C++: Abstraction mechanisms

Basic classes

• Concrete: ‘just like built in types’
  • user would recompile if representation changes in a significant way
  • representation is part of definition
  • efficient
    • place objects in stack (statically allocated memory)
    • refer to objects directly (not just through pointers and references)
    • initialize objects immidiately and completely
    • copy objects
  • destructor ~ClassName()
  • RAII: acquire resources in constructor and deallocate in destructor
• Abstract: a class with a pure virtual function
  • user does not care at all about representation. representation can change without user recompiling
  • usually used through pointers or references
  • virtual keyword means ‘may be redfined later in a class derived from this one’
  • = 0 or pure virtual means ‘some class derived from this must define this function’
  • :public means ‘derived from’
  • a class that provides an interface to a variety of other classes is called a polymorphic type
  • Classes with virtual function have a virtual function table or vtbl
• Class hierarchy: ‘a set of classes ordered in a lattice created by derivation’
• The vtbl is used when a user calls functions of an ‘abstract’ interface to resolve and invoke the correct function from the most derived class.
• Class hierarchies offer
  • Interface inheritance (base class is an interface for derived class)
  • Implementation inheritance (base class provides functions or data to derived class)
• Avoid not deleteing objects in the free store: use unique_ptr
• Copying and Moving
• when to memeberwise copy
  • concrete type: yeah
  • sophisticated concrete type: no
  • abstract type: almost never
• Copy of an object of a class is defined by:
  (for class X argument is const X&)
  • copy constructor
  • copy assignment operator
• for move:
  (for class X argument is X&&)
  && means rvalule reference. i.e. no one can assign to it
  • move constructor
  • move assignment operator
• =delete to suppress operations like copy and move for class hierarchy when object members are unknown given a pointer to a base.

Templates

‘A class or a function that is parametrized with a set of types or values’. A compile-time mechanism (no runtime overhead).

• Parametrized types
  • use template<typemame T>
    a mathematical ‘for all types T’

  • use T for typing

  • Use parametrized functors to strategy pattern algorithms.
    [ capture_list ]( arguments ){ body } names from local scope in capture list are ‘captured’ into the scope of body
    • capture nothing: []
    • capture everything by reference: [&]
    • caputre evrything by value: [=]
    • capture a name by reference: [&name]
• Variadic templates
  • template<typename... VariadicTypenames>
  • First argument can be separated like: template<typename T, typename... VariadicTypenames>
• Aliases: synonym for a type or template
  • using keyword