lecture02-cpp.ppt

lecture02-cpp.ppt

• 1.
  A Condensed Crash Courseon C++ ECE 417/617: Elements of Software Engineering Stan Birchfield Clemson University
• 2.
  Recommended C++ resources •Bjarne Stroustrup, The C++ Programming Language • Scott Meyers, Effective C++
• 3.
  Why C++? • Popularand relevant (used in nearly every application domain): – end-user applications (Word, Excel, PowerPoint, Photoshop, Acrobat, Quicken, games) – operating systems (Windows 9x, NT, XP; IBM’s K42; some Apple OS X) – large-scale web servers/apps (Amazon, Google) – central database control (Israel’s census bureau; Amadeus; Morgan- Stanley financial modeling) – communications (Alcatel; Nokia; 800 telephone numbers; major transmission nodes in Germany and France) – numerical computation / graphics (Maya) – device drivers under real-time constraints • Stable, compatible, scalable
• 4.
  C vs. C++ •C++ is C incremented (orig., “C with classes”) • C++ is more expressive (fewer C++ source lines needed than C source lines for same program) • C++ is just as permissive (anything you can do in C can also be done in C++) • C++ can be just as efficient (most C++ expressions need no run-time support; C++ allows you to – manipulate bits directly and interface with hardware without regard for safety or ease of comprehension, BUT – hide these details behind a safe, clean, elegant interface) • C++ is more maintainable (1000 lines of code – even brute force, spaghetti code will work; 100,000 lines of code – need good structure, or new errors will be introduced as quickly as old errors are removed)
• 5.
  Efficiency and Maintainability 90/10rule: 10% of your program will take 90% of the processor time to run optimize what needs to be optimized, but no more focus on design
• 6.
  Design goals ofC++ • Backward compatibility with C (almost completely – every program in K&R is a C++ program – but additional keywords can cause problems) • Simplicity, elegance (few built-in data types, e.g., no matrices) • Support for user-defined data types (act like built-in types; N.B. Standard Template Library (STL)) • No compromise in efficiency, run-time or memory (unless “advanced features” used) • Compilation analysis to prevent accidental corruption of data (type-checking and data hiding) • Support object-oriented style of programming
• 7.
  Compatibility with C C++does not allow • old-style C function declarations void f(a) int a; {} • generic function declarations void f(); void g() { f(2); } • setting enum to int enum Dir {Up, Down}; Dir d=1; • multiple declarations int i; int i; • assigning to void * int* p = malloc(10); • “implicit int” signed a = 7; Other differences: • const global variables have internal linkage in C++, external in C • extra keywords in C++ void main() { int catch = 5; } • bizarre comments int f(int a, int b) { return a//**/b ; } How is C++ not backward compatible with C (C89)? (For these, C++ is backward compatible with C99)
• 8.
  Purpose of aprogramming language • Programming languages serve two purposes: – vehicle for specifying actions to be executed “close to the machine” – set of concepts for thinking about what can be done “close to the problem being solved” • Object-oriented C++ excels at both
• 9.
  Learning C++ • Goal:Don’t just learn new syntax, but become a better programmer and designer; learn new and better ways of building systems • Be willing to learn C++’s style; don’t force another style into it • C++ supports gradual learning – Use what you know – As you learn new features and techniques, add those tools to your toolbox • C++ supports variety of programming paradigms
• 10.
  Programming paradigms • procedural– implement algorithms via functions (variables, functions, etc.) • modular – partition program into modules (separate compilation) • object-oriented – divide problem into classes (data hiding, inheritance) • abstract – separate interface from implementation (abstract classes) • generic – manipulate arbitrary data types (STL: containers, algorithms)
• 11.
  • Encapsulation “black box”– internal data hidden • Inheritance related classes share implementation and/or interface • Polymorphism ability to use a class without knowing its type What is object-oriented? © SDC “C++ is an object-oriented language” = C++ provides mechanisms that support object-oriented style of programming
• 12.
  Some C++ concepts •constructor / destructor / copy constructor • initialization list • inheritance • exceptions • overloading operators (e.g., assignment operator) • namespace • const • virtual function • pure virtual (abstract) function • friend • template • standard template library (STL) • pass by value, pass by reference • composition versus derivation
• 13.
  A simple C++program #include <iostream> // std::cout #include <cstdio> // printf int main() { int a = 5; // 'a' is L-value float b = 0.9f; printf("Hello world %d %3.1f n", a, b); std::cout << "Hello world" << a << " " << b << " " << std::endl; return 0; }
• 14.
  Declarations and definitions •Declaration: – extern char c; – struct User; – double sqrt(double);  Okay to have many • Definition: – char c; – int count = 1; – double abs(double a) { a>0 ? a : -a; }  Must have exactly one
• 15.
  Fundamental types • bool(true  1, false  0) • char (could be signed or unsigned – implementation-defined) • int (signed by default) • double • void (“pseudo-type”) • enum • class • also short, long, struct, float, wchar_t, etc.) Do not rely on sizes of these! (Implementation-dependent) INTEGRAL ARITHMETIC USER-DEFINED
• 16.
  Macros • Dangerous: – compilernever sees them source code  translation unit – global • Instead, use – const – inline – template – enum • Ok to use for include guards (“header wrappers”) • If you must use a macro, give it a long ugly name with lots of capital letters Example: template<typename T> inline T max(T t){ t>0 ? t : -t; }
• 17.
  Memory allocation • “onthe stack” – block delimited by {} – object alive till it falls out of scope – calls constructor / destructor • “on the heap” – new and delete replace malloc, calloc, free – object exists independently of scope in which it was created – also “on the free store” or “allocated in dynamic memory” – be careful: new  delete, new[]  delete[] – for safety, same object should both allocate and deallocate • “local static store” void foo() { static int i=0; }
• 18.
  Global variables • Built-intypes initialized to 0 (but local variables uninitialized) • Initialized before main() invoked • Initialization order: – within translation unit, same as definition – between translation units, arbitrary order file1.cpp double pi = 3.14; file2.cpp double twopi = 2*pi; Bad! No guarantee that twopi will be initialized correctly
• 19.
  A class class Date{ public: enum Month {Jan, Feb, Mar, ...} Date(int year, Month month, int day); int GetDay() const; void SetDay(int day); Date& operator+=(int days); private: Month m_month; int m_year, m_day; }; member functions (methods) member variables
• 20.
  Struct vs. class •In C++, no difference b/w struct and class (except default public vs. private) • In C++, struct can have – member variables – methods – public, private, and protected – virtual functions – etc. • Rule of thumb: – Use struct when member variables are public (just a container) – Use class otherwise
• 21.
  OO in C •In C, a struct can have both member variables and methods: • In C++, syntax is simpler: void CreateFoo() { } struct Foo { void (*Construct)(); int m_data; }; int main() { struct Foo a; a.Construct = &CreateFoo; a.Construct(); } struct Foo { Foo() int m_data; }; Foo::Foo() { } int main() { Foo a; }
• 22.
  Names • Maintain consistentnaming style – long names for large scope – short names for small scope • Don’t start with underscore; reserved for special facilities • Avoid similar-looking names: l and 1 • Choosing good names is an art
• 23.
  Access control • Public:visible to everyone • Private: visible only to the implementer of this particular class • Protected: visible to this class and derived classes • Good rule of thumb: – member functions (methods): • if non-virtual, then public or protected • if virtual, then private – member variables should be private (except in the case of a struct)
• 24.
  The big four •By default, each class has four methods: – constructor Date(); – destructor ~Date(); – copy constructor Date(const Date& other); – assignment operator Date& operator=(const Date& other); • These call the appropriate functions on each member variable • Be careful: If this is not what you want, then either override or disallow (by making private)
• 25.
  Constructor and destructor •(Copy) constructor creates object • Destructor destroys (“cleans up”) object • Be aware of temporary objects class Matrix { Matrix(const Matrix& other); Matrix operator+(const Matrix& other) const; Matrix& operator=(const Matrix& other); }; void foo() { Matrix a, b, c, d; a = b + c + d; } What functions get called? (Note: There are ways to speed this up while preserving the syntax)
• 26.
  Example Suppose we havea simple class. class A { public: A() { printf("conn"); } A(const A& other) { printf("copyconn"); } ~A() { printf("desn"); } A& operator=(const A& other) { printf("assignn"); return *this; } };
• 27.
  Example 1 What isthe output of the following program? 01 { 02 A a; 03 A* b = new A(); 04 *b = a; 05 delete b; 06 A c = a; 07 } 02 con 03 con 04 assign 05 des 06 copycon 07 des 07 des
• 28.
  Example 2 What isthe output of the following program? 01 void F(const A& f, A* g, A h) 02 { 03 *g = f; 04 } 05 { 06 A a, b; 07 F( a, &b, a); 08 } 06 con 06 con 01 copycon 03 assign 04 des 08 des 08 des
• 29.
  Example 3 What isthe output of the following program? 01 A F() 02 { 03 A tmp; 04 return tmp; 05 } 06 { 07 A a = F(); 08 } (VC++ 6.0 -- Windows) 03 con 07 copycon 05 des 08 des (g++ 3.4.3 -- Linux) 07 con 08 des
• 30.
  Avoid new anddelete • Whenever possible, avoid ‘new’ and ‘delete’ • Instead create object on stack • Automatic destructor makes things easier • No need to worry about forgetting to delete the object (memory leak) or deleting the wrong object (crash) • If you must use ‘new’, then try to keep the ‘delete’ nearby • This helps code maintenance – otherwise it is hard to keep track of the new/delete pairs
• 31.
  When to usenew and delete • Sometimes you have to use new and delete • And sometimes the pair cannot be close together • Oh well • The next slide shows an example where we need to break both of these rules
• 32.
  An example ofnew/delete • You have a base class: class Command { virtual DoSomething(); }; • You have several derived classes: class CommandAdd : public Command {}; class CommandMove : public Command {}; class CommandSet : public Command {}; • You have a list of objects whose types are unknown at compile time (polymorphism): std::vector<Command*> undo_list; • Must put pointers in list – not actual objects – because the objects may be of different sizes (among other reasons) • Someone creates the object and puts its pointer on the list: undo_list.push_back( new CommandAdd() ); • Later the object is removed from the list and deleted: Command* com = undo_list.back(); undo_list.pop(); com->DoSomething(); // call a virtual method delete com;
• 33.
  Initializer lists Matrix::Matrix(const Matrix&other) : m_n(0), m_a(0) { } Matrix::Matrix(const Matrix& other) { m_n = 0; m_a = 0; } Use initializer list: Assign values inside constructor:
• 34.
  Concrete classes • Aconcrete class – does a single, relatively small thing well and efficiently – hides data members (encapsulation) – provides clean interface – acts like a built-in type – is a “foundation of elegant programming” – Stroustrup • Don’t underestimate the importance of this basic C++/OO feature!
• 35.
  Class relationships • OK: –A calls function from B – A creates B – A has a data member of type B • Bad: – A uses data directly from B (without using B’s interface) • Even worse: – A directly manipulates data in B
• 36.
  Pointers, arrays, references •Use 0, not NULL (stronger type checking) • Name of array is equivalent to pointer to initial element • Access array using * or []; same efficiency with modern compiler • use std::vector, not built-in array, when possible • Reference is like a pointer
• 37.
  References • Reference: alternate namefor an object (alias) • There is no null reference • No reference to a temporary • Syntax confusing • Basically a const dereferenced pointer with no operations int &a; int* c = &a; int& a = 1; “get address of” (not a reference) int b; int &a = b; (Now use ‘a’ as ‘b’)
• 38.
  Confusing syntax int a,b; int c = a * b; int* d = &a; int e = *d; int& f = a; * means • multiplication, or • pointer, or • dereference pointer & means • get address of, or • reference Same symbol, different meanings!
• 39.
  Pass by X voidf(int a, int* b, int& c) { // changes to a are NOT reflected outside the function // changes to b and c ARE reflected outside the function } main() { int a, b, c; f(a, &b, c); } pass by value pass by pointer pass by reference DOES make a copy does NOT make a copy PBP and PBR are different syntax for the same functionality
• 40.
  Argument passing /return • Pass / return by value – calls copy constructor – ok for built-in types int foo(int a) { return 0; } – performance penalty for structs and classes (temporary objects) • Pass / return by reference or pointer – does not call copy constructor – pass inputs by const reference – never pass inputs by “plain” reference void update(int& a); update(2); // error – pass outputs by pointer int x = 1; next(x); // should not change x int x = 1; next(&x); // may change x – ok to return a ref, or const ref
• 41.
  C++ function mechanisms •Overloaded function names – Cleaner and safer print(int); print(float); – But beware print(int); print(int*); print(0); • Default parameters void print(int a, int b=0, int c=0); • Operator overloading Matrix& operator+=(const Matrix& other); • Implicit conversion operator operator int() const {} // converts to int – Provides convenient syntax, but potentially dangerous so use sparingly
• 42.
  Opaque pointers • Anopaque pointer is used to hide the internal implementation of a datatype • Also called Pimpl (pointer to implementation) idiom, or Cheshire Cat • Example: The d-pointer is the only private data member of the class and points to an instance of a struct defined in the class' implementation file http://en.wikipedia.org/wiki/Opaque_pointer
• 43.
  Explicit type conversion •C++ casts – static_cast between 2 related types (int/float, int/enum, 2 pointers in class hierarchy) – reinterpret_cast between 2 unrelated types (int/ptr, pointers to 2 unrelated classes) – const_cast cast away constness – dynamic_cast used for polymorphic types Run-time type info (RTTI) • Avoid casts, but use these instead of C casts – e.g., compiler can perform minimal checking for static_cast, none for reinterpret_cast
• 44.
  Namespaces • Namespace expresseslogical grouping • using declaration – Don’t use global using except for transition to older code – Ok in namespace for composition – Ok in function for notational convenience • Namespaces are open • Unnamed namespaces restrict code to local translation unit • Aliases ( namespace ShortName = LongName; )
• 45.
  Const • Const preventsobject from being modified (orig., readonly) • Avoid magic numbers char a[128]; const int maxn = 128; char a[maxn]; • Logical constness vs. physical constness • Const is your friend; use it extensively and consistently • can cast away constness, but be sure to use mutable • const pointers: – const int * const ptr = &a[0]; // const ptr to a const int – int const * const ptr = &a[0]; // ” – int * const p2 = &a[0]; // const ptr to an int – const int * p1 = &a[0]; // ptr to a const int – int const * p2 = &a[0]; // ”
• 46.
  Assert macro • Assertallows the programmer to explicitly type assumptions about expected inputs and values • Use assert generously; it is your friend • Assert helps to catch bugs early during development • Assert is removed by precompiler before final release, so no run- time penalty • Use assert only to check values; do not change values!!! #include <assert.h> int GetValue(int index) { assert(index >= 0 && index < array.size()); if (index < 0 || index >= array.size()) return -1; // value should make sense return array[index]; } If performance is not a concern, then it is okay to augment (but not to replace) assert with an extra check that will remain in the final version.
• 47.
  Inheritance • Subclass derivedfrom base class • Two classes should pass the “ISA” test: derived class is a base class class Shape { }; class Circle : public Shape { }; • Class hierarchy: means of building classes incrementally, using building blocks (subclass becomes base class for someone else) • Facilitates code reuse
• 48.
  Inheritance vs. composition •Inheritance: “is a” class Circle : public Shape { }; • Composition: “has a” class Circle { private: Shape m_shape; }; • Decision should be based on commonality of interface
• 49.
  Virtual functions • Functionof derived class is called even if you have only a pointer to the base class File.h class Shape { virtual void Draw(); }; class Circle : public Shape { virtual void Draw(); }; File.cpp void Func1() { Circle mycirc; Func2(&mycirc); } void Func2(Shape* s) { s->Draw(); } // calls Circle::Draw()
• 50.
  How a virtualfunction works Shape vtable vfunc1 addr vfunc2 addr ... vfuncN addr vfunc1 addr vfunc2 addr ... vfuncN addr Circle vtable var1 ... varM vtable ptr mycirc var1 ... varN shape member variables circle member variables { {
• 51.
  What is thepenalty of a virtual function? • Space: – one vtable per class with virtual function(s) – one pointer per instance • Time: – one extra dereference if type not known at compile time – no penalty if type known at compile time (ok to inline a virtual function)
• 52.
  Pure virtual function •Pure virtual function – Function intentionally undefined – Same penalty as regular virtual function • Abstract class – Contains at least one pure virtual function – Cannot instantiate; must derive from base class and override pure virtual function – Provides an interface (separates interface from implementation) • Advice: virtual functions should always be pure virtual – i.e., “Make non-leaf classes abstract” (Scott Meyers, Item 29) – Also, “Don’t derive from concrete classes” (Herb Sutter, p. 137) • More advice: Make virtual functions private (Herb Sutter, p. 134). This separates the override implementation details from the public interface. class Shape { virtual void Draw() = 0; };
• 53.
  Multiple inheritance • C++allows you to inherit from multiple base classes • Works best if – exactly one base class passes ISA test – all other base classes are interfaces • Advanced feature that is rarely needed class MyDialog : public CDialog, Observer {}; MyDialog is a CDialog MyDialog needs a single method from Observer (lightweight class) (see MVC architecture)
• 54.
  Polymorphism • Polymorphism – “abilityto assume different forms” – one object acts like many different types of objects (e.g., Shape*) – getting the right behavior without knowing the type – manipulate objects with a common set of operations • Two types: – Run-time (Virtual functions) – Compile-time (Templates)
• 55.
  Exceptions • Error handlingin C: – Half of code is error handling – Dangerous: Easy for programmer to forget to check return value void Func() { int ret; ret = OpenDevice(); if (ret != 0) error(“Unable to open device”); ret = SetParams(); if (ret != 0) error(“Unable to set params”); }
• 56.
  Exceptions (cont.) • Errorhandling in C++: – try-catch blocks safer – separate “real code” from error handling code void Func() { try { OpenDevice(); SetParams(); } catch (const MyException& e) { e.ReportToUser(); } catch (...) { abort(1); } } void OpenDevice() { if (bad) throw MyException(“Cannot open device”); }
• 57.
  Templates • Define aclass or function once, to work with a variety of types • Types may not be known until future • Better type checking and faster (cf. qsort) • Specialization can be used to reduce code bloat • Templates support generic programming template<typename T> T Max(T a, T b) { return a>b ? a : b; } template<typename T> class Vector { Vector(int n, T init_val); T* m_vals; };
• 58.
  Generic programming • Drawbacksof qsort in <stdlib.h> – requires a compare function, even if trivial – loss of efficiency b/c dereferencing pointer – lost type safety b/c void* – only works with contiguous arrays – no control over construction / destruction / assignment; all swapping done by raw memory moves
• 59.
  Standard Template Library(STL) • Containers: – Sequences • vector – array in contiguous memory (replaces realloc) • list – doubly-linked list (insert/delete fast) • deque (“deck”) – double-ended queue • stack, queue, priority queue – Associative • map – dictionary; balanced tree of (key,value) pairs like array with non-integer indices • set – map with values ignored (only keys important) • multimap, multiset (allow duplicate keys) – Other • string, basic_string – not necessarily contiguous • valarray – vector for numeric computation • bitset – set of N bits
• 60.
  STL (cont.) • Algorithms(60 of them): – Nonmodifying • find, search, mismatch, count, for_each – Modifying • copy, transform/apply, replace, remove – Others • unique, reverse, random_shuffle • sort, merge, partition • set_union, set_intersection, set_difference • min, max, min_element, max_element • next_permutation, prev_permutation
• 61.
  std::string • Example: #include <string> voidFunc() { std::string s, t; char c = 'a'; s.push_back(c); // s is now “a”; const char* cc = s.c_str(); // get ptr to “a” const char dd[] = "afaf"; t = dd; // t is now “afaf”; t = t + s; // append “a” to “afaf” }
• 62.
  std::vector #include <vector> void Func() { std::vector<int>v(10); int a0 = v[3]; // unchecked access int a1 = v.at(3); // checked access v.push_back(2); // append element to end v.pop_back(); // remove last element size_t howbig = v.size(); // get # of elements v.insert(v.begin()+5, 2); // insert 2 after 5th element } • Example:
• 63.
  std::vector (cont.) #include <vector> #include<algorithm> void Func() { std::vector<int> v(10); v[5] = 3; // set fifth element to 3 std::vector<int>::const_iterator it = std::find(v.begin(), v.end(), 3); bool found = it != v.end(); if (found) { int three = *it; int indx = it - v.begin(); int four = 4; } } • Example:
• 64.
  Iterators • iterator –generalized pointer • Each container has its own type of iterator void Func() { stl::vector<int> v; stl::vector<int>::const_iterator it = v.begin(); for (it = v.begin() ; it != v.end() ; it++) { int val = *it; } }
• 65.
  Types of iterators template<classInputIterator, class Type> InputIterator find( InputIterator _First, InputIterator _Last, const Type& _Val ); • Each container provides a different type input forward bidirectional random access output Types
• 66.
  Allocators • STL writtenfor maximum flexibility • Each container has an allocator • Allocator is responsible for memory management (new/delete) template < class Type, class Allocator = allocator<Type> > class vector { ... }; • Advice: Ignore allocators
• 67.
  Streams • C – flush,fprintf, fscanf, sprintf, sscanf – fgets, getc • C++ – cout, cin, cerr
• 68.
  Buffer overrun • Neveruse sprintf! • Use snprintf instead to avoid buffer overrun • Or use std::stringstream
• 69.
  Numerics • valarray – matrixand vector (not std::vector) – slices and gslices • complex • random numbers