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A Consolidated Intensive lesson on C

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  1. A CondensedCrash Course on 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++? • Popular and 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 90/10 rule: 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 Maintainability(programmer time) Efficiency(processor time)

  6. Design goals of C++ • 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. C++ does not allow old-style C function declarationsvoid f(a) int a; {} generic function declarationsvoid f(); void g() { f(2); } setting enum to intenum Dir {Up, Down}; Dir d=1; multiple declarationsint 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 commentsint f(int a, int b) { return a//**/b;} Compatibility with C How is C++ not backward compatible with C (C89)? (For these, C++ is backward compatible with C99)

  8. Purpose of a programming 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. What is object-oriented? • Encapsulation“black box” – internal data hidden • Inheritancerelated classes share implementation and/or interface • Polymorphismability to use a class without knowing its type © 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. 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 Declarations and definitions

  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.) INTEGRAL ARITHMETIC USER-DEFINED Do not rely on sizes of these! (Implementation-dependent)

  16. Macros • Dangerous: • compiler never sees themsource 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 • “on the 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-in types initialized to 0(but local variables uninitialized) • Initialized before main() invoked • Initialization order: • within translation unit, same as definition • between translation units, arbitrary order Bad! No guarantee that twopiwill be initialized correctly file1.cpp double pi = 3.14; file2.cpp double twopi = 2*pi;

  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. In C, a struct can have both member variables and methods: In C++, syntax is simpler: OO in C 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 consistent naming 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 have a simple class. class A { public: A() { printf("con\n"); } A(const A& other) { printf("copycon\n"); } ~A() { printf("des\n"); } A& operator=(const A& other) { printf("assign\n"); return *this; } };

  27. Example 1 What is the 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 is the 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 is the output of the following program? (VC++ 6.0 -- Windows) 03 con 07 copycon 05 des 08 des 01 A F() 02 { 03 A tmp; 04 return tmp; 05 } 06 { 07 A a = F(); 08 } (g++ 3.4.3 -- Linux) 07 con 08 des

  30. Avoid new and delete • 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 use new 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 of new/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 methoddelete com;

  33. Initializer lists Assign values inside constructor: Matrix::Matrix(const Matrix& other) { m_n = 0; m_a = 0; } Use initializer list: Matrix::Matrix(const Matrix& other) : m_n(0), m_a(0) { }

  34. Concrete classes • A concrete 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. Reference: alternate name for an object (alias) There is no null reference No reference to a temporary Syntax confusing Basically a const dereferenced pointer with no operations References int b; int &a = b; (Now use ‘a’ as ‘b’) int &a; int& a = 1; int* c = &a; “get address of”(not a reference)

  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 pass by pointer pass by value pass by reference void f(int a, int* b, int& c) { // changes to a are NOT reflected outside the function // changes to b and cARE reflected outside the function } main() { int a, b, c; f(a, &b, c); } 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 typesint 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” referencevoid update(int& a); update(2); // error • pass outputs by pointerint x = 1; next(x); // should not change xint x = 1; next(&x); // may change x • ok to return a ref, or const ref

  41. C++ function mechanisms • Overloaded function names • Cleaner and saferprint(int); print(float); • But beware print(int); print(int*); print(0); • Default parametersvoid print(int a, int b=0, int c=0); • Operator overloadingMatrix& operator+=(const Matrix& other); • Implicit conversion operatoroperator int() const {} // converts to int • Provides convenient syntax, but potentially dangerous so use sparingly

  42. Opaque pointers • An opaque 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 typesRun-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 expresses logical 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 prevents object from being modified (orig., readonly) • Avoid magic numberschar 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 • Assert allows 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 derived from base class • Two classes should pass the “ISA” test:derived class is a base classclass 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 • Function of derived class is called even if you have only a pointer to the base class File.hclass Shape { virtual void Draw();};class Circle : public Shape { virtual void Draw();}; File.cppvoid Func1() { Circle mycirc; Func2(&mycirc);}void Func2(Shape* s) { s->Draw();} // calls Circle::Draw()

  50. How a virtual function works Shape vtable Circle vtable vfunc1 addr vfunc1 addr vfunc2 addr vfunc2 addr ... ... vfuncN addr vfuncN addr mycirc { var1 shape member variables ... varM vtable ptr { var1 circle member variables ... varN