lecture5-cpp.pptintroduccionaC++basicoye

A Condensed Crash Course on C++ ECE 417/617: Elements of Software Engineering Stan Birchfield Clemson University

Recommended C++ resources • Bjarne Stroustrup, The C++ Programming Language • Scott Meyers, Effective C++

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

C vs. C++ • C++ is C incremented (orig., “C with classes”) • C++ is more expressive (number of C++ source lines less than C 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)

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 Efficiency (processor time) Maintainability (programmer time)

Design goals of C++ • Backward compatibility with C (almost completely – every program in K&R is a C++ program) • 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) • Compliation analysis to prevent accidental corruption of data (type-checking and data hiding) • Support object-oriented style of programming

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)

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

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

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)

• 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

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

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; }

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

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

Macros • Dangerous: – compiler never 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; }

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; }

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 file1.cpp double pi = 3.14; file2.cpp double twopi = 2*pi;

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 member variables

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

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 public or protected – member variables private

Default class functions • By default, each class has member functions: – 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)

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); 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)

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:

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!

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

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

References • Reference: alternate name for an object • There is no null reference • No pointer to a reference • No reference to a temporary • Syntax confusing • Basically a const dereferenced pointer with no operations int &a; int a; int& b=a; b++; int* c = &a; int &a = b; int* c= &a; int& a = 1;

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

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

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

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

Const • Const prevents object 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]; // ”

Inheritance • Subclass derived from 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

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

Virtual functions • Function of 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()

How a virtual function works Shape vtable vfunc1 addr vfunc2 addr ... vfuncN addr vfunc1 addr vfunc2 addr ... vfuncN addr Circle vtable var1 ... varM vtable ptr mycirc

What is the penalty 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)

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) class Shape { virtual void Draw() = 0; };

Multiple inheritance • C++ allows you to inherit from multiple base classes class CalculatorWatch : public Calculator, Watch {}; • Works best if – exactly one base class passes ISA test – all other base classes are interfaces • Advanced feature that you will not need in this course

Polymorphism • Polymorphism – “ability to 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)

Exceptions • Error handling in 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”); }

Exceptions (cont.) • Error handling 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”); }

Templates • Define a class 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; };

Generic programming • Drawbacks of 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

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

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

std::string • Example: #include <string> void Func() { 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” }

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:

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:

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; } }

Types of iterators template<class InputIterator, class Type> InputIterator find( InputIterator _First, InputIterator _Last, const Type& _Val ); • Each container provides a different type input forward bidirectional random access output Types

Allocators • STL written for 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

Streams • C – flush, fprintf, fscanf, sprintf, sscanf – fgets, getc • C++ – cout, cin, cerr

Numerics • valarray – matrix and vector (not std::vector) – slices and gslices • complex • random numbers

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