::f()\n”;} }; int main() { int X::*ptiptr = &X::a; //pointer to data member void (X::* ptfptr) (int) = &X::f; //pointer to member function X xobject; xobject.*ptiptr = 10; (xobject.*ptfptr) (20); }

What is message passing?  Messages are transfers of data or requests for another object to take an action.

Message passing example   Adapter pattern